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I built myself an e-bike from a kit bought on Amazon. It works great but I have a question regarding batteries for which I can't find an answer online.

I found a great deal on 4.4 Ah (36V) batteries usually for small electric scooters. Since 4.4 Ah is not enough (same with their max current of 16 A), I put 2 packs in parallel. I am aware that there are drawbacks, that my batteries won't last as many cycles as one 8.8 Ah pack but for the price I got them, I simply don't care.

I hooked them up after equalizing their voltages and charge them up in parallel. There is no problem with that. I eventually reach 41.8 V and batteries don't get hot at all while charging.

My question is about discharging. Whenever I reach my destination, one of the two batteries (always the same) is hot to the touch (not burning, simply hot) while the other one isn't. Why is that?

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Batteries must be match in V, mAh and ESR.

This mismatch results in a hotspot. The cheaper batteries are likely rejects from excessive mismatch. A load test can compare full charged cells.

Also the capacity must be matched since the lowest capacity rises in ESR first so the stronger battery gets more load.

The best solution is to have extras and sort them by Ah and ESR then choose a cutoff voltage before one or the other rises in ESR from undercharge.

Next is put on 2 thermal sensors like a car water temperature but with an alarm.

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  • \$\begingroup\$ If I understand correctly, the "bad" battery rises in ESR first and that rise in resistance is what causes more heat dissipation (Joule effect)? I don't have extras around so that solution isn't an option unfortunately. Anyway, this is more of a curiosity thing than trying to fix it. Thank you. \$\endgroup\$ – user126881 Jan 23 at 20:08
  • \$\begingroup\$ The bad string rises in ESR so the good battery gets warmer I^2ESR but if the better string (lower ESR overall) but has 1 bad cell with higher ESR in sereies with much better ones, the bad cell is the one getting hotter, so it depends on start of charge and ESR \$\endgroup\$ – Sunnyskyguy EE75 Jan 23 at 20:10
  • \$\begingroup\$ From what I've read, a bad cell causes heat during charging because of overvoltage which I don't see. I had it the other way around with I^2*R. Anyway, I think my question has been answered. It's a matter of internal resistance mismatch probably caused by a mismatch in capacity. Thank you! \$\endgroup\$ – user126881 Jan 23 at 20:15
  • \$\begingroup\$ mismatch is a process tolerance due to foil acid etching, material content and contaminants \$\endgroup\$ – Sunnyskyguy EE75 Jan 23 at 20:30
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Your two batteries are not exactly the same. Even though you equalize their voltages before charging, they do not stay equalized. So, one of the batteries is supplying most of the current and getting hot.

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  • \$\begingroup\$ I equalized their voltage before hooking them up, not everytime I recharge. Shouldn't their voltage stay exactly the same as they are in parallel? Why would one battery supply more current than the other? \$\endgroup\$ – user126881 Jan 23 at 20:01
  • \$\begingroup\$ @user126881 because the batteries are "not exactly the same" as stated in the answer - if the internal resistance is different then the batteries behave differently, perhaps a controller that deals with two batteries may be a possibility, but that is not how your original system is designed... \$\endgroup\$ – Solar Mike Jan 23 at 20:06
  • \$\begingroup\$ Yes, their voltage will stay the same if they are in parallel but that doesn't mean they both provide the same current. There are other relevant parameters, such as the internal resistance of the connections, the physical area of the plates, etc. \$\endgroup\$ – Elliot Alderson Jan 23 at 20:07

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