Overcharging batteries

Submitted: Wednesday, Jul 27, 2005 at 22:41
ThreadID: 25122 Views:4931 Replies:18 FollowUps:31
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I keep reading on 'battery websites' that overcharging batteries is one of the big sins that will lead to short battery life.

I charge my deep cycle (both flooded and AGM) batteries in two ways:
1. Via the alternator in the vehicle
2. Using a CTEK 3 stage charger from 240V or (occasionally) from a 1KVA generator.

I believe the 3 stage charger is smart enough to ensure I don't overcharge, but what about from the alternator? Is it possible to overcharge here? I had been led to believe by auto electricians that as the battery nears full charge it will decrease it's current draw, eventually to zero. Overcharging would therefore not occur. Is this right?

Having paid a heap for my batteries I want to extend their life as much as I can.

With respect to AGMs, different web sites have entirely different info. Ranging from AGMs are intolerant to overcharging, to they are better than Gels and flooded, and can be charged to 16V without damage. Confusing huh?

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Reply By: V8troopie - Wednesday, Jul 27, 2005 at 23:51

Wednesday, Jul 27, 2005 at 23:51
Norm, if the alternator puts out the correct voltage then overcharging is unlikekly. Your auto electricians gave you the correct information. The car alternator employs a voltage regulation, IOW, the output from it is pegged at a constant 14V or thereabouts. Once the battery comes up to this voltage no more charging current will flow into it.

I do not own any AGM batteries so I have not researched them in depth to confirm your 16V chaging statement.

Normal wet cell batteries can be occasionally taken up to 16v for a short time, this 'equalises' the charge in each cell. The very expensive chargers do that automatically once a month or so.

AnswerID: 122446

Follow Up By: drivesafe - Thursday, Jul 28, 2005 at 00:20

Thursday, Jul 28, 2005 at 00:20
Right on the money V8troopie, and the 16 volt charge should be just that.

You can take most of the AGM type batteries up to 16 volts while charging but you will damage them if you leave them at that voltage.

According to most of the battery manufactures web sites, once fully charged the voltage should then be take back down to a maintenance voltage, usually no more than about 14.5.

Here's the thing though, how do you know when a sealed battery, AGM or otherwise, is fully charged.

With standard type cranking and deep cycle batteries you can remove the filler caps and use a Hydrometer to check the charge state.

With sealed batteries, the only way to check the charge state of the battery is to measure the voltage BUT you can not get an accurate voltage reading unless the battery has been sitting without any charge or discharge occurring for a minimum of 72 hours.

Go figure.

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Follow Up By: Member - Norm C (QLD) - Thursday, Jul 28, 2005 at 07:52

Thursday, Jul 28, 2005 at 07:52
Thanks V8troopie and drivesafe. Sounds like overcharging should not be a problem given my charging methods Hopefully undercharging shouldn't be a problem either.

Drivesafe you a right on the sealed batteries. I love them for a number of reasons, but to some extent you have to take their condition on trust and judge it from performance. Not a problem so far. When you are really concerned about the charge (out camping) they will never be at rest for long enough to take an accurate voltage reading.

But how many people carry a hydrometer in the 4B anyway?
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Follow Up By: Grungle - Thursday, Jul 28, 2005 at 07:58

Thursday, Jul 28, 2005 at 07:58
Very few batteries die due to overcharging. Almost all batteries die because of constant under charging or over discharging.

Actually AGM's may explode if you take them to 16V (I have one here that looks like a baloon) You have to remember that the higher the voltage the more batteries gas and heat up. AGM batteries still gas but nowhere near the rate of flooded (due to starved electrolyte levels). They are valve regulated and if the voltage is taken too high they cannot vent the excess gases quich enough and coupled with the heat, they distort the case and move plates and glass mats.

Also AGM's float at 13.5V and max charge rate for Boost (constant current) and Absorbtion (constant voltage) is around 14.2-14.5V.

Manufacturers design alternators to prevent overcharging hence the reason why a flooded battery is only ever charged to 70% capacity by an alternator. Alternators are quite primitive and current usually tapers of to couple of amps by around 14-14.4V. Flooded batteries need to be at around 14.8V to be fully charged.

Equalising batteries (flooded only as you will destroy AGM's) should only be done once every couple of months. This is when the voltage is pegged 1V higher that the Boost/Absorption voltage which in most cases is 14.5-14.8V. Therefore you are equalising the voltage in each cell of the battery by charging at around 15.5V for around 2 hours max. The batteries will gas furiously and need to be well ventilated and away for ignition sources or you will get an explosion.

I would not recommend any general charging higher that 15V due to the above reason. I would also not recommend mixing batteries - especially flooded and AGM's as the AGM will not even last 6 months. You have the right idea of using a smart charger which is the only way to go to ensure maximum battery life and optimum state of charge.

Leave the flooded to start the car and put any AGM's in the camper or in the car but seperate from the flooded. With AGM's it is eiter all AGM's or none.

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Follow Up By: Member - Jeff M (WA) - Thursday, Jul 28, 2005 at 11:55

Thursday, Jul 28, 2005 at 11:55
"With sealed batteries, the only way to check the charge state of the battery is to measure the voltage BUT you can not get an accurate voltage reading unless the battery has been sitting without any charge or discharge occurring for a minimum of 72 hours. "

And you need to take into account the ambiant tempeture and adjust accordingly too!

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Follow Up By: drivesafe - Friday, Jul 29, 2005 at 09:05

Friday, Jul 29, 2005 at 09:05
Folks the argument about whether a battery is only charged to 70% or can be charged much higher using no more than the garden variety vehicle alternator, is being muddied because people are not taking into account the situations in which the batteries are being used.

If we were taking about battery use in a taxi like environment then there is probably a valid argument that the battery will never get fully charged and will have a short life span.

A taxi is use for continuos short trips, there motors are stopped and started dozens of times a day and their motors can be started and then run for no more that a few seconds at a time, as many as 3, 4, 5 even 10 times in a short period, the to allow the taxi to be moved up in a taxi rank and yes they are never going to be fully charged.

Now correct me here but aren’t most of the people on this site talking about long trips which at a minimum would entail driving continuously for at least 2 hours a day.

This is in now way similar to city driving and the 70% charge is based on city drive conditions. For someone who is driving continually for a few hours at a time, the battery charge conditions are completely different.

Here’s a quick way to dispel the “No more that 70% charge” myth for anyone doing hourly continuos drives and have maintenance free batteries.
simply check the state of charge in the battery indicator after a days driving.

This type of battery usually has a coloured indicator that shows the state of charge of the battery.

These indicators work in exactly the same manner as a Hydrometer.
They have two or three small balls that are set to float at a specific gravity point, when the colour in the indicator is RED, the battery will be at or bellow 50 or 60% charged ( the difference depends on the make of battery ).

When the colour is CLEAR or WHITE, the battery is somewhere between 50% and 90% charged.

When the indicator colour is GREEN, the battery is at or above 90% charged.

Now even if there is a 10% margin of error, this is still a minimum of 80% charged indication ( BTW, none of the battery sites I have looked at list a likely margin of error ).

Why would the manufactures give a charged indication that is set at 90% if they thought that the batteries would never get over 70% charged. They would be over run with people complaining that they could not get their battery charged and so.

My point is that, and is I have stated, the Inverter/charger set up will work, but this set up is not only unnecessary, it is inefficient, added expense expensive set set up even if you already have the gear and an increased load factor for the alternator particularly when the alternator can do the same job by itself and more efficiently.

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Reply By: Member - Barry W (VIC) - Thursday, Jul 28, 2005 at 00:07

Thursday, Jul 28, 2005 at 00:07
Hi Norm
I'm not a battery expert !!!!
check out " Collyn Rivers" web site he is the "Gurroo" of everything electrical ??
I have a couple of his books - Motor Home Electrics & Solar that really works - they are very good.
You can get his publications on this site I think ??
Hope this helps
AnswerID: 122452

Follow Up By: Member - Norm C (QLD) - Thursday, Jul 28, 2005 at 07:46

Thursday, Jul 28, 2005 at 07:46
Good point Barry,
I'm familiar with Collyn's work. He is also a regular poster on this forum.
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Reply By: Mainey (WA) - Thursday, Jul 28, 2005 at 09:00

Thursday, Jul 28, 2005 at 09:00
(QUOTE) I charge my DEEP CYCLE, *BOTH* flooded and AGM batteries, in two ways: (end quote)

It appears by your post you have more than one Deep Cycle battery, an AGM battery and also a conventional flooded Deep Cycle battery ?

If you have these different types of Deep Cycle batteries wired in parallel, then your Deep Cycle system will give you major problems, because as you have seen from other posts above, both of these different batteries will accept charge at very different rates ! !

This is not a recommended set-up by any ‘qualified’ and ‘knowledgeable’ auto elec, so I take it that it was only a typo and I have read it all wrong :-(
AnswerID: 122482

Follow Up By: Mad Dog (Australia) - Thursday, Jul 28, 2005 at 09:22

Thursday, Jul 28, 2005 at 09:22
What are these major problems Mainey. I've heard of people combining AGM and conventional batteries with no problems at all, some are on this forum.
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Follow Up By: Member - Roachie (SA) - Thursday, Jul 28, 2005 at 10:10

Thursday, Jul 28, 2005 at 10:10
I too have often read about the potential for major hiccups if using different batteries together.
This could be one reason why a 33 month old Exide Orbital battery of mine died recently. I had originally used it under the bonnet for about 12 months as my auxillary battery, wired via a simple solenoid system. So it was being charged by the alternator at the same time my normal Nissan wet-cell starter battery was being charged.
I now have another question.....Advice seems to be that if we're gunna use deep cycle or AGM or SLA batteries, it is best to keep them separate (eg: in a camper trailer etc). Well, this is now the set-up I have. The Nissan has 2 Exide Extremes under the bonnet using the same solenoid system for isolation. The camper trailer has 2 SLA batteries Super Charge Gold Series I think (106a/h each). I have heavy wiring (8mm) in place from the aux Nissan battery, to the camper trailer batteries via a fusible link. Does the fact that these batteries are 'connected' to the 2 flooded cell batteries when I'm travelling, mean that I have 4 incompatible batteries linked at run the risk of damage?
When I'm at home, the 2 SLAs in the trailer get charged by a Durst 3 stage charger, but whereas i used to keep that on 24/7, I've recently decided to only switch it on a day or 2 before I go on a trip, just to ensure the batteries are in top condition prior to leaving.
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Follow Up By: Mad Dog (Australia) - Thursday, Jul 28, 2005 at 11:55

Thursday, Jul 28, 2005 at 11:55
Premature failure of those orbital batteries seems to be a common occurence, I have no idea why.

The 8mm cable is too thin for the long run back to the camper Roachie.

>it was being charged by the alternator at the same time my normal Nissan wet-cell starter battery was being charged.

Was it really being charged by the alternator...ie you had both the starter battery and aux in parallel with the alternator (not good) or was the alternator charging the main battery which charged the Aux...that is ok
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Follow Up By: Mainey (WA) - Thursday, Jul 28, 2005 at 18:45

Thursday, Jul 28, 2005 at 18:45
(QUOTE)Was it really being charged by the alternator...ie you had both the starter battery and aux in parallel with the alternator (not good) or was the alternator charging the main battery which charged the Aux...that is ok (end quote)

Ummm, Mad Dog it doesn’t work that way, the main battery doesn’t charge the Aux battery.
Charging in parallel with a quality solenoid or electronic isolator as the 'switch' is the same as charging solid wired in parallel, technically it should not be done, yes I know it is, but technically it’s wrong, because different batteries charge at different rates and also Discharge at different rates, causing the smaller less powerful one to drain charge from the larger battery, read any battery fact or information web site to clarify the situation further.

Roachie, quick answer, yes, you have two (2) sets of identical batteries, the alternator/regulator sees the two SLA's as one large battery, (because they are hard wired in parallel ?) and it is connected to the second Exide Extreme which is separated by a solenoid from the starter battery, by "not thick" lead.
Definitely leave the SLA's on the 3 stage charger 24/7 as they will sulphate if left uncharged for any length of time.
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Follow Up By: Mad Dog (Australia) - Friday, Jul 29, 2005 at 10:19

Friday, Jul 29, 2005 at 10:19
That may be the case with batteries wired in full time parallel but having an isolator between batteries where during charging the batteries are always non identical even if they look the same because of the diffferent potentials is a different ball game. Roachies batts under the bonnet are isolated. When two batteries of different potentials are connected in parallel they will attempt to equalise at a rate depending on the resistance, current will flow from the higher voltage battery to the lower voltage battery. Battery fact sites generally discuss full time paralled batteries and not the more complex situation of isolated batteries.
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Follow Up By: Mainey (WA) - Friday, Jul 29, 2005 at 18:23

Friday, Jul 29, 2005 at 18:23
Under the bonnet there is a start battery, and a (smart) solenoid that connects/disconnects the second "cranking battery" which happens *only* when the start battery has been fully charged first.
Most 4wd's have this system, it's the most common method :-(

The two Deep Cycle batteries in the camper Trailer are hard wired in parallel, and therefore are effectively only one large Deep Cycle battery, they are permanently charged and discharged as only one battery.

When the Deep Cycle battery 'system' is connected to the vehicle at the back bumper connection plug it (they) will equalise instantly with the second cranker battery which is under the bonnet, but NOT with the start battery, *unless*, the motor is running and the start battery is fully charged and the solenoid has closed and the second "cranker battery" has been joined to the start battery and has equalised with the start battery and is being charged by the alternator.

Assuming the motor is NOT running and the CT is connected to the vehicle, when you start the motor the start battery is isolated from all other batteries till it is fully charged, then the solenoid closes and all three (cause the two DC are only one) batteries will equalise with the fully charged start battery.

The second cranker battery is being equalised (charged/discharged) each time the vehicle starts and also each time the trailer (and DC battery) is connected/disconnected, it has a very hard life by comparison to the other batteries and yes will die first unless the DC system suffers from sulphation due to being undercharged.
The fully charged battery/s also suffer due to the equalisation process because they have to be recharged again due to their loss of power to the lesser capacity battery :-(
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Reply By: Member - Norm C (QLD) - Thursday, Jul 28, 2005 at 10:55

Thursday, Jul 28, 2005 at 10:55
Interesting. I've also read heaps on the issue of not mixing batteries.

Mainey, to clarify my set up:

What I have is a 105 AH Federal Deep Cycle in the engine bay isolated from the started battery by a Cole Hersee solenoid (with diode for spark protection).

In the camper trailer, I have two 120 AH Full River AGMs connected in parallel. When the Anderson plug (175 Amp) is connected, the AGMs are in parallel with the flooded deep cycle. This is all the time when driving and charging. They do not remain connected in discharge mode for long periods. Generally a few minutes, but may be up to an hour if we stop for lunch and I forget to disconnect. If this is a big problem, I can connect another solenoid to isolate the AGMs on the CT from the flooded deep cycle when the ignition is turned off.

I have been told by a number of sources (but not all; there is no way to get general agreement on this subject) that my set up is fine, as long as I don't leave the batteries in parallel for long periods in discharge mode.

Happy to get other opinions on this, particularly from those of you with real knowledge on this subject.

I was originally going to install a top end Rotronics system to properly isolate the different batteries, but at over $1,000 including installation, I couldn't justify the cost at the moment.

My other option was the one currently under debate on another thread. Install an inverter and 3 stage charger on the CT to look after the AGMs and allow the simple solenoid system to look after the starter and engine bay deep cycle. This is still an option for the future as I have a 25 Amp CTEK 3 stage charger, and will probably get an inverter at some stage for general use.
AnswerID: 122505

Follow Up By: Flash - Thursday, Jul 28, 2005 at 11:32

Thursday, Jul 28, 2005 at 11:32
connecting them together like that whilst charging IS fine.
No drama at all.
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Follow Up By: drivesafe - Friday, Jul 29, 2005 at 09:34

Friday, Jul 29, 2005 at 09:34
Hi again NormC, my post above was about both the 70% charge myth and inverter/charger set up debate in the same post.

But this is another point most people are unaware of. Even if you have two batteries of the same make, type and size, they will not be identical and as such will go through equalisation at some time after you remove the charge ( in this case I am referring to the alternator be stopped ).

When batteries are first removed from a charging device they will start to drop in voltage. You can check this yourself, next time you are doing a trip and know you will be stopping for around the same time on a couple of lunch breaks, when you pull up for one lunch break, go to your paralleled batteries and measure the voltage at the battery terminals but leave them connected. Just before you start up to leave, measure the batteries again.

Next time you stop for a break of about the same time period, as soon as you stop, separate the batteries and then measure both batteries and before you start off again, measure both batteries just before you reconnect them again.

This will give you a very, very rough idea of how the batteries are reacting against one another.

Also providing you will drawing power off the batteries overnight, this discharging will keep the batteries equalised anyway so leaving them together should not be a problem.

I would not recommend leaving them together for long periods of time if there was not going to be some form of load on them.

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Reply By: Member - Collyn R (WA) - Thursday, Jul 28, 2005 at 14:38

Thursday, Jul 28, 2005 at 14:38
Firstly, thank you for kind comments on my books.

The vast majority of batteries are destroyed by undercharging. Few even approach full charge - and even then a mild overcharge now and again will do more good than harm as long as the electrolyte stays over the plates. Gassing (often mistaken for 'boiling') only begins at around 70% charge. It is a natural and indeed desirable effect.

A god rouigh guide to charging is that a non-sealed battery should use about one cm of water every 8-10 weeks. If it doesn't it odds on that it's chronically undercharged.

A car charging system is designed such that the charge automatically tapers off as the voltage rises - and in practice few batteries go much over 70% charge. This is done to protect vehicles (like taxis on shift work) in non-stop service.

AGM makers advice is not to parallel charge their products with conventional batteries - and I included this in earlier editions of 'Motorhome Electrics'.

I cannot totally confirm this but I have good reason to believe the concern is about direct parallel connection - where the AGM battery will grab the starter battery charge. In recent years though, voltage sensing relays (such as the Redarc) ensure the starting battery has priority - and that will solve that problem.

The other possible concern is that a big AGM battery bank (say >200 Ah) may load up the alternator excessively - because AGMs will accept high charging current. There it is probably prudent to upgrade the alternator (or use a continously rated unit) especially if charging a big battery bank in hot places.

Studies show that very high charging voltages (and hence current) will shorten the life of AGMs. There's no point at all in doing this as an adequate charge - to 100% - can be obtained as low as 14.1 volts. The uusal 14.2-14.4 volts is fine. There is some case for charging AGMs via a three-stage charger, but less so than for conventional batteries.

(For the technically minded all this is because AGMs have very low internal impedance and thus accept high charge currents from lower applied voltage).

The other concern with AGMs is that their float voltage is very low. One maker quotes 12.9 volts at 50 degrees C, 13.4 at 25 degrees C and 13.9 at 0 degrees C.
Trust this helps
Collyn Rivers
AnswerID: 122551

Follow Up By: Spade Newsom - Thursday, Jul 28, 2005 at 23:51

Thursday, Jul 28, 2005 at 23:51

Have been told that a lead to an AGM in a trailer from a parallelled deep cycle will provide sufficient voltage drop to make the set up compatible.

Any comment? Is this a bit too simplistic?


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Reply By: Member - Collyn R (WA) - Friday, Jul 29, 2005 at 11:35

Friday, Jul 29, 2005 at 11:35

It IS a bit simplistic, but often not far wrong because AGM batteries are very tolerant in regard to charging voltage. As long as there's more than 13.9 -14.0 volts available with a load of (say) 10-15 amps, then an AGM should charge well enough.

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Reply By: Member - Collyn R (WA) - Friday, Jul 29, 2005 at 11:51

Friday, Jul 29, 2005 at 11:51
The energy required to start a taxi's (petrol) engine is negligible. It is about the same draw as a sidelight globe for 10-15 minutes (or in fact of the taximeter).

The alternator typically replaces the starter current drawn in less than a minute. In practice starter batteries typically move between 67-70% charged.

It is true that an instantaneous voltage reading may show otherwise but, as you point out, this is due to the lag in the battery's electro-chemical reaction masking the reality within the electroyte. Whilst this lag is far shorter than with deep cycle batteries it is very much still there.
Collyn Rivers
AnswerID: 122694

Follow Up By: drivesafe - Friday, Jul 29, 2005 at 17:55

Friday, Jul 29, 2005 at 17:55
The starting of any vehicle is going to be about the same and any vehicle start, will use a huge current draw in a few seconds while the side light, as you used in your example, will take 10 to 15 minutes to use the same amount of power.

This type of use don’t compere to the use of a fridge drawing power from an auxiliary battery. The starter motor power usage will take around 5 minutes driving to replace the power used in a few seconds.

this situation is not occurring in the case of people who are using their vehicles for camping and what ever.

Far more power is going to be used by the fridge but spread over a much, much longer period and then once the vehicle is travelling again, the battery is now going to be continually charging for a far greater period.

The very nature of the way a taxi is used, will limit it’s ability to recharge the battery properly whereas with the 4by’s type of use that requires it to be driven for hours on end will not only replace the power used by the fridge but will go a long way to fully charging the battery, certainly way above 70%.

If this same 4by was to be used as day to day transport around a town or city then yes 70% charged may be the level the battery is going to end up at. The point I have raised is that this is not the case, the type of vehicle use being discussed here relates to a situation where there is a lot of continuos driving going to be done.

The charging cycles are completely different and is far more advantageous to the way the batteries need to be recharged after their specific type of use.

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Follow Up By: Spade Newsom - Friday, Jul 29, 2005 at 18:38

Friday, Jul 29, 2005 at 18:38
Can a standard vehicle alternator charge a cranking battery any where near full charge??

Does anyone know the phone number for the "myth busters".

I want to know the answer.
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Reply By: drivesafe - Friday, Jul 29, 2005 at 19:42

Friday, Jul 29, 2005 at 19:42
Hi spade Newsom, joke or not, if by standard, you mean by a wet cell with filler caps, you don’t need to ring anybody.

Next time you go for a decent drive somewhere, take along a hydrometer and measure the battery yourself. In this way it won’t matter what is said here, you will know for sure for for yourself.

AnswerID: 122759

Follow Up By: brett - Friday, Jul 29, 2005 at 20:07

Friday, Jul 29, 2005 at 20:07
Did this the other day, popped the bonnet and got a reading of 1265 for the starter. Car is driven every day short trips no longer than 30mins Then tried the battery in my camper which had been on float charge for several days got a reading of 1275.
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Follow Up By: drivesafe - Friday, Jul 29, 2005 at 23:58

Friday, Jul 29, 2005 at 23:58
Hi brett, a reading of 1265 gives you a full charged battery. ( near to 100% )

A couple of questions, did you check all the cells and had you just taken the camper battery off the charger.

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Follow Up By: brett - Saturday, Jul 30, 2005 at 18:52

Saturday, Jul 30, 2005 at 18:52
Checked all the cells and they were all very close, and the camper battery had been off the charger for a few hours.
I basically posted these figures to say there is not a lot of difference between the alternator and the fancy charger. As far as I see the only differnce between the two is time. When your talking about voltage regulation charging the battery doesn't see the 14.2V from the charger, solar panel or alternator any different.
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Follow Up By: drivesafe - Saturday, Jul 30, 2005 at 20:46

Saturday, Jul 30, 2005 at 20:46
Hi Brett, reason I asked was out of curiosity. Some people check just one cell while others check them all.

I think it is up to your own choice as to what you do but I, like yourself, check them all.

Thanks brett.
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Reply By: Member - Collyn R (WA) - Saturday, Jul 30, 2005 at 09:45

Saturday, Jul 30, 2005 at 09:45

12.65 SG is about right for a well-charged deep-cycle battery.

The increasing trend to more CCA has led to higher SGs for cranking batteries. I'd suggest that wioth today's batteries, 12.65 SG is more typical for a state of charge of 70-75%.
Collyn Rivers
AnswerID: 122838

Follow Up By: drivesafe - Saturday, Jul 30, 2005 at 14:12

Saturday, Jul 30, 2005 at 14:12
I’ve always respected the info you published, I may not agree with how you advise people to use this info but up until now I considered the info to be accurate.

That is till I read this post.

For the specific gravity of the acid to change it’s reading, there has to be a change in the ratio of the acid content, and as there has NOT been a change, how can you say that a standard means of testing a battery for over one hundred years has now suddenly changed and not only has it changed but no one in the industry has been told and those making hydrometers have not been told BUT YOU HAVE.

This one is way over the top.

You have a responsibility by the very nature of your business of selling information, to stick to the truth.

Again, if anybody disagrees with me and has a fairly new bottle of battery acid handy, test it with your hydrometer.

New unused battery acid shows on a hydrometer as being 100% charged.

I don’t mined being corrected when I’m wrong but for someone of your standing to come out whit this sort of statement is nothing short of ludicrous.

Brett, don’t take my word, go to any battery site on the web and you will find identical info relating to Specific Gravity readings from one battery manufacture to another and nowhere will you find anything that says there has been a change in SG readings.

Your battery is 100% charged.

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Follow Up By: drivesafe - Saturday, Jul 30, 2005 at 18:22

Saturday, Jul 30, 2005 at 18:22
For the benefit of those following this posting, there is an Australian book that states “The specific gravity of a fully charged battery will be 1.250 - 1.280”

The book is called “Motorhome Electrics & Caravans too!”

It’s put out by a bloke called Collyn Rivers


FollowupID: 378049

Reply By: Member - Collyn R (WA) - Saturday, Jul 30, 2005 at 10:46

Saturday, Jul 30, 2005 at 10:46
I introduced the starting energy issue to hopefully demyth the belief that starting a car engine requires a lot of energy. It doesn't. The practical evidence used to be the ability of my then 51 kg girl friend's ability to hand-start my 4.5 litre vintage Bentley with singular ease. More recent engines have higher compression - but not 1.25 litre cylinders!

Drawing (say) 400 amps from a starter battery for a typical two seconds will deplete it to a greater extent than drawing 40 amps for twenty seconds (due to internal heat losses etc) but only by two/three times. It is still a tiny amount of energy and a few sums will show that it takes only a minute or two to replace it.

Its relevance is simply that most people assume the contrary and are unnecessarily concerned about the perceived time that replacing that energy takes. Thus parallel charging is safe enough if you use a Redarc or other such device to give starter battery priority for the first minute or so - and the alternator can cope with the increased load.

One does not need to ring Mythbusters to acertain maximum charging percentages. There's any number of other ways to check this.

These include reading the definitive texts on this subject; doing a controlled discharge test, plotting charging rate over time from a constant voltage source, charging from a three-stage charger after the same from a constant voltage charger, even wondering why on earth people pay a lot more for constant current chargers. Anyone using alternator charging with solar backup (via a good regulator) likewise will rapidly find that the solar input will increase battery charge way beyond the capability of the alternator.

My OKA uses both - and I'm happy to demonstrate this at any time.

Even adding an amp meter and watching what happens after the first few minutes driving will give some indication.

Why does this matter?

It's because a vast number of RV charging problems stem from a fundamental and widespread misunderstanding of battery charging basics. Not least of these is that the constant voltage charging system is used on motor vehicles and cheap battery chargers ONLY because it is cheap, reliable and rugged. (During my years with GM Research Division we came up with any number of alternatives, but which were rejected because they cost more).

Happily its era is passing as the new hybrid cars use constant current Pulse Width Modulated charging, and it is likely the new 36-42 volt vehicle systems will do likewise.

This archaic charging technique is rarely if ever used in any serious application outside thiese areas because of it's serious and fundamental limitations - not the least of which is a serious predisposition to thermal run away were the charging voltage to be high enough to be truly effective (three-stage chargers typically charge at up to 15 volts - but at constant and controlled charging current).

The point is that it works fine for starting because the whole system is designed accordingly - but it's one hell of a lot less than fine when you attempt to use the same system for parallel charging batteries used for non-starting purposes.

Most of this was well-known by the early 1900s and can be found in any basic battery text book.
Collyn Rivers

AnswerID: 122850

Follow Up By: Mainey (WA) - Saturday, Jul 30, 2005 at 18:19

Saturday, Jul 30, 2005 at 18:19
Now guy's take a deep breath..... :-)

(QUOTE) Anyone using alternator charging with solar backup (via a good regulator) likewise will rapidly find that the solar input will increase BATTERY charge way beyond the capability of the alternator (end quote)

I have a solar system with a Steca regulator, it shows via the LCD screen the amps at the panel, amps coming from the panel, amps going to the Aux batteries, voltage of the batteries, amps used by the fridge, I can sit and watch it change in front of me when the various different items take power from the batteries and then turn off and the amps change with each different change in use of the batteries.
I also use an expanded 10 amp ammeter, solid wired between the Steca and the batteries, which shows amps going to or from the DC battery system, and I can see as Collyn has stated, the solar system charges the DC battery system higher than the alternator.
FollowupID: 378047

Follow Up By: brett - Saturday, Jul 30, 2005 at 18:59

Saturday, Jul 30, 2005 at 18:59
" the solar system charges the DC battery system higher than the alternator. "

What does that actually say? I'm tipping if your batteries are reaching full charge off your solar panel and drawing say 4amps, and you disconnect the solar panel and connect it to your alternator you will still see a reading of 4 amps, I don't think it will drop to say 3amps and charge slower. The batteries internal resistance is the constant at that time and whether you supply 14.2V from your solar panel or your alternator makes no difference. Current=E/R. The only thing I see is your solar panel will provide that current for maybe 8 hours in a day but you probably won't drive for that long so you will get a better charge from your solar panel but only because of time
FollowupID: 378053

Follow Up By: Mainey (WA) - Sunday, Jul 31, 2005 at 00:48

Sunday, Jul 31, 2005 at 00:48
(QUOTE)I'm tipping if your batteries are reaching full charge off your solar panel and drawing say 4amps, and you disconnect the solar panel and connect it to your alternator you will still see a reading of 4 amps, I don't think it will drop to say 3amps and charge slower(end quote)

brett, when the two 80 A/h DC Aux batteries, solid wired in parallel, are fully charged by the solar panel via the solar regulator they are at 13.9v minium and no (zero) amps will be sent to the Aux batteries from the solar system because the solar regulator is set to trickle charge at 13.9v and Amps will only flow to the Aux battery when it is below 13.9v, boost charge is 14.4v and equalisation charge is 14.8v.

During the day when the fridge is actually running, Amps will go from solar panel to Aux DC batteries, only till the Aux batteries again reach 13.9v minium, so as you can see the DC Aux batteries are basically always at 13.9v during the day, unless the fridge is running, so the Aux battery recharge is happening all day every day in sunshine, even when the fridge is using Amps they are being automatically replaced by the solar system.
When the fridge is drawing, for example, 3 Amps the solar system is also showing 3 Amps being sent to the Aux batteries on a sunny day to replace it and keep the DC Aux battery at 13.9v minium.
I sometimes only start my vehicle once a week and that is to go to town for food an water etc.
FollowupID: 378087

Reply By: Member - Collyn R (WA) - Sunday, Jul 31, 2005 at 18:02

Sunday, Jul 31, 2005 at 18:02
I need to respond to the above one at a time.

Brett. I'm not about to argue with Ohm's Law. The whole point is that the batteries are charging at a higher rate because the solar regulator typically applies a higher voltage across them (than does a car alternator/regulator) and hence a higher current flows.

Many solar regulator have boost voltages of up to 15 volts. Whilst this may not seem that much higher than 14.4 volts of the car system the charging rate of a constant voltage system is a function of the difference between the applied voltage and the battery voltage. The extra 0.6 volt or so is a high proportion of this.

This again is a substantinally pointless argument as what I am saying can readily be checked by anyone with an alternator plus solar system.

I do not propose to respond in the manner of your posting.

Here's what a wide range of others state on the same subject. The 'others' include battery makers and one the world's most respected makers of battery chargers. I leave it to others to decide whether my statements are valid.

The Ample Power Company states: 'Specific gravity for fully charged deep cycle batteries falls in the range of 1.250-1.280.' Also: 'A heavier (say) 1.300 electrolyte can provide more capacity ..but at a reduced rate.'

'Therre is no specific gravity which indicates a full or empty battery.... the higher the initial SG, for instance 1.280, the higher will be the SG for a discharged battery... whatever it is, that's the SG for the new batteries should be when they are fully charged."

US Authority, Harold Barre: (page 170 of '12-Volts') "In temperate climates the specific gravity reading for most fully charged batteries is 1.250-1.280."

North-West Energy Storage: In a fully charged cell the electrolyte has a specific gravity that varies from 1.260 to 1.285 (depending on type and manufacture.'

Powerstream's 'Capacity of Storage Batteries- Chapter 7: "[Speaking of temperature climates] "A question which must be considered at this point is why in automobile work, a specific gravity of 1.280-1.300 is adopted for the electrolyte of a fully charged cell." The paragraph from which this quote is taken concludes: "The density of 1.280-1.300 is therefore a compromise between various factors.."

The chapter finishes with a discussion of the results of sulphations that is very significant.

"When a battery has been in use for some time, a considerable portion of the active material will have fallen off the positive plates, and a decrease in capacity will result. Such a battery will charge faster than a new one because the amount of sulphate which has formed when the battery is discharged is less than in a newer battery. ...the battery will come up faster on charge, although the specific gravity of the electrolyte may not rise to 1.280."

Trojan Batteries: "A hydrometer reading of 1.277 or greater indicates full charge for Trojan batteries .... based on a specified temperature of 77-80 degrees F.

New Energys (previously Yuasu). Quotes 1.290.

Century quotes 1.30.

Any or all of the above is readily verifiable.

I welcome the opportunity to correct any error in my work (and publish such corrections on my website), but would prefer it it could be offered in a more genteel fashion.

AnswerID: 123023

Reply By: drivesafe - Monday, Aug 01, 2005 at 11:46

Monday, Aug 01, 2005 at 11:46
So if we are to take you at your word, then what your saying is that you have been selling books with information in them that you know to be incorrect.

As fast as you can come up with data to back which ever story you want to push at the time, in this case the same can be done by anybody checking the different battery sites and finding different results

To simply say that’s Brett's battery is not fully charge even though this statement fly's in the face of data available from a whole host of publications and web sites shows you to be someone who will say anything to try and score brownie points or what ever.

Contrary to what you may think, I’m not hear to score brownie point against you or or anybody else. I put forward an opinion base on first hand experience that says that a battery CAN be charged way above 70% by an alternator and at the same time put it to anyone interested in finding out for themselves an industry accepted way to do it.

You have now posted that these reading are incorrect even though you sell a publication that states otherwise. You say that the very use of a hydrometer is a waste of time while, even though they may not give a 100% accurate indication, they are the best way to get a quick and comparatively accurate indication of the state of charge and condition of battery and I might add again, the industry accepted way.

Yet while you say that the hydrometer indication is not viable you try to use info from different battery data using SG reading to validate your claim. So tell me how does one prove or disprove your claim if one does not have a tool to do it with, thats convenient.

And I still stand behind my opinion that the maximum charge level that an alternator can get a battery to is 70%, is a myth.

AnswerID: 123119

Follow Up By: Member - Collyn R (WA) - Monday, Aug 01, 2005 at 12:55

Monday, Aug 01, 2005 at 12:55
This is a dynamic situation in which industry approaches are clearly changing as alternator and battery makers attempt to cope with the ever-increasing electrical demands of today's computer-equipped cars.

To cope with this my books are constantly revised and updated and this is made very clear in each edition.

Regarding my published statements re SG, I quote directly from page 30 of Motorhome Electrics under 'Hydrometer Reading'

"The specific gravity of a fully-charged deep-cycle battery will be 1.240-1.250. That of a starter battery will be 1.280-1.300."

AGMs are well over 1.300 but this cannot readily be measured.

In the US charging voltages are increasing - commonly to 14.7 volts, in Europe they are lowering (to <14.2 volts).

There is also a forthcoming and inevitable trend to 36/42 volt systems - and those that have emerged so far appear to use pulsed constant current charging.

When/if a clear trend emerges I shall, update the book and post the change on my website. To make changes right now would only confuse.

Regarding errors in my books. The first edition of Motorhome Electrics had a few errors (mostly misplaced commas etc, but incorrectly gave the rotational speed for 50 Hz gen sets at the US 60 Hz (1800 and 3600 rpm) instead of 1500 and 3000 rpm. I posted the correct data on my website immediately the error was spotted. Do you publicaly admit to error? Or do you not make any mistakes?

Regarding SG testing. Whilst still common, the primary information it provides is disparity between cells. It cannot reliably be used to indicate charge (except on newish batteries of which the original electrolyte SG was known) because the degree of sulphation affects charging voltage vis a vis SG.

Load testing is more reliable - but is only really relevent with starter batteries.

Regarding Brett's experience, you seem to totally miss the point.

Brett was totally correctyly stating that 14.2 volts is 14.2 volts and that changing from one source of it to another cannot affect current flow. No-one in their right mind would dispute this (assuming they were aware of Ohms Law). The whole point is that if the applied voltage is raised (as in constant current charging) then current flow rises accordingly.

I suspect the reality is that I have upset a set of views on battery charging that you have no doubt held in good faith for many years. May I respectfully suggest you check around to see whether or not there might be something in what I'm saying - rather than lashing out at things and people that your prior practice leads you to disagree with.
Collyn Rivers

FollowupID: 378213

Reply By: drivesafe - Monday, Aug 01, 2005 at 13:35

Monday, Aug 01, 2005 at 13:35
We have been taking about batteries that are here and in use right now, not what’s possibly coming.

Now your saying that Brett's ( and sorry for bringing your post into this again Brett ) battery reading might be correct if his battery is "newis", talk about a copout, as Brett never mentioned whether his battery was new or not, why didn’t you bring up this possible difference in the first.

As I posted, you are just continuing to try justify what you have posted by moving the goal posts.

None of this explains why his battery is way over 70% charged nor have you put up any viable argument that says that this can not be achieved.

I reiterate, from hands on experience, it is my opinion that “the maximum charge level that an alternator can get a battery to is 70%”, is a myth.

I am quite happy to leave it to others to prove or disprove this for themselves.

AnswerID: 123133

Follow Up By: Member - Captain (WA) - Monday, Aug 01, 2005 at 16:19

Monday, Aug 01, 2005 at 16:19
I have been following this post with quite some interest. Both Drivesafe and Collyn have been regular posters with good info. Standing back from a distance, I believe both of you are right about the SG and state of battery charge!!!

Its my understanding that all "SG vs state of charge" graphs are based on a certain % H2SO4 acid concentration. Now depending on the initial % H2SO4 used, you will have a different SG per % charge. I'm sure you will find that traditionally all battery makers probably used the same base % concentration. But if a particular battery maker used a different % H2SO4, then its state of charge would have a different SG. For example,
50% H2SO4 has SG of 1.38
65% H2SO4 has SG 1.55
92% H2SO4 has SG 1.83
(note: taken from various websites, so may be minor inconsistencies in SG)

So, my interpretation of what Collyn is saying is that some battery makers have taken to using higher % acid hence SG readings will not correlate to historical "SG vs % charge" graphs.

Hope my interpretation of the above is correct :)


FollowupID: 378244

Follow Up By: drivesafe - Monday, Aug 01, 2005 at 19:27

Monday, Aug 01, 2005 at 19:27
Hi Captain, up until this morning I was, wrongly or rightly, of the OPINION that all battery acids were of the same standardised composition.

Made a few calls this morning just to see if I was correct and the info I have been getting says just that, I might add if I had have been wrong I would have posted an apology, at this time I don’t think an apology is going to be necessary.

It seems, as I thought, that if you have brand X battery, you can use brand Y acid and know they are totally compatible.

This is also why you can use any brand of hydrometer with any brand of battery and you will have a comparatively accurate indication of the battery’s state of charge and each cells condition.

Hydrometers are not give you a 100% accurate indication of the state of charge of the battery but in the situation that has been discussed here, they will give an accurate enough difference in reading to show if a battery is 70% charged or more likely over 90% charged which still indicates that the battery has ( if it shows such a reading ) far more than the 70% charge, that is constantly muted as the up limit that an alternator is capable of achieving.

I do not expect anybody to take my word as gospel but by carrying out a test of the battery’s charge using a hydrometer, will prove or disprove my statement.

FollowupID: 378277

Follow Up By: Member - Captain (WA) - Monday, Aug 01, 2005 at 21:44

Monday, Aug 01, 2005 at 21:44
Hi Drivesafe,

I am not in the electrical industry, but as a Chemical Engineer I have used Sulphuric Acid in industrial applications. The thing about sulphuric acid is that you measure its SG to determine its concentration.

So, to use SG to measure the state of charge of a battery, you are really measuring the concentration of the acid and then relating that to a battery charge (ie. an inferred reading). Now if you use a different concentration acid in the first place, you will get a different SG reading for the same state of charge. A hydrometer only measures SG and its the graph derived from experience that gives you the state of charge.

I guess it gets back to what concentration acid is used in batteries. if every battery maker used the same acid concentration, then SG can be correlated to %charge (as has been done for years). But if makers start to play with the %acid concentration and thus vary the "base" SG, then who knows what state of charge the battery is in from SG readings?

Now common sense (and NOT facts) says to me that as some battery makers try to push their battery performance, they would use different %acid for different types of batteries. I appears to me this is the point Collyn was trying to make.

Regardless, I have found this a very interesting thread


FollowupID: 378305

Reply By: Member - Norm C (QLD) - Monday, Aug 01, 2005 at 20:44

Monday, Aug 01, 2005 at 20:44
A fascinating debate although at times a bit more personal and vitriolic than might be necessary.

I claim no particular knowledge in this are, quite the contrary. But I recall from my youth learning that a fully charged battery would have a SG of 1.260 to 1.265. That was a long time ago and having seen the posts on higher SGs, I decided to do the 10 minute Google test.

Found many references to higher SGs, but mostly for AGM and Gel batteries.

Here are some that refer to variable and higher SGs for normal flooded batteries:

If you get new batteries, you should fully charge them and equalize them and THEN take a specific gravity reading for future reference, as not all manufacturers use exactly the same SG, and SG may also vary for the same battery sold in different climates.

Most Lead-Acid batteries will be in the range of 1.1 to 1.3 specific gravity, with most fully charged batteries being about 1.23 to 1.30 (some hydrometers multiply this number by 1000, so 1.23 would read as 1230.) Some batteries manufactured for use in very hot or very cold climates may have stronger or weaker acid

http://www.cyb.com.au (Century Yuasa Batteries)
The fully charged specific gravity readings should fall between 1.280 to 1.310

I put no interpretation on this. I don't have the knowledge to do so. Just found it interesting, so thought I'd share it.
AnswerID: 123225

Follow Up By: drivesafe - Tuesday, Aug 02, 2005 at 08:55

Tuesday, Aug 02, 2005 at 08:55
Hi Norm C, to be completely honest with you I have never bothered to check what the SG reading would be for another type of battery as I have no way of being able to take readings on these types of batteries.

As for battery manufacturers upping the acid content of their batteries, well this my opinion and just that, but to do anything like even the slightest change in the acid to water ratio would mean that the manufacture would most certainly have to notify, if not the general public and those associated with this market, but they would have to notify industries that depend on these batteries and use them in heavy duty applications such as folk lifts where the use of deep cycle batteries is common place and the continuos maintenance of these batteries would require the service staff to be notified of any changes and a similar situation applies to the mining industry.

Even if there is the slightest difference in acid content it would not be so much as to give around a 25% error reading and thats more what I challenged.

The argument I put forward was that the statement “An alternator can not charge a battery above 70%” was a myth. I will be the first to say that I would not be surprised if there are vehicles running around with only 70% charged batteries in them.
The point is, taking into account of the situation that was being discussed, the batteries were going to be subjected to climate of use that lends itself perfectly to allowing an alternator to be able to charge a battery to 100% fully charge or as near as to that as to not be able to notice any difference ( all other things being equal such as cable size, battery condition and so on ).

I did not challenge the ability of an inverter/charge set to charge the battery to a full state. What I did and still do challenge is the cost effectiveness of such a set up as compared to simpler and cheaper set ups. Ultimately, at the end of the day the aim is to have as much stored power as possible. The type of set up is down to the individual but the posting was being steered to a point where someone reading it would have come away with the opinion that the inverter/charger set up was the only way to charge you batteries.

The fact is, and this is a very rough comparison as there a number of factors that would have to be taken into consideration in each individual set up, but if you drive for less than 4 hours a day and you compared the amount of stored power achieved by a single battery being charged by and inverter/charger set up to that of a two battery set up being charged straight from the alternator, even though neither set up would have charged the batteries to any where near full, I would lay money that you have between 50 to 75% more stored power in the two battery set up and after 8 to 10 hours driving it would be more like 70 to 100% more stored power.

If you already have an inverter and battery charger then there may be little reason to get another battery for your caravan / camper trailer.

If you are just starting out and this is what I was pushing, you will end up with double the stored power for around a little more than half the cost. That’s at least a 3:1 cost ratio for each amp/hour stored.

I’ll get off the soap box now. Cheers
FollowupID: 378354

Reply By: Member - Collyn R (WA) - Tuesday, Aug 02, 2005 at 13:55

Tuesday, Aug 02, 2005 at 13:55
Thank you - that is exactly the point I am making. I'm becoming increasingly puzzled re this SG issue as the evidence for what I am saying is so readily available by anyone who cares to look for it: that original SGs vary considerably hence it is simply not possible to generalise re SG and state of charge.

As I've said it generally is in the range of (for starter batteries) 1.280 - 1.300) but there are no doubt some batteries still around where 1.265 is applicable.

Collyn Rivers
AnswerID: 123341

Reply By: Mike DiD - Saturday, Aug 06, 2005 at 13:05

Saturday, Aug 06, 2005 at 13:05
It is not valid to assume that just because current is flowing into a battery that you are increasing the amount of usable charge in the battery.

By "conservation of energy" the electrical energy flowing into the battery must go somewhere, and for a Lead Acid Battery there are three main options -

a. Converting the chemicals to store energy for later use during discharge.
b. Heating up the battery.
c. 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.
AnswerID: 124068

Reply By: Mike DiD - Saturday, Aug 06, 2005 at 13:35

Saturday, Aug 06, 2005 at 13:35
How many of you have charged a battery using a 12.7 volt charger ?

If not, I'd like you to explain how two batteries connected together will equalise their charges ?

A fully charged battery under no load puts 12.7 volt. So if you connect it to a fully discharged battery (10.6 volts) some current will flow for a few minutes (maybe one AmpHour) until the discharged battery builds up a bit of surface charge.

You just can't put any significant charge into a discharged battery by connecting it to a charged battery. If you have a look at the Voltage-Currnet Charge curve for any battery you will see that the voltage quickly rises above 13 volts if any useful current is flowing into the battery.

So is this different if I have two batteries connected to an Alternator ? The Alternator always puts out a CONSTANT voltage (provided the current drawn is within its lmits) regardless of what is connected to it. The Alternator doesn't see two batteries, an ignition system, headlights etc. An Alternator is connected by two wires - it doesn't see anything, except the total current drawn by ALL the loads.

If the total current demand exceeds the Alternator's limits, then the voltage will drop and so one battery may charge faster than another. As the first battery charges level increases, the ssytem voltage will rise and the second will charge.

So if your electrical system is in good condition and you drive for long enough both batteries will charge.

If you don't drive for long enough to replace the charge needed in the batteries then -
- if you have two identical batteries both will be undercharged and start to sulphate.
- if you have two different batteries one may charge fully and the other not - is this really a worse situation than above ?

Mike DiD

AnswerID: 124069

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