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christern avatar image
christern asked

Explain the rule of max 3-4 batteries in parallel

In another thread there was someone who pointed at a statement in the Wiring Unlimited document saying there should be a maximum of 3 or maybe 4 lead acid batteries connected in parallel. Reason, as stated in the document, is that large battery banks become tricky to balance and that imbalance is created because of wiring and due to slight differences in the internal resistance of the batteries.

My experience, but also the result of some tests that I have seen reported, is that imbalance is created in bigger banks but when the batteries are on float charging quite soon these imbalances are reduced into a level which is similar to as if the bank was only fewer (3?) batteries.

So, as you understand I only partly agree with this statement in the WU-document and have seen several installations with many batteries working just fine.

Could someone explain, in other words than the WU-document, how this imbalance would be of such importance that it would make installations with a higher number of batteries bad/useless/non-recommendable?

battery charging
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Dušan Klofutar avatar image Dušan Klofutar commented ·
In my off grid system i have 18 x 12V/100Ah agm batteries in parallel.Total 1800Ah and they work just fine for 3 years...
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Alexandra avatar image
Alexandra answered ·

@ChristerN

Make separate 'banks' and balance each of them. Connected with a good bus bar for good current share setup.

Keep DC runs as short as possible.

Balancing is always an issue. Good current sharing is always an issue. Even in lifepo4.

Battery bank rotation with lead acid is also good practice.

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christern avatar image christern commented ·
Thanks @Alexandra. I was perhaps unclear in my question because what I wondered was what it is that makes a bank of i.e. 6 batteries such a bad alternative compared to a bank of 3.

I know the imbalance is likely to increase in bigger banks but have seen test that says the bank will be as balanced as smaller when float is allowed for slightly longer than with a smaller bank.

For LiFePo the challenge is even higher and, thus, need a balancer due to those batteries “take whatever they get” compared to lead acids which takes what they can absorb.

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Alexandra avatar image Alexandra ♦ christern commented ·

@ChristerN

So you also need to look at energy unlimited as well. It explains more about lead acid charging.

Usually there are not enough amps when charging larger banks. If you trickle charge lead acid they sulphate. This tends to happen when there are more in parallel. Cant charge too fast or too slow or too long, or not long enough.

You need charge correctly to reverse all the chemistry. Not just get voltages correct.

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Alexandra avatar image Alexandra ♦ Alexandra ♦ commented ·
@ChristerN

Periodically equalisation charges as well are used for bank maintenance.

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christern avatar image christern Alexandra ♦ commented ·
Thanks, energy unlimited was an unknown document to me. Will study it too :-)
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klim8skeptic avatar image
klim8skeptic answered ·

I wondered was what it is that makes a bank of i.e. 6 batteries such a bad alternative compared to a bank of 3.

The number of battery interconnects for one. A 6 parallel battery bank will have 10 interconnects. A 3 parallel battery bank only has 4 interconnects.

Each one of those interconnects has to be sound and clean. LA batteries tend to leak, and if your batts are mobile, are subject to movement and vibration.

6p-batt.png

Current balancing with paralleled batteries is also harder to deal with. Smart Gauge explains the current sharing problem, and gives some solutions.

Also if you are after a large battery bank, why choose (example) 6 x 12v 100Ah batteries over an easier to implement 3 x 12v 200Ah batteries? Or even a 6 x 2v 600Ah cells bank??


PS, in the photo, the batteries appear to be paralleled as cross diagonal. However the charge and load connections dont allow proper current balancing.


6p-batt.png (1.5 MiB)
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christern avatar image christern commented ·

Thanks for your explanations. I will read and try to understand your link.

As for: Also if you are after a large battery bank, why choose (example) 6 x 12v 100Ah batteries over an easier to implement 3 x 12v 200Ah batteries? Or even a 6 x 2v 600Ah cells bank??

My experience is mainly from installations in boats (and a few in vans) and there are often dedicated, small, places for the batteries to fit where the space is just tailor-made for 95-100 Ah batteries and just a few of that kind of batteries will fit. The rest has to be placed outside the dedicated slots. And, of course, the rest of the batteries have to be of the same size as fitting the original slots. Also batteries of that size give more Ah per spent money.

And, apart from the case with more connections (= small losses) is it wrong that the batteries balance themselves during float, which some tests say they do? Still talking about LA.

Maybe the answer is in the Smart Gauge.

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JohnC avatar image
JohnC answered ·

Hi @ChristerN

Even new Pb batteries can come with differing resistance. Those small differences can result in quite significant effects on the current flow through each. Then the ones taking the most get a little warmer, the chemical reaction speeds up, and you get the 'thermal runaway' effect.

Lithium batts suffer this really badly, so they come with a BMS to help prevent it. But even then their management systems can detect something out of whack and raise alarms or baulk.

Pb users don't have that luxury. All looks well, while one string is boiling off electrolyte and the others, while still at the same Absorb V, haven't reached the same SOC. And may never, even under Float on a limited length solar day. Aware owners might touch-test or use an IR scanner to check T, but they can only monitor, not control, what's happening.

This gets worse if the batts are of different brands and age, sulphation, etc. Lucky owners might have a midpoint terminal to check V and switch them around as they see fit, but really just prolonging the inevitable. This can be managed somewhat by backing off Abs V, but for some reason owners are reluctant to do that.

The result is usually failure from loss of electrolyte, high temp, sulphation, under and overcharge, and whatever else you can think of. Think I'm exaggerating? Nope, I've been there. It was an expensive nightmare never to be repeated again. I now run 48V in a single string. I can accept 2x strings if wanted for redundancy, and maybe 3x for the desperados. More is for the foolhardy, unless they're prepared to lightly load and charge them very conservatively. And have perfect wiring design, installation and maintenance. Rare to see this, good design wouldn't spec parallel strings.

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christern avatar image
christern answered ·

I pick up this old question to present some of my experiences. Someone who reads this at a later stage might be interested in the results.

I charged all my 6 batteries full and had them sitting on "storage" charging for one day. Disconnected them all and waited a couple of days before I measured their voltage. The difference was 0,02 V. This means that all of these 6 batteries got fully charged with no significant difference between them.

Looking back at the time the batteries were charging, even at storage (13,2V) there were some charging going on even if it was very low, 1-3 A.

Talked to a distant friend who has been teacher in electro (-something) at a university and he confirmed my view and said that the "rule" of not having more than X number of batteries in parallel is an old "truth" from the NiCd days. He also said that there are some major errors in the docs and they are based on the fact, if I understood him right, that the resistance in lead-acid batteries actually helps the charging to equalise between them also in bigger parallel banks due to that the resistance changes during charging. He also said that the experience I had that charging continues also on lower voltage charging where the batteries with the highest SOC are not charged and the ones with lower SOC takes the charging, and hence, equalises the bank.

He also said a lot of other things that I do not cover here but all supported having several batteries in parallel would not cause any issues.

I will not debate this further in this forum but write this to present another view within this thread.

I will also take the opportunity to thank the people who responded to this. At least it made me dig very deep into the subject.

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