Combining dissimilar lead acid batteries

[Mark62]

Hello. I have 4 of the same make/model marine 12v sealed lead acid batteries. I started with 2 that I used for a few months, and then bought but never got around to installing the second two. So two of them are probably in better condition than the other two. I think that if I wire them all in parallel then the whole bank would only perform as well as the poorest quality battery, but that if wiring in series it doesn't matter as much. So, could I wire in series two pairs of an old and a new (two 24v pairs), and then wire those two pairs in parallel to avoid get the full capability of all of the batteries? That is, will the series-wired pairs of old and new avoid the degraded performance of wiring them all in parallel? Thanks.

 

[sunshine_eggo]

Hello. I have 4 of the same make/model marine 12v sealed lead acid batteries. I started with 2 that I used for a few months, and then bought but never got around to installing the second two. So two of them are probably in better condition than the other two. I think that if I wire them all in parallel then the whole bank would only perform as well as the poorest quality battery

Nope. You have it backwards. You would get to sum the approximate total capacity of all 4 batteries in 12V.

Let's say they're all 100Ah batteries. Old are really 90Ah due to age/use. You would have roughly 380Ah of capacity in 12V.

, but that if wiring in series it doesn't matter as much.

Matters far more. Series elements should be as close as possible in every way. Having a 90Ain in series with a 100Ah would definitely limit you to the 90Ah battery. Furthermore, since you're only charging at 24V, the individual 12V may not be in sync. At 28.8V peak charge, you might have one at 14.8 and the other at 14.0. Egon bad.

So, could I wire in series two pairs of an old and a new (two 24v pairs), and then wire those two pairs in parallel to avoid get the full capability of all of the batteries? That is, will the series-wired pairs of old and new avoid the degraded performance of wiring them all in parallel? Thanks.

Nope.

You should wire the new in series with new. Old in series with old. New 24V in parallel with old 24V. This way the "new" 24V 12V batteries would be well matched, and you'd get your 100Ah. The "old" 24V would have 90Ah, but are also well matched. You'd now have about 190Ah of 24V.

If you did your scenario, each 24V would be limited to 90Ah, and you'd only have 180Ah.

 

[Mark62]

Thank you. That's very helpful.

 

[sunshine_eggo]

Worth noting that it's important to connect batteries with best practices in Victron Wiring Unlimited in Resources section.