I have four lithium-ion APC batteries. They are all from the same facility and their condition seems to be alright.

I'd like to connect all of them together in parallel, but my question is this: Will the new batteries be used more than the old batteries, going towards an equilibrium? Or will the old batteries take and give more current causing some kind of run-away effect?

  • \$\begingroup\$ I have these graphs from some papers, but I don't know how to interpret them. everexceed.com/js/htmledit/kindeditor/attached/20211023/… researchgate.net/profile/Kuk-Cho-3/publication/312355070/figure/… \$\endgroup\$ Commented Nov 8, 2022 at 15:54
  • \$\begingroup\$ Are "these Lithium-Ion APC batteries" single cells or multi-cell packs? What's their voltage? \$\endgroup\$ Commented Nov 8, 2022 at 15:56
  • \$\begingroup\$ They are 13.4V so I think they're multiple lithium cells in series, but I'm planning to treat them like they are 4 13.4V cells. \$\endgroup\$ Commented Nov 8, 2022 at 16:01
  • \$\begingroup\$ If you try to draw more current than a single pack can provide, you may over-stress the newer batteries due to unequal current sharing (the new batteries will have lower esr). This won't permanently put them out of balance, since the older packs will recharge the newer ones once the current drops, but it still causes stress and potentially failure. \$\endgroup\$
    – Drew
    Commented Nov 8, 2022 at 17:28

2 Answers 2


This is not a good idea, the discharge curves will be unbalanced and the current through them will also. If your doing this for an APC product it could also throw off the charging circuit. I personally would not do this if the APC was operating in an area that I didn't want things to catch fire (like a house). If you just want to experiment then you could try it but not recommended

  • \$\begingroup\$ Yes, but will those unbalanced currents typically tend towards "aging" the newer batteries faster, such that in the long term they'll be the same "age", or is more likely the old batteries will be used the most, while the newer batteries stay new, thus causing the entire thing to be unstable? I inferred that newer batteries will draw more current based on increasing internal resistance with time, but I don't know how valid that is. \$\endgroup\$ Commented Nov 10, 2022 at 15:25
  • \$\begingroup\$ At minimum you need a bms to do the balancing. The other problem is the charger, what is it going to do when it sees double the current when charging? It's likely that it won't be able to handle double the current. \$\endgroup\$
    – Voltage Spike
    Commented Nov 10, 2022 at 15:56

In general, you should NOT connect batteries in parallel. In case of slightly different voltages, there might be a very large short-circuit current because of the low internal resistance.

In addition, do NOT experiment with Lithium batteries. In case something happens, the fire will be giant. Lithium is a metal that easily burns. In the old days, metals like lithium and magnesium were used as photographic flashlights. On youtube you will find many examples of horrific fires caused by lithium batteries.

Mixing old and new batteries is also a good one. In case you have a "safe" application, one bad battery, when starting to burn, will ignite all the other batteries.


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