# Balancing a 48V battery bank built on multiple series-strings of batteries

I have 12 identical AGM batteries, however they have been used in different locations so the age and wear may vary slightly (but not much) between each battery. They have also been sitting around for around 3-4 months without any topping charge. So they were at different states of charge to begin with.

They are now connected in 3 strings of 4 in series, then those 3 strings are paralleled at the far ends of each string (0V and 48V are paralleled, but NO parallel connects at 12, 24, 36V). I have connected my Multiplus II to the battery bank, and as I expected, some of the batteries charge to 14.40V sooner than others that linger at 14.00V.

Do I need a battery balancer for each string? Some guides say to make parallel connections across the series strings. However there are mixed opinions about whether making parallel connections like that is a good idea because it can imbalance the load put on each string. There is also a safety risk about it because if something happens, like an internal battery/cell short, current can rush from one battery to another without any fuse (with my setup, each string is fused individually and a faulty string should in theory blow a fuse). And adding parallell connections with fuses is both messy and expensive. Also, paralleling batteries and connecting at the opposing diagonal terminals, doesn't balance correctly with more than 2 batteries.

So how do people normally deal with balancing battery banks where there are more than 2 sets of batteries in parallel? Is one balancer for each string the only way to go? I might add more batteries later if I get hold of more of them.

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Matthias Lange - DE answered ·

First you should charge each battery individually with a 12V charger to have all at the same level.

I think you should balance each string -> you need 9 balancer.
https://www.victronenergy.com/battery-monitors/battery-balancer

If all batteries are used individually before, all of them aged differently and I think this setup will not last very long before it starts to make trouble.

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