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I have three 3.7 V li-ion batteries 4 Ah each. I need total voltage to reach 8.4 V at full charge, and use the higher capacity of the one left battery.

Is it safe if I connected two of them in parallel then connected the 3rd one in series with them?

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2 Answers 2

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There is nearly no interest in doing so, and a lot of risks:

Best case: you have a good BMS and a good charger. Once you have drawn 4 Ah, the battery that is alone is empty, and the BMS shut down your load. So you got 4 Ah in a 2S configuration (i.e. 2 batteries in series). Exactly the same as if you used just 2 batteries in series. The only small difference being that the 2 batteries in parallel will discharge less, so their voltage will drop a little bit less. So you might gain half a volt or so when fully discharged.

Non ideal case (quite possible):

  • if your BMS isn't monitoring (well) individual cells: the total voltage will remain high enough to continue discharging (because the parallel cells are still 50% full), but you will start deep discharge on the single cell, destroying it
  • your charger just charges the full string, without active balancing (or with limited balancing, which is enough for a normal battery): due to the difference between the cells, it's likely to either not charge fully part of the battery, or to overcharge the other.

And don't forget that an overcharged or deeply discharged lithium battery can catch fire. So with minimal gain and huge risks, I would strongly discourage you to do so.

If you want all of your capacity, put all 3 in series and add a buck converter, or all 3 in parallel with a boost converter : it will be safer and more efficient.

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  • \$\begingroup\$ It's exactly the same problem: 2 (strictly identical) 4Ah batteries in parallel or one 8Ah is the same (nb you can have some additional problems with 2 4Ah batteries in parallel if not well matched)). So no, it is not OK to have a 4Ah and a 8Ah battery in series. When putting batteries(or blocs of batteries) in series, they have to be nearly perfectly matched AND actively balanced. The only thing you might do with different batteries (provided same technology and same nominal voltage) is to put them in parallel. NB: there are several constraints still, ask a new question about it if interested \$\endgroup\$
    – Sandro
    Feb 2 at 11:05
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No, you will seriously unbalance them when you charge and discharge your pack.

Find another solution, using either all three in series with BMS or all three in parallel.

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