Old and new lead batteries in a bank?

[GTom]

Often heard that a single dead battery can kill the entire bank. Is there a way (balancer?) to keep older batteries, still in good condition, without paying with the life of the newer ones?

[Compass790]

Yes there are battery balancers. We have one that keeps them balanced volts wise but as to effect on life I'll leave to an expert to comment.

[john61ct]

Well "in the same bank" means used concurrently, which is what causes the weakest link to determine the health of the whole.

Keep the old ones in a separate bank, only connected while charging, and use a bank 1-2 switch (no Both) to feed your consumer loads.

Or recycle as Starters?

Not a great idea IMO, just sell off or scrap the older ones.

[GrowleyMonster]

Mixing old and new in parallel without some sort of BMS is a pretty bad idea. Mixing a new one into a series bank is not quite so bad. If you have a 12v system and use 12v batteries in parallel, then I recommend you don't use them in parallel. Connect them as two separate banks with an A/B switch is what I would do.

I have a 48v bank for propulsion, made up of 6v golf cart batteries in series. A year after installing, a "helper" drilled through the cover we were making and into the case of one battery. I tried plugging but the battery went bad and I replaced it with not only a different month, but a different brand. Careful monitoring has detected no problem. So far. SG matches, no load voltage matches. and even individually charging them with a 6v charger shows similar charge current and times. After discharging to 60% or 50% the readings are very consistent among the batteries. YMMV but careful matching can be okay in a series string if they aren't TOO dissimilar in age. 3 years later, still no problems.

Batteries of unequal performance in parallel can be a problem because all current can pass through just one battery. This can overheat the battery, leading to premature failure of the "good" battery. With FLA it is usually not a catastrophe. With some other types things can get noisy.

With FLA, one big bank is somewhat more efficient than two separate banks, due to Peukert effect. Peukert gives you more discharge capacity than the 20 hour rate capacity, at current below the 20 hour discharge rate. Less, at higher discharge current. So you get more bang for the buck by having a higher capacity bank, and one way this is done is to wire batteries in parallel. I'm not saying don't do this, but I am saying don't do this with a mix of batteries. For more capacity use two or more separate banks if you have differently aged or rated or branded batteries. The extra Peukert losses are not a total dealbreaker except at extremely high discharge currents.