The 70% Charge Myth (long msg)

Submitted: Thursday, Aug 18, 2005 at 22:26

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Mike DiD

When I started reading that “car batteries are never charged above 70% capacity”, I had trouble believing it for several reasons

More accurate statements about charging a battery in a car would be that the battery will not charge fully if -
I) You install a battery needing a high charge voltage (Calcium-Calcium needs 14.8 volts)
ii) Your Alternator or regulator is faulty.
iii) Your Regulator Alternator is an older design that puts out less than 14.2 volts.
iv) You don’t drive long enough to replace the charge taken from the battery (charge time for a discharged battery can range from 2 to 12 hours). www.absorbedpower.com/battery/support/applications/ www.batteryuniversity.com/partone-13.htm
v) You add heavy loads (winch, high-power audio, HF Radio) and your Alternator can no longer provide the power.

a. Does it really matter if your battery is charged to 70% or 95% ?
If you just use your battery for Starting in a reasonably warm climate and the battery is in good condition, then it doesn’t matter if you have 70% or 95% charge (ignoring sulphation damage) because starting only uses up between 2 and 5% of battery capacity.
But if you are going into remote areas and/or you are using the battery to power fridge, lighting, winch etc then you really need to have lots of battery capacity. If your alternator is in reality only charging to 70% then you might be looking at spending good money on better charging, better Isolator or bigger batteries - but if your Alternator is in fact charging close to 100%, this would be wasted money that you could be spending differently to get a real improvement.
If you increase your battery charge level from 70% to 100%, you are more than doubling your available capacity because you should never regularly discharge any battery below 50% (yes, even DEEP cycle) so discharging from 70 to 50 gives you 20% available but discharging from 100 to 50 gives you 50% available.

b. These are articles that suggest “car batteries aren’t charged above 70% capacity”-
1. “With common machine sensed alternators, batteries are, in most cases, charged to only 60 - 70% of their capacity, however long the engine is running” www.adverc.co.uk/technical/technical-altmis.asp This article states you will get closer to 100% charge if you buy ADVERC’s special regulator !!!!
2. “once a battery has been on charge for more than about 1/2 hour, changes take place . . . . which have the effect of increasing the internal resistance. This build up of internal resistance will inhibit the flow of charging current, . . . . It is estimated that batteries recharged in this way never reach more than about 70% full charge.” www.acornengineer.com/kestrel_alt_controller.php This article states you will get closer to 100% charge if you buy Kestrel’s special regulator !!!!
3. “In practice, starter batteries spend most of their life somewhere between 65% and 70% of full charge” www.caravanandmotorhomebooks.com/articles/lead_acid_batteries.htm
4. “Vehicle alternator/regulators . . . work this way and . . . charging tapers almost to zero once the battery is 70% 'full' “. www.exploroz.com/vehicle/electrics/solar.asp
5. “Car batteries are seldom charged above 70% of their capacity, but they are designed for shallow discharge.” Deep Cycle Charger - Silicon Chip, page 34, Nov 2004 (SC) www.siliconchip.com.au/cms/A_103239/article.html
6. “Vehicle charging systems stop charging when batteries are around 70% 'full' “ www.absorbedpower.com/battery/support/faq/
7. “Standard car and truck alternators are designed, for various good reasons to recharge car batteries to only 70-80%”, www.fridge-and-solar.net/agm.htm
8. “idling the engine or short stop-and-go trips during bad weather or at night will not fully recharge a car battery” - At least this one states the conditions under which a car battery will not be fully charged. www.uuhome.de/william.darden/carfaq14.htm Sect 14.2

c. Possible explanations for the 70% Charge Myth
1. These statements may have been true in the days of 30 Amp Generators with Relay regulators giving varying voltages. But with today’s high-capacity Alternators, Electronic Regulators, Temperature Compensation and remote Battery Voltage Sensing, overcharging is more likely. Overcharging would boil a battery in weeks whereas undercharging would cause a slower death by sulphation - since Relay Regulators could not control voltage accurately, they chose to undercharge rather than risk overcharge.
2. “During the first Stage of charging the battery is charged to 70%-80%” is widely accepted www.batteryuniversity.com/partone-13.htm , Somehow this leads people to erroneously believe the simple Constant-Voltage charging in a car can only provide the first 70% of charging in Stage 1 (current-limited) charging mode. To understand the rest of this article, you really need to understand the basics of Lead-Acid Battery Charging, so I suggest you read this reference.
3. References 3 and 4 are using the statement that a normal Alternator only charges to 70% to convince you to buy their improved Alternator Regulators. References 6 and 7 use this statement to convince you to buy their AGM batteries rather than standard wet-cell batteries.
4. If you connect a battery that has been charged in a car to another charger that produces a higher voltage, then current will flow into the battery. But people assume that this current is increasing the charge level of the battery, and therefore the battery was not fully charged, but this is not necessarily true. By the "conservation of energy" principle, the electrical energy flowing into the battery must go somewhere, and for a Lead Acid Battery there are three main options -
i. Converting the chemicals to store energy for later use during discharge.
ii. Heating up the battery.
iii. Electrolysis of the water, visible as gassing.
If 1 amp is flowing into a fully-charged car-size battery and all this energy is converted to heat, there will be a barely noticeable temperature rise of the battery.

d. The incorrect assumptions
But in fact a car actually provides only Stage 2 Constant Voltage charging (unless your Alternator is seriously under-rated), NOT Stage 1 constant-current charging !!!!. Stage 1 isn’t provided for in a car because starting the engine only uses 2 to 5 % of the capacity and Stage 1 current-limiting is only necessary for batteries that have been discharged to below 70% remaining.
So if you have a deeply discharged battery (Starting or Deep Cycle) connected to a Car Alternator. it won’t be charged in Stage 1 mode (current limited to a level safe for the battery) but rather it will charge at whatever current the battery will take from 14.4 volts - this could be 50 amps initially. So instead of taking 2 to 5 hours to get up to 70% in Stage 1, the battery is fast charged is much less time (and shortening the battery life). Charging current will continue to taper off as is normal for Stage 2 until the battery is fully charged. Also a car Alternator never switches to the lower (Stage 3) Float Voltage, (except due to compensation from temperature rise) so charging does not even slow down dramatically when 90 to 95% charge is reached, as happens in a 2 or 3 Stage charger.
So this following quote is wrong in concluding that charging tapers to almost zero when charge reaches 70%. In a car, charging actually tapers to almost zero when charge reaches 95%, but does not reduce to float voltage (that is why Taxi owners go to the expense of relocating the battery to the boot, because modern Alternators will over charge on vehicles driven on long shifts). “If the charging voltage is fixed, as the battery charges and its voltage rises, charge rate automatically falls. Vehicle alternator/regulators . . . work this way and . . . charging tapers almost to zero once the battery is 70% 'full'. This prevents overcharging with vehicles like taxis on shifts”. www.exploroz.com/vehicle/electrics/solar.asp
The graph here shows clearly that current tapers to zero at the end of Stage 2 (95%), NOT at the end of Stage 1 (70%) - www.batteryuniversity.com/partone-13.htm
All modern car charging systems use remote battery voltage sensing http://www.autoshop101.com/forms/h8.pdf and ambient temperature compensation, so you would be wasting money to “change” to one of these “improved” Alternator Regulators.

e. Why is it obvious that modern Car batteries are charged to near 100% ?
The average car battery lasts four years and a battery that is only ever charged to 70% maximum will Sulphate very quickly and therefore will not last 4 years.
From www.uuhome.de/william.darden/carfaq.htm -
- Section 13.2 - "sulfation will begin occurring when the State-of-Charge (SoC) drops below 100%"
- Section 14.2 - "Generally, idling the engine or short stop-and-go trips during bad weather or at night will not fully recharge a car battery. When a dead battery needs to be recharged, it is best to use an external battery charger because you could over heat and damage your vehicle's charging system and your will save a lot of gas and wear and tear on your engine". But this does not mean that long drives won't fully charge a battery, or that an adequately sized Alternator will not safely charge a partially discharged battery.
“Recharging less than 100% may result in premature battery failure” “loss of capacity can become permanent if the battery remains in a discharged or undercharged state for even a short period of time” - www.absorbedpower.com/battery/support/

h. Is my Alternator big enough to charge my battery ?
The general guideline seems to be that the Charging current should be no less than 1/3rd of the battery capacity - so for a 125 Amp Alternator (assuming 25 amp is needed to run the loads) you can charge up to a 300 AmpHour battery. www.adverc.co.uk/technical/technical-teched-5.asp#Battery%20Facts
“A simple test to determine if the charging system is large enough is to check the battery's State-of-Charge after the surface charge has been removed. If the State-of-Charge is consistently above 95%, then the charging system is fully recharging the car battery based on your driving habits and electrical load.” Sect 5.3 from www.uuhome.de/william.darden/carfaq.htm

i. How can I be sure what the level of charge is ?
Measure your battery voltage early in the morning without ANY load connected to it (NO interior light on etc) - is it 12.3 volt (70% ) or is it closer to 12.6 volt (90%). Here are two opinions on using voltage to determine capacity..
“Instantaneous measurement of deep cycle battery voltage is meaningless and often leads to quite wrong conclusions: all you can really be sure of is that the meter's working.” www.exploroz.com/vehicle/electrics/solar.asp
“The state-of-charge of a lead-acid battery can, to a certain extent, be estimated by measuring the open terminal voltage. Prior to measuring, the battery must have rested for 4-8 hours after charge or discharge and reside at room temperature.” www.batteryuniversity.com/partone-13.htm
To be really sure of charge level, measure the SG of the battery - messy, but much more accurate than voltage measurements. It is still important to wait several hours after the last charge or discharge before measuring SG. The actual SG that indicates a full state of charges varies for different battery types, so you may need to “calibrate” for your battery by float charging it for a while. Here are some standard figures from www.uuhome.de/william.darden/carfaq9.htm -
1.260-1.270 for wet Starting batteries with pasted plates
1.267-1.284 for wet motive Deep Cycle batteries with solid lead positive plates
1.300-1.310 for wet motive deep cycle batteries with tubular positive plates
Of course if you have Gel or AGM Battery you will have to use Voltage Measurements because you can’t measure SG. There are some Capacity meters based on pulse load testing, but these meters cost more than a battery.
More complex methods based on measuring the battery internal impedance exist but the equipment is expensive and needs to be calibrated for specific battery types.

i. Don’t confuse State of Charge with Available Capacity
If you bought a 100 Amphour battery four years ago it may well show 100% remaining capacity on voltage and SG tests, but that doesn’t mean you have 100 Amphour available. As the battery ages its available capacity reduces, so it may now be a fully charged 40 Amphour battery. The only way to verify Available Capacity is to do a discharge test.

j. So before deciding on what you need to do, to have your battery charged above 70%, do some simple tests to see if anything really needs to be done at all. Changes should only be needed if your car is very old, has a very large electrical load or has a fault.

Mike DiD

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Reply By: Flash - Thursday, Aug 18, 2005 at 23:01

Thursday, Aug 18, 2005 at 23:01

I'm impressed.
Sounds very believable. I must admit I never really believed the 70% figure either. BTW you should be a starter for a job with "Myth Busters".

AnswerID: 125840

Reply By: Member - Blue (VIC) - Thursday, Aug 18, 2005 at 23:04

Thursday, Aug 18, 2005 at 23:04

2 things...

1. My brain hurts
2. Are your fingers sore?

No disrespect, just that by the third paragraph, I think my brain was nicely browned on both sides and I'm still confused...

AnswerID: 125841

Reply By: Crackles - Thursday, Aug 18, 2005 at 23:43

Thursday, Aug 18, 2005 at 23:43

Sory Mike but really who cares what the state of charge is.............While you're busy measuring volts I'm off 4x4ing or out catching a fish.
Cheers Craig..........
Two 115 amphour Trojan deep cycle batteries in parallel. Simple, no fancy electrics, no complicated systems, no worries :-)))

AnswerID: 125846

Follow Up By: Mike DiD - Friday, Aug 19, 2005 at 06:54

Friday, Aug 19, 2005 at 06:54

"Two 115 amphour Trojan deep cycle batteries in parallel. Simple, no fancy electrics, no complicated systems"
- yes, with AGM Deepcycle being able to supply 1000Amp starting current, I'm beginning to think that this system, with a cutoff at 50% (voltage AND current sensed) would be better for several reasons than the traditional Main-Switch-Auxiliary approach.
There is only one disadvantage - reliability. If there is a big switch disconnecting the loads from the Main, I can be very sure that 50% of my battery capacity will be avaialble for starting and emergencies.

Mike

FollowupID: 380542

Follow Up By: Mike DiD - Friday, Aug 19, 2005 at 06:56

Friday, Aug 19, 2005 at 06:56

"Sory Mike but really who cares what the state of charge is."
- if you read carefully, you'll notice that I was expecting your post. :-)

Mike

FollowupID: 380543

Follow Up By: Crackles - Friday, Aug 19, 2005 at 20:10

Friday, Aug 19, 2005 at 20:10

Having just taken the time to read your entire post you seem to have squashed a rumour I never believed in anyway. I do like your thinking on 2 big AGM batteries with a low volt disconnect to isololate. 2 batteries of the same size & construction both discharged (no more than 50%) & charged up together should give both reliability & long life.
Cheers Craig.............

FollowupID: 380662

Reply By: Member - Davoe (Widgiemooltha) - Friday, Aug 19, 2005 at 01:06

Friday, Aug 19, 2005 at 01:06

I will wait till i am at work to get paid to read all that

AnswerID: 125851

Follow Up By: Member - Jeff M (WA) - Friday, Aug 19, 2005 at 11:25

Friday, Aug 19, 2005 at 11:25

LMAO, Classic Davoe, I like your style! Your a true Ozy mate.

FollowupID: 380595

Follow Up By: Member - Matt Mu (Perth-WA) - Friday, Aug 19, 2005 at 13:06

Friday, Aug 19, 2005 at 13:06

Gee doesnt everyone do their surfing at work?? Cant let work get in the way of a good read, its surprising any production exists now we have the internet!!

hahahahha!

FollowupID: 380615

Reply By: Nick - Friday, Aug 19, 2005 at 01:16

Friday, Aug 19, 2005 at 01:16

Mike DiD

Thanks for this post and your exhaustive research. This topic interests me too and, as you know, it gets a lot of air play. I'd like to believe in full charging, especially for an auxilliary, but we hear regular conflicting advice from seemingly qualified people (not me)

As you suggest, a test measuring rested no-load voltage or a hydrometer reading after a drive should give the answer. Have you (or anyone else) done a test? I guess you would have after that much research. You can't beat an experiment to prove or disprove the various theories. For the starter battery I suppose it would have to be disconnected for the rest period to ensure no load from clocks, stereos etc.

I have an AGM auxilliary which charges to a high level, but have not tested a wet cell (yet).

Nick

AnswerID: 125852

Follow Up By: Mainey (WA) - Sunday, Aug 21, 2005 at 00:44

Sunday, Aug 21, 2005 at 00:44

Mike,
you have done some work there, is deep thinking material.

I use two Delkor 80 A/h Deep Cycle batteries wired in parallel, I do get them fully tested when I go to Perth by the company who sold them to me.
I tell them straight, "if they are suspect, I want them replaced with two AGM's" and four years later they are still waiting to give me the AGM's.
A benefit I have is they are powered by a Solar system 24/7, so the power is always there and sulfation and lack of charge is not a worry.

FollowupID: 380759

Reply By: Member - John C (QLD) - Friday, Aug 19, 2005 at 07:53

Friday, Aug 19, 2005 at 07:53

Thanks Mike,

Good summary, but not exhaustive. There will be a few out there who will find a few holes :-)
Good enough for me though.

AnswerID: 125857

Follow Up By: Mike DiD - Friday, Aug 19, 2005 at 08:10

Friday, Aug 19, 2005 at 08:10

"Good summary, but not exhaustive."
- I had to do enough research to convince myself firstly "beyond REASONABLE doubt". I know there are others who don't work that way :-).

Mike DiD.

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Follow Up By: Member - John C (QLD) - Friday, Aug 19, 2005 at 20:06

Friday, Aug 19, 2005 at 20:06

Sorry, I know you most probably have spent a few weeks chasing info.
But some highly technical people would not appreciate that:-).

FollowupID: 380661

Reply By: G-wizz - Friday, Aug 19, 2005 at 08:13

Friday, Aug 19, 2005 at 08:13

An excellent post Mike.

I've always been sceptical of many claims about battery chargers and my standard car system has always given me good battery life which seems to debunk the need for these high tech gadgets.

AnswerID: 125859

Reply By: CubCamperMan - Friday, Aug 19, 2005 at 08:45

Friday, Aug 19, 2005 at 08:45

never believed the 70% crap
hydrometer shows fully charged

AnswerID: 125864

Reply By: Willem - Friday, Aug 19, 2005 at 09:29

Friday, Aug 19, 2005 at 09:29

Mike

Good research that I would normally have given scant notice to but it being a drizzly morning thought I had better read it. I was tempted to say "Who cares" but in the end went out to the garage and tested my batteries.

Both batteries are 700cca Century 4wd Overlander 17 plater under 12 months old.
The GQ hasn't been started for a week
The auxilliary battery has three pilot lights of undetermined low wattage on all the time
Main battery 12.65 volts
Aux battery 12.37 volts

Batteries are run in parallel through a simple solenoid with dash mounted switch

I grew up with the 30amp generator and had various battery failures over the years. Bought my first 4by in the mid '70's and went bush with a single battery and only converted to a dual battery set-up about 5 years ago.

I always make sure that my batteries are tested before going on an extended trip.

The modern vehicle ie later than 2000 model is becoming too complicated for me and I must stay with my mid 90's model.

In the end all I really care about is that the battery performs to its requirements and starts the truck every time.

AnswerID: 125876

Reply By: drivesafe - Friday, Aug 19, 2005 at 10:00

Friday, Aug 19, 2005 at 10:00

Hi Mick DiD, good to see that there are others out there that are prepared to think for themselves and take some measurements to see if things are as they are posted to be.

Years of working in the industry has shown the 70% as nothing more than a myth but try and tell people here that, and a number of supposedly learned individuals go to extreme lengths ( and lies ) to push info that is nothing short of fantasy as being the gospel according to who ever and ends up leaving most every day 4x4 users either convinced that they need to spend a fortune to do something that their alternator is already doing or leaves them totally bewildered.

The time you have taken and the fact that you were NOT prepared to accept the 70% myth, I’m sure, is going to be of financial benefit to a lot of people and as is already happening, going by the posts above, getting a lot more people to go out to their vehicles and check for themselves

An excellent posting, keep up the good work.

AnswerID: 125885

Reply By: davros_who - Friday, Aug 19, 2005 at 11:00

Friday, Aug 19, 2005 at 11:00

Firstly I must say that I haven't read all of the first post, as my head started to hurt a little bit. :(

I had beleived the 70% statement for many years. My good friend (now retired) has been an auto electrician all of his working life, and insists on the 70% thing, so that was good enough for me. He has always told me to whack any new battery on a good charger, before using in vehicle, and get it topped up. I did this last month, with a new battery, that was supposed to be fully charged. I have a very good heavy duty, fully auto charger, that showed the battery was only at 60% of capacity, so I left it on overnight.

That is all I can say, because that is the limit of my personal experience in this regard.

AnswerID: 125894

Follow Up By: Mike DiD - Friday, Aug 19, 2005 at 11:17

Friday, Aug 19, 2005 at 11:17

There can be many exceptions to a rule, but that doesn't make the rule useless.

If anyone can suggest additions to situations I've listed where a battery will not be fully charged, I'd like to learn more.

What voltage is across the battery during normal driving, what type of battery is it.

FollowupID: 380592

Follow Up By: Member - Matt Mu (Perth-WA) - Friday, Aug 19, 2005 at 13:13

Friday, Aug 19, 2005 at 13:13

Davros..how did you know this new battery was fully charged? Did they take it off a charger and give it to you? No battery sitting in a shop on a shelf or whatever is fully charged?
They will have enough charge in them to satrt the car but they are far from fully charged!

I fill new batteries with acid and while they will start a veh they are green and still need to be fully charged.

FollowupID: 380617

Follow Up By: Mainey (WA) - Sunday, Aug 21, 2005 at 00:51

Sunday, Aug 21, 2005 at 00:51

Matt,
.......you have that correct 100%

ALL "new" batteries should be left on a decent 240v charger at least for a few hours if not over night before use to get them to their peak condition as it is all down hill from that day

FollowupID: 380761

Reply By: Member - Jeff M (WA) - Friday, Aug 19, 2005 at 11:34

Friday, Aug 19, 2005 at 11:34

I have also done alot of research on this topic myself and came to a lot of similar conclusions as yourself. My alternator only puts out 14.2V MAXIMUM (it very rarley get's up there as there is always some kind of load. I have 210 amp hours of batteries on board, they are calcium - calcium. I was origionally told not to use calcium to calcium as they need a 14.8V charge. There is also conflicting information regarding this and I have since learnt that it is a similar to what you have stated regarding standard wet cell batterys. Sure 14.8v will charge the calcium battery more efficiantally, but leavin the batteries on a 14.2v charge will eventually fully charge them. I have proven this with my solar regulator. We run calcium calcium batteries off quite and expensive electronic charge controller that used computer algerrithims and it only ever pushed 14.2v but still managed to full charge the batteries.

The problem I face is a slightly under rated alternater for the ammount of battery power I have under the bonnet combined with short city driving for the most part, so I still use a 240v charger to top them up once a week.

AnswerID: 125900

Reply By: flappa - Friday, Aug 19, 2005 at 13:17

Friday, Aug 19, 2005 at 13:17

Everytime I read a post like this . . . . I just stand back . . . and wait for the action to start.

the "experts" will come out of the woodwork over this.

Ask a dozen "experts" what the answer is . . . get a dozen different answers.

Anyway , interesting read , even if I dont understand half of it.

I just go about it my own way know . . . I have become my own . . "expert"

AnswerID: 125907

Follow Up By: drivesafe - Friday, Aug 19, 2005 at 21:52

Friday, Aug 19, 2005 at 21:52

Hi flappa, you are not alone when it come to not understanding what is happening with your vehicles electrical system and Mick DiD has hit the nail on the head when he puts it that battery data is being misinterpreted.

The statement “An alternator can not charge a battery above 70%” is an incorrect statement of fact.

If it read that there are vehicles running around with batteries that will probably never get charged over 70% then this may be closer to the real facts.

The point is that somewhere back a few years ago or so, the statement was changed to what is now commonly accepted as fact but those making the statement have obviously not checked to many batteries and by far a more important consideration especially where this forum is concerned, is the fact that most of the time, the batteries here are being used in a way that puts them in operating conditions that will most definitely fully charge them.

We are not taking about short distance city use over short periods of time, we are using these batteries in situation where they are being charged continually for hours on end and when set up properly and in most cases this does not require huge amounts of money to set up a good efficient system, the batteries are regularly fully charged even when they have been considerably discharged the night before.

The point Mick DiD has made is to go and check your own battery. You don’t have to be a rocket scientist to get a rough idea as to the state of charge of your battery(s), using nothing more than a cheap multi meter.

Flappa there is really little to understand other than your battery is being charged from your alternator while you drive to your favourite fishing spot OR it isn’t charging.

Can’t get much simpler than that and this is all most people need to get by on.

Cheers.

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Reply By: arthurking83 - Friday, Aug 19, 2005 at 21:11

Friday, Aug 19, 2005 at 21:11

I wanna know.......what's so complicated about driving a post 2000 model vehicle?
:p
I thought that they were designed to be backward compatible, that is once you've driven a 1901 model, it's requires the same dynamics to achieve the desired result.

BTW, I liked the first post about the batteries too!

AnswerID: 125963

Reply By: Member - Collyn R (WA) - Saturday, Aug 20, 2005 at 10:25

Saturday, Aug 20, 2005 at 10:25

Mike Did

It is not that a conventional battery CANNOT be charged beyond 70% by a traditional alternator regulator (because given long enough it can). It is that in practice, few batteries ARE charged beyond that approx level (excepting in exceptionally hot places where charge acceptance is thus increased). The rapid tapering off of charge rate is inherent with a constant voltage charging system.

As I've stated many times before, when researching my book 'Motorhome Electrics', draft copies were sent to over 20 of the world's leading battery makers seeking comment. The only dissenters noted that, with RVs, a figure of 70% charge was possibly a little high.
Collyn Rivers

AnswerID: 126004

Follow Up By: Mike DiD - Saturday, Aug 20, 2005 at 13:12

Saturday, Aug 20, 2005 at 13:12

Hi Collyn
Yes, that's the point I was trying to emphasise at the start - it's not so much what's written, it's the way that people interpret it, that can be the problem. People invariably mis-interpreted these statements as - "a car battery wil never, ever be charged above 70%" !

But then that's what surprised me, when I started learning about 12 volt battery systems - there are so many factors to consider. After RF engineering (we've met at Roger Harrisons decades ago) having to consider complex impedances and the square-root of -1, I thought that designing a good DC system would be very easy. What I have learnt I am trying to share with others who are deciding what is the best configuration for THEIR situation.

You have done an excellent job of making solar power and living off batteries understandable to non-techical people. I am merely trying to improve people's understanding of the ONLY sentence in your voluminous writing that I thought people might misinterpret.

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Reply By: Mike DiD - Saturday, Aug 20, 2005 at 14:25

Saturday, Aug 20, 2005 at 14:25

This article is a bit easier to read with formatting of the text - see

www.pajeroclub.com.au/forum/forum_posts.asp?TID=8397&PN=1

AnswerID: 126021

Reply By: Member - Collyn R (WA) - Saturday, Aug 20, 2005 at 17:13

Saturday, Aug 20, 2005 at 17:13

Mike Did

I think I remember you - way back in the 70s - in Balmain!

I emphasise this point (that in PRACTICE, batteries rarely get much past 70% charge) because it is probably the biggest single cause of motorhome and caravan electrical woes. Compounding this is that magazine article after article suggests (or states outright) that a 100 Ah battery is good for 100 Ah - when the reality is closer to 40-50 Ah.

I'd like to have your thoughts on the NZ 12-volt to 12-volt charger than I currently have on trial. Most interesting - constant current to about 80% charge and the switches to pulse width modulated short pulses at up to 15.5 volts. Charges to beyond the maker's 100% - (typically to 11-120% and with virtual zero gassing!

Also on test shortly is a new three-stage Redarc charger.
Collyn Rivers

AnswerID: 126035

Follow Up By: Mike DiD - Saturday, Aug 20, 2005 at 22:33

Saturday, Aug 20, 2005 at 22:33

"I'd like to have your thoughts on the NZ 12-volt to 12-volt charger than I currently have on trial. Most interesting - constant current to about 80% charge and the switches to pulse width modulated short pulses at up to 15.5 volts. Charges to beyond the maker's 100% - (typically to 11-120% and with virtual zero gassing! "
3-stage chargers are the fastest way of charging a battery - it sounds like this is intended to go beyond the normal 3-stage approach with the pulse width modulation. If it runs off DC, then it will allow the auxiliary battery to be charged as fast as possible - even if the Alternator voltage is down, or the battery is in a trailer connected by 6mm2 cable :-)

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Reply By: Member - Collyn R (WA) - Monday, Aug 22, 2005 at 10:39

Monday, Aug 22, 2005 at 10:39

MikeDid

I believe that what tends to cloud this issue is the difficulty in establishing percentage charge by a basic instantaneous voltage measurement - not least due the electrochemical time lag in the charge being distributed homogeonously throughout the electrolyte.

Instant measurement may well show a battery as being (say) 12.8 volts or more) but that measurement reflects only the local conditions around the plates. Measuring that battery an hour or two later is likely to show a lower (and truer) voltage. Ditto with a hydrometer (and that is now less than reliably meaningful unless one knows the original SG of the electroyte - which may be anything from 12.65-13.1).

With deep-cycle batteries, that lag may be up to 48-hours particularly in cold places.

Another and classic example is a battery that is heavily sulphated. This will show a high voltage on charge - but which will drop rapidly over time. That battery may in fact be only charged to 50% of its nominal capacity - but will present as fully charged.

I'm interested in your thoughts on this Mike but outside a research lab about the only truly meaningful measure of percentage charge is by a controlled discharge test (as the C20 rate) down to about 11.4 volts with both voltage and current being measured throughout.

I am certain that many systems are chronically undercharged. I have written the Tech Notes colunm for the now-45,000 member CMCA member monthly magazine for the past seven years - and receive huge numbers of queries from members with electrical system problems.

Most stem from one cause or another of undercharging/undervoltage.

Certainly inadequate wiring is responsible for most fridge problems, and also poor charging of trailer batteries, but the biggest issue is the belief that batteries routinely charge close to 100% and can be discharged virtually till they go flat.

I am not attempting to do is not to score points - but to assist the vast numbers of people with this type of problem.

If I am wrong in the above I agree it is a serious issue, as my book 'Motorhome Electrics' has over the past four years become a standard reference for auto sparkies in Aust and NZ.
Collyn Rivers

AnswerID: 126266

Follow Up By: Mike DiD - Monday, Aug 22, 2005 at 21:17

Monday, Aug 22, 2005 at 21:17

Establishing Percentage Charge - a "Fuel Gauge" is what we all want, but is hard to achieve. It's nice to know how much is available, but I agree with you that it is most important so people stop discharging at 50% to avoid shortening battery life.

There are some expensive solutions - tracking all charge and discharge totals; measuring the impedance; are possible but they still seem to need to calibration for the battery type according to Buchmann.

If you use Gel or AGM then SG just isn't an option !

People living off 12 volts just can't disconnect all loads for 4 hours to rest the batteries for a voltage test.

So I'm wondering of Voltage+Discharge current can give a useful indication (even though it in no way considers electrochemical time lag ).

I've been collecting some info on Voltage+Discharge current versus capacity and not surpisingly it shows inconsistencies. -

Discharge Amps
0x 0y 0z 0w 3 4.5 22 90 270 340
100% 12.70 12.66 12.80 12.80 12.5 12.8 12.5 12.2 11.6 10.4
80% 12.42 12.49 12.64 12.55 10.2
60% 12.20 12.32 12.48 12.30 9.9
50% 12.06 12.24 12.40 12.18 12.4 12.2 12.0 11.8 11.3 9.4
40% 11.90 12.16 12.32 12.06 9.5
20% 11.58 12.06 11.96 11.82 8.9
0% 10.50 11.89 11.80 11.60 10.6 10.8 10.4 9.5 9.4 6.0

Sources
0x - unspecified battery www.windsun.com/Batteries/Battery_FAQ.htm#AGM,%20or%20Absorbed%20Glass%20Mat%20Batteries
0y - Low Maintenance (Calcium Lead) Battery - www.uuhome.de/william.darden/carfaq4.htm
unspecified battery http://www.batteryuniversity.com/partone-13.htm and www.ibsa.com/www_2001/content/faqs/tech_talk/maintenance/faq_tech_maint.htm

0z - Maintenance Free (Calcium-Calcium) or VRLA (AGM) - www.uuhome.de/william.darden/carfaq4.htm

0w - Odyssey Staring Battery Handbook.

4.5A-270Amp Absorbed Power N70-size 90AHr AGM deep cycle battery - http://www.absorbedpower.com/battery/pdf/AP-GT12-90C-01Sep05.pdf

3A, 340A 60 AHr unspecified battery - www.cyb.com.au/automotive/batt_facts/undbatt_2.htm

For people who don't want to spend $400 on a battery meter and for those who can't measure SG, I'm wondering if Voltage+Discharge current measurements can give a practical indication of approximate capacity ?

I am sure that many battery failures occur due to undercharging - there are many reasons - undersized wires to the trailer with heavy loads, load current exceeds the Alternator's total ouput, etc, etc Car electrical systems are certainly not designed to charge big batteries that are deeply discharged. I wonder if they have Voltmeters which are accurate enough, hich are wired in to give accurate readings and the owners have enough information to understand whether enough voltage is applied for long enough to charge the batteries fully ?

I think what is needed is to explain to people the complexities of what superficially seems to be a simple case of charging a battery with an Alternator. All the "ifs" and "buts" need to be detailed - what I'm trying to do is to understand what some of the important factors are that need to be considered. I'll keep on proposing them and looking for critica feedback !

FollowupID: 380981

Reply By: drivesafe - Monday, Aug 22, 2005 at 19:34

Monday, Aug 22, 2005 at 19:34

Collyn, I have not criticised your books in general, just one statement you make over and over and your not the only one saying it.
Some go as far as saying that an alternator can not charge a battery above 70% and your statement “ in practice few batteries go much over 70% charge “ Both of these statements are incorrect when you take into account the way these batteries are intended to be used by the people on this forum.

I have been supplying devices that not only uses the auxiliary battery’s power based on the fact that these batteries are storing capacities of well over 90% but these devices also exploit the fact that the cranking battery will also have well over 90% capacity and these devices were developed over 17 years ago after a considerable R & D period and as we have continually carried out our R & D, I can assure very little has changed in the way batteries are able to be charged well over 85 to 95% regularly basis, by the standard vehicle alternator.

As I have stated in this post and others on the site and on a number of other forums, Because of the fact that no two uses will be the same, the only way each individual can see whether the system is working or not is to test it themselves.

No matter what type of battery you are using, cranking, deep cycle AGM and so on, if you what to find out what state of charge you alternator is capable of in regards to your auxiliary battery, the next time you use your set up in the manor you installed it for in the first place, this simple test will tell you.

As it is more likely that a battery in a caravan or camper trailer is going to more readily show that it is not being charged properly, this is aimed at the caravan or camper trailer user, but it can also be carried out on an auxiliary battery mounted in the vehicle and the results will be similar.
On arriving home after a trip, after disconnecting the caravan/trailer from the vehicle, remove the negative lead of the battery.

For a number of safety reasons DO NOT remove positive lead unless you intend to remove the battery.

By removing one of the lead you will be guarantied that there is not charge or load applied to the battery.

Simply leave the battery stand for 72 hours and then use an ordinary garden verity multi meter and measure the voltage across the battery’s terminal.

You will need to check the battery manufactures web site to get the exact results for the particular battery you are using, but no matter what type it is, after 72 hours the voltage reading will give a 95% accurate indication of the state of charge of your battery.

AnswerID: 126337

Reply By: Mainey (WA) - Monday, Aug 22, 2005 at 23:24

Monday, Aug 22, 2005 at 23:24

As has been previously stated a starter battery is charged enough to start a vehicle with only a short amount of vehicle running time due to its construction.

Whereas the average "large capacity Deep Cycle wet cell" battery will take far longer to recharge fully, because it's (when it’s used to power a fridge etc) often close to being flat eg; 12v, generally after running the fridge and accessories at night and before starting the vehicle or generator to recharge it!

Sulphation is also a very major contributing factor for DC batteries failing, as they often sit in an uncharged state @12.2v – 12.5v for long periods, and by not allowing the DC battery to retain voltage even though it’s getting the power, it drops back to the mid 12’s or thereabouts after the charger is turned off.

AnswerID: 126378

Reply By: Member - Collyn R (WA) - Tuesday, Aug 23, 2005 at 11:14

Tuesday, Aug 23, 2005 at 11:14

MikeDiD

A bit of history may be of interest.

It started for me in the mid-1970s whilst I was running my magazine Electronics Today International (ETI) when we started to look at battery chargers as a constructional project. You may even have been at meetings at Roger's (my then editor) house in Balmain when this was being discussed?

Up to that time by general thoughts on batteries and battery charging were much as (I think) those of Drivesafe. But the more we looked into this the more we became to suspect the 'conventional wisdom', and found a gulf between what most people believed and what battery makers were actually saying.

The issue came to a head again in 1996 whilst converting my OKA into a motorhome: in particular when I found that upgrading the alternator by close to 50% made no measurable difference in charge rate beyond what I'd assumed to be about 55% or so charge. I had not fully realised back then that (once past 50% or so charge) the full implications of the regulator rigidly controlling the charge rate, nor the inherent failings of constant voltage charging.

It became increasingly obvious that, if an alternator can fully charge a battery within a few hours, it must inevitably overcharge it after that length of time, and discussion with companies like Bosch and Lucas confirmed that this was fully understood and that charging voltage was deliberatly limited to ensure this does not happen unless the vehicle is driven for vast distances.

This is never an issue unless attempts are made to parallel charge extra batteris and particularly deep-cycle batteries.

A year or two later I began to research 'Motorhome Electrics' - and a lot started to fall into place.

Drivesafe quotes me accurately enough regarding batteries rarely charging beyond 70% - but he omits to state that I make it totally clear that this statement does not apply to charging generally - but only to vehicle and home constant voltage chargers.

At this point I suspect there may be some confusion with terminology.

Most respondents are taking it for granted that a voltage measurement taken properly will indicate the 'state of charge'. Ditto SG.

But does this mean that (say) at 12.84 (or 1.280 SG or whatever) a 100 Ah battery now 'holds' 100 Ah? No - it doesn't. What it means, and ALL that it means, is that at (say) 12.84 volts that battery is holding 100% of what is possible for it at the time of charging. This may be the full Ah for a brand new battery, but most likely it is not.

So why not? Every time a deep-cycle battery is discharged much below 50% it loses 0.5% of its battery becomes an 85 Ah battery. Even if charged to an improbable 85% that battery is now an effective 72 Ah battery.

In practice a battery can only ever deliver it's claimed Ah rating just once (if measured by the method the battery maker uses). And that's to fully charge and then discharge to essentially zero (10.4 volts). Which immediately drops its available capacity by 0.5%.

Re measuring remaining capacity. I totally agree with Drivesafe's contention that the battery be left off-load for 72 hours (in fact that's what I say in my book!).

But that measurement may have little to do with remaining capacity because that depends hugely on battery condition etc. I have in my workshop right now a nominally 120 AHbattery that shows 12.8 volts across it and an SG of 1.285. It is massively sulphated. It's effective capacity is less than 5 Ah!

My own technique uses Plasmatronic PL 40 regulators. These measure incoming amps and amp-hours, and outgoing amps and amp-hours.

My battery bank is 24 by 640 Ah 2-volt cells.

I do not believe there is any other practical nor realistic way of establishing charge percentage nor remaining charge without full scale measuring gear.

As a matter of interest (and converting the data to 12-volts) the systems charges at constant current at up to 15-volts, then drops to about 14.2 volts to allow charge to be absorbed. This results in 98% charge.

As a matter of interest if the charge rate is cut back, by limiting charging to 14.4 volts, both PL 40s show that the maximum achievable charge (over several days) is about 75%.

As a matter of interest, the NZ system I am currently trialling monitors the steepness of the pulsed final charge waveform as an indicator of charge acceptance. The pulsing technique seems to enable an actual 'overcharge' without gas formation. Very interesting.

(Also about to be on test is Redarc's new three-stage battery charger and cooler running Redarc relay).
Collyn Rivers

AnswerID: 126421

Follow Up By: Mike DiD - Tuesday, Aug 23, 2005 at 22:24

Tuesday, Aug 23, 2005 at 22:24

Collyn

Thinking about your battery installation in your OKA made me realise I need to another statement to the list I put at the start- "More accurate statements about charging a battery in a car would be that the battery will not charge fully if - "
" Batteries are not installed in the Engine Compartment e.g. Vehicle Rear or Camper Trailer." EXPLANATION - modern Alternators have the Regulator in the Engine Compartment and they trim the Alternator Voltage to compensate for the different charging voltage needed as the battery temperature can range between -20 (14.9 volts) and 80 degrees (13.8 volts).
This temperature compensation as the Alternator heats up (that's where the sensor is) due to incoming air and internally-generated heat, makes some people believe that it is sensing battery charge level and is switching from Absorption to Float Voltage. But if the battery is in the rear or in a trailer, then the Alternator is heating - but the battery is NOT !.
The Alternator might have heated to 60degrees internally and reduced its voltage to 14.0 volts (which is the Absorption Volatge for a battery at 60 degrees) BUT the battery is still at 20 degrees needing an Absorption Voltage of 14.4 volts to charge up.
If the wiring to the rear is undersized and/or there is a Fridge drawing current over the same wire, then there will be even less than 14.0 volts - it will take a LONG time to charge the battery !
I might have to put some more thought into Alternator Voltage raising schemes that are safe !!!!

FollowupID: 381129

Follow Up By: Mike DiD - Tuesday, Aug 23, 2005 at 22:47

Tuesday, Aug 23, 2005 at 22:47

Collyn

What plate composition (Antimony, calcium) are your natteries and what electrolyte (Wet, Gel, AGM ) are they ?

Does the Plasmatronic take battery aging into account - does it regularly recalibrate itself to full batteries and discharged (how ?) batteries.

Yes - I agree many batteries are below their original capacity due to over-discharging - here is a "recommendation" from someone who should know better "Deep Cycle - Best features:- long life, and can be flattened many times without significant damage to the plates " - no wonder it's hard to convince people not to discharge below 50% !
http://www.piranhaoffroad.com.au/products/dualbattery/dualbatteryinfo_isolator.htm.

FollowupID: 381133

Reply By: Member - Collyn R (WA) - Wednesday, Aug 24, 2005 at 08:45

Wednesday, Aug 24, 2005 at 08:45

Mike Did

Re your first posting (temperature).

The role of the alternator/regulator is of course changing - from that of ensuring a safe and reliable method of maintaining charge in a starter battery - to that of also providing a stable reference voltage for any number of transducers. Further, the electrical demands of engine management and other computers pushes the limit of what can be produced from a 12-volt system. Hence the already happening shift to 36/42-volt systems.

It is thus not feasible for a vehicle charging system to respond to the temperature/related charging needs of a remote battery. Nor is it desirable as this is likely to interfere with the vehicle electronics.

No problem with older vehicles and vegicles such as my just-bought Nissan 4.2 litre, but it becoming one with the more complex cars.

I believe the days of straight parallel charging with or without varying alternator output are coming to an end (for new vehicles). Several companies (including Stirling in the UK) are producing what in effect are dc-dc converters that isolate the alternator system from the auxilliary system - and produce whatever is optimally needed by the auxilliary battery. That's how the NZ system that I'm currently trialling works. (I will send you details).

Re my big house battery bank - they are conventional lead acid BP two-volt cells, now about six years old and still in remarkably good condition (typical life is 14 years). They normally cycle from 100% down to about 87% (remaining) - close to battery heaven!

The Plasmatronic PL 40s are initially preset for battery Ah capacity and recalibrate their % charge indication whenever the batteries go into Float mode (usually by mid-day on most days). There is no correction for Peukerts Equation but this is of little consequence where such a large reserve (640 Ah at 48-volts) carries relatively low peak loads.

The batteries have an equalising charge every month or so (at about 65 volts).
Collyn Rivers

AnswerID: 126575

Follow Up By: Austravel - Wednesday, Aug 24, 2005 at 10:08

Wednesday, Aug 24, 2005 at 10:08

Hi,

I know it's a slight change of subject. I read some time ago were you suggested using a plasmatronics solar reg and a basic battery charger to get a smart/3 stage charger. I have a PL60 and considered buying a cheap power supply (40amp), hooking it to the PL60 and when at a powered site I would then have a good 3 stage charger.

I rang plasmatronics and they strongly advised against it due to the chance of large spikes during the switching phase of the switch mode charger which will damage the PL60 or any of there units.

How do you get around this??

Thanks

FollowupID: 381161

Follow Up By: Alloy c/t - Wednesday, Aug 24, 2005 at 10:37

Wednesday, Aug 24, 2005 at 10:37

Austravel , dont know about the plasmatronics , but was told on this forum that would be ok to do ,, Steca also state DO NOT ,, what would they know ????? cost of new Steca was $259 ,,,, stick with what the manufacturer advises and save the price of having to replace your regulator.

FollowupID: 381163

Follow Up By: Austravel - Wednesday, Aug 24, 2005 at 11:11

Wednesday, Aug 24, 2005 at 11:11

Thanks Alloy c/t, thinking I'll do the same.

FollowupID: 381168

Reply By: Alloy c/t - Wednesday, Aug 24, 2005 at 10:27

Wednesday, Aug 24, 2005 at 10:27

Great debate on if or can /lots of great research and even the "deleted" was fun to read , so just for a laugh I checked my batts this morning , vehicle has not been moved/started for 4 days [96 hrs] so batts should be fully rested !!?? batts = 2x 80amp fullriver brand AGM made in china , 2yrs old , Start batt reading 12.79 volts, only load is clock ,, Aux batt [thru rotronics] reading 12.84 volts ,nil/zip/no load ,, Also tested c/trailer batts =3x 80amp fullriver AGM ,no load/no recharge for over 30days [swmbo broken ankle so no camping unless at the Hilton ] reading 12.84 volts ,,

so MikeDid ,, Collyn R,, Drivesafe ect,ect ect,,, tell me are my 5 batts 100% fully charged or only 70% ,or somewhere in between LOL and in the real world it dosn,t realy matter does it.

AnswerID: 126593

Follow Up By: Mike DiD - Wednesday, Aug 24, 2005 at 11:10

Wednesday, Aug 24, 2005 at 11:10

It certainly sounds like they're well charged - but then not everyone has your Alternator, your driving pattern, your useage pattern, your trailer wiring, and AGM Battery etc.

A bit like skinny people looking at overweight people and saying - just eat properly.

FollowupID: 381167

Follow Up By: flappa - Wednesday, Aug 24, 2005 at 11:13

Wednesday, Aug 24, 2005 at 11:13

Quote: A bit like skinny people looking at overweight people and saying - just eat properly.

Why Not ?

It seems obvious to me the "experts" cant agree , what what chance to us laymans have ?

FollowupID: 381169

Follow Up By: drivesafe - Wednesday, Aug 24, 2005 at 19:38

Wednesday, Aug 24, 2005 at 19:38

Hi Alloy c/t, I have no intention of dragging you into this ridiculous debate but from you post, I take it they were all charged buy your vehicles alternator. Could you give us an idea of both your normal type of use and what sort of driving time occurred prior to the vehicle standing for the 4 days.

By the way they may or may not be fully charged but I think it is blatantly obvious that they are way WAY over 70%.

Cheers mate.

FollowupID: 381242

Reply By: Mike DiD - Wednesday, Aug 24, 2005 at 11:45

Wednesday, Aug 24, 2005 at 11:45

We would all like to have the clear answer to our problems "If your auxiliary battery is failing after two years you definitely need to . . ."

This thread is not one of those explaining the final answer, we are still working on it :-). At the start I suggested 5 situations where a battery wonlt be charged fully - I have just learnyt of a 5th !

Unfortunately it takes a while to build up knowledge like this. One problem is that there are so many variables - Alternator types, Battery types, cabling size, usage pattern, driving pattern etc. It's a matter of working through the actual problems that people have, to see which factors can be eliminated.

Esatblishing clear guidelines depends on people coming forward with facts about where they have had problems with battery power, considering possible reasons and improvements and then testing the theories to prove or disrove them. That's how all the "facts" we accept today have been established - slowly. :-)

AnswerID: 126601

Reply By: Member - Collyn R (WA) - Wednesday, Aug 24, 2005 at 15:20

Wednesday, Aug 24, 2005 at 15:20

Re running battery charger input through a solar regulator.

Plasmatronics' full installation manual states:

"Although the PL is primarily a device to control the charging of batteries from solar electric panels, it can also be used with other energy sources such as wind, microhydro and fuel driven generators". In other words what Plasmatronic is saying is that 'dc is dc'. The regulator neither knows nor cares where that dc comes from as long as it's within the specs.

In practice they cope very well with the input from a conventional rectified output tranformer charger - and I've run mine like this for the better part of six years.

Switch-mode devices (and anything that has a capacitive ouput) are different matters however and I imagine that in time Plasmatronics will revise that part of the handbook.

Some other regulator makers make a generally similar comment - others don't.

My strong advice is don't attempt this unless you really know what you are doing as you must know the rms value of the output (if it's not pure dc) and also the nature of the waveform and any transients.
Collyn Rivers

AnswerID: 126630

Follow Up By: Austravel - Thursday, Aug 25, 2005 at 12:53

Thursday, Aug 25, 2005 at 12:53

Thanks, it's what I thought. They'll handle DC from a transformer charger but not a switch mode unit.

Thanks

FollowupID: 381349

Follow Up By: Member - Collyn R (WA) - Thursday, Aug 25, 2005 at 13:06

Thursday, Aug 25, 2005 at 13:06

Austravel
Swithed mode supplies have curious habits - one very well known inverter charger that has switched mode operation will not run from an equally well known motor generator unless its input protection circuitry is partly disabled.
Collyn Rivers

FollowupID: 381351

Follow Up By: Austravel - Thursday, Aug 25, 2005 at 13:10

Thursday, Aug 25, 2005 at 13:10

Thanks Collyn,

Had initially thought I might be able to make a good high amp charger using the PL60 but generally nothing is that simple I guess..

Regards

FollowupID: 381353

Reply By: Member - Collyn R (WA) - Wednesday, Aug 24, 2005 at 15:34

Wednesday, Aug 24, 2005 at 15:34

Alloy c/t

The 70% or so thing relates to conventional lead acid batteries.

AGM batteries have far greater charge acceptance and will readily go up to 12.8 volts (some go to 12.9 volts) from charging voltages as low as 14.1 volts. This corresponds as close as makes no odds to 100% charge.

They stabilise after only an hour - and have next to no internal self-discharge.

In my own opinion these batteries are by far the best bet for auxiliary use. Gel cells have generally similar characteristics but are irrepairably damaged if charged above 14.4 volts. AGMs are more forgiving - they only need 14.1 or so but will withstand a lot more.

They do need to be parallel-charged via a voltage sensing relay - to ensure the starter battery has priority.

There is also some minor concern about their use with a standard vehicle charging system in that their float voltage is typically 134.2-13.3 volts at 25 or so degrees C, and as low as 13.0 volts at higher temps. Theoretically the alternator's output is too high, but in practice I am not aware of that many recent problems due to this.
(Would welcome others' experience's though. Their makers are still cagey about parallel charging).
Collyn Rivers

AnswerID: 126631

Follow Up By: drivesafe - Wednesday, Aug 24, 2005 at 19:58

Wednesday, Aug 24, 2005 at 19:58

I will argue that the 70% myth is not aimed specifically at conventional lead acid batteries.

From my experience of working on dual battery systems for many years show that this myth is quoted as being any battery can not be charged properly over 70% by an alternator.

I further suggest that, again, from info gained from hands on experience, that ALL vehicle batteries can easily be regularly charged over 90% and commonly as near to 100% as one can expect the individual battery to be charged.

Your vehicle’s alternator is the most cost effective way to FULLY charge any vehicle battery providing the simplest of requirements is meet and that is the use of suitably sized cable is used to connect batteries to the alternator NOTE that is both the positive and negative cables.

Simply test your own batteries and find out for yourself.

Cheers

FollowupID: 381248

Reply By: Member - Collyn R (WA) - Thursday, Aug 25, 2005 at 09:50

Thursday, Aug 25, 2005 at 09:50

For the record - my statements regarding the 70% or so typical limit on charging make it absolutely clear that I refer only and specifically to conventional lead acid batteries, charged by a standard alternator/regulator.

They do not apply to gel cell or AGM batteries.

Page 13 of Motorhome Electrics reads in part:

.. "14.2-14.4 volts is too low to charge any conventional deep-cycle batteries to an adequate level for what Americans call 'recreational vehicle use'. [The charging system} was not designed for this purpose nor is it reasonable to expect it to do so."

The book further states (on page 27), referring to gel cell batteries:

"They can be fully charged ... by applying a constant voltage of 13.8 volts."

Also (on page 27)

"A real benefit of gel cell and AGM batteries is that, if the charging capacity is available, they can be charged faster and more deeply than conventional lead-acid batteries from a standard vehicle alternator/regulator system."

I agree completely that a standard vehicle alternator/regulator WILL fully charge most conventional lead acid batteries GIVEN ENOUGH TIME. If continuously applied it will overcharge such batteries.

In PRACTICE however, and given a typical driving situation of (say) three/four hours, typical alternator load, and auxiliary conventional lead-acid caravan/motorhome battery bank of (say) 150-250 Ah, a tapering current charge will not and indeed cannot bring that battery from a typical 40% charge to much over 70% without suspending the more fundamental laws of physics.

With a typical tapering charge (ie constant voltage) system, once past 50%-55%, or an hour or so charging, the average charge level will be only a few amps, by 70% it will be down to an amp or so - no matter how massive the interconnecting cables (although I totally agree that adequately-sized cables are essential).

Gel cell and AGM batteries have very much lower internal resistance (or more correctly, impedance) and whilst charging (via a standard alternator/regulator) will still taper off, the falloff is far less. This enables them to charge as close to 100% as makes no odds.

These statements can readily be checked by anyone with a suitable multimeter or clamp ammeter.

Taking such a check over say two/three hours will convince anyone who can read such a meter and do simple arithmetic.
Collyn Rivers

AnswerID: 126759

Follow Up By: Mike DiD - Thursday, Aug 25, 2005 at 10:20

Thursday, Aug 25, 2005 at 10:20

Collyn

Thanks for clarifying another one of those uncertainties.

"A real benefit of gel cell and AGM batteries is that, if the charging capacity is available, they can be charged faster and more deeply than conventional lead-acid batteries from a standard vehicle alternator/regulator system."

I was never sure whether faster charging of AGM's only applied because in Stage 1 a charger COULD be set to a higher current for AGM than wetcells.

The "Charge Characteristic" graph here suggests that you can charge from 0 to 90% in 4.5hrs at 0.25C Amps (25 Amps for 100Amphour) from a 13.8 volts charge voltage. The graph confirms your measurements that 0.25C current will not drop off until 80% charge level.
http://www.absorbedpower.com/battery/pdf/AP-GT12-100C-01Sep05.pdf.

To summarise for those not interested in whys or wherefore -

If you have a problem with limited charging voltage (trailer, high electrical loads, low output Alternator) or cool batteries (rear compartment, trailer) and you want your battery to charge quickly and fully and but not overcharge easily, spend the extra money and get an AGM Battery.

FollowupID: 381327

Reply By: Member - Collyn R (WA) - Thursday, Aug 25, 2005 at 10:57

Thursday, Aug 25, 2005 at 10:57

Mike Did
Could not agree more!

Apart from cost, AGMs solve most charging issues in one hit.

They will charge even faster with a three-stage constant current charger (with an AGM cycle) but the real benefit of that charger is the lower and possible temperature compensated float voltage speciified for AGMs.

If considering AGMs here's where DriveSafes's advice is particularly relevant. Do use heavy charging cables as an AGM will absorb whatever the alternator can spare. Even over a metre or two, 16 or even 25 sq mm is far from overkill.

One electrical engineer that I know has run two 90 sq mm cables the length of his Nissan Patrol!

I also support DriveSafe's advice to use a negative return cable - ie, do not rely on an earth return unless done superbly well - even then I prefer and recommend dedicated cabling.

Am intrigued to find that Redarc's new 'three-stage' charger (that the company has supplied for me to trial) actually has a fourth stage - it equalises for a short time before entering float mode.

Am also trialling the company's new voltage sensitive relay - lower current consumption - so runs much cooler.
Collyn Rivers

AnswerID: 126772

Follow Up By: Mike DiD - Thursday, Aug 25, 2005 at 11:10

Thursday, Aug 25, 2005 at 11:10

Collyn

I assume the Redarc Three-stage charger runs off DC.

Can the Current / Voltage for each stage br programmed

Can Equalisation be turned off

I wonder how much they Reverse-Engineered the Silicon Chip design - it has all these features - all they had to do was add a Boost-mode switching supply :-)

FollowupID: 381337

Reply By: Member - Collyn R (WA) - Thursday, Aug 25, 2005 at 11:27

Thursday, Aug 25, 2005 at 11:27

Mike Did
Redarc is a mains voltage charger. Has preset programs for various types of battery. Equalising only operates on conventional lead acid batteries.

A lot of thought has clearly gone into it - it starts up always on the lower voltage gel cell cycle for example if not instructed otherwise.

I'll do a full report on it - but need at least a month or so's trials.
Collyn

AnswerID: 126778

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