question

@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.

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.

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.

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.

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.