Overcharging batteries

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.

Klaus

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
Barry

(QUOTE) I charge my DEEP CYCLE, *BOTH* flooded and AGM batteries, in two ways: (end quote)

Norm,
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 :-(

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.

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

Spade

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.

Drivesafe
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

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.

Cheers.

Drivesafe

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

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

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.

Drivesafe
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.

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.

Cheers.

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.

Cheers.

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:

http://www.windsun.com/Batteries/battery_Glos.htm
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.

http://www.magnacharge.com/mchg_gls.htm
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.

Captain
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

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.

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