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Can I wire 2 lithium batteries of the same make but different amp hours in parallel? I need 300 Ah in my battery bank. Can I wire a 12 V 200 Ah lithium battery and a 12 V 100 Ah lithium battery in parallel?

Null
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Tess Gallagher
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    If both batteries have their own bms then likely yes. However I went with two identical batteries for my son’s system and a bit of spare capacity. – Solar Mike Nov 06 '21 at 06:10
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    I mean, maybe. But there could be problems. One thing I will say absolutely positively for sure do not try to put them in parallel unless they are at exactly the same voltage (say within 0.01 or even 0.005 V difference). If possible, ask the manufacturer if it is OK. – user57037 Nov 06 '21 at 08:01
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    These are LFP batteries, right? (aka lithium iron phosphare or LiFePO4)? – user57037 Nov 06 '21 at 08:02
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    @Solar Mike "If both batteries have their own bms then likely" NOT! When one of the BMSs turns back on later, there is a high current inrush between the high voltage battery and the low voltage battery, resulting in degradation at least, damage at worst. – Davide Andrea Nov 06 '21 at 19:27

2 Answers2

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It depends:

  • Cells permanently, directly in parallel: very good: the stronger one will help the weaker one
  • Cells in cell holders: very bad, because nothing prevents someone from placing a full cell in one holder, and an empty one in the other
  • Self protected 18650's directly in parallel: very bad, see the next point
  • Batteries (each with its own BMS) just the 2 power terminals: very bad: as soon as the protector BMS of one of them turns on, excessive current will flow from the higher voltage battery to the lower voltage battery, resulting in damage
  • Batteries with balance connectors not paralleled together: bad: each battery will need its own BMS plugged into the balance connector, and that's expensive, plus the problems listed in the point above
  • Batteries with their balance connectors directly paralleled together: bad: due to variations in internal cell resistance (which get worst with time), at the start and end of high battery current, the differences in cell voltages with cause an inrush of current through the balance connector, much higher than the small gauge wires can handle; they may melt
  • Batteries with their balance connectors directly paralleled together and each with its own BMS: very bad: as soon as one of the protector BMSs opens, current will flow through the balance connectors, and that battery is not protected.

Summary: either connect cells permanently, directly in parallel (except for protected 18650's), or find batteries that already have the larger capacity you want

Davide Andrea
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I wouldn't do that.

Parallelizing lithium primary or rechargeable batteries is dangerous unless you use dedicated chips and you know the risk you are taking.

Enrico Migliore
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