I have a Li-ion battery pack 18650 cells with a BMS, 10 in series by 7 parallel. The pack I bought was defective from day one and had a 7 cell row that got hot when charging now I just repaired the defective row. Now a new problem occurred with overheating everything else. Since one row was defective I never had any problems with overheating but my BMS let me charge the pack anyways till one day it would not charge the pack anymore.

The motor is rated for 36 volts. 350 watts.

The controller I bought has an auto voltage mode 36v or 48v so I wonder if that had something to do with putting too much juice into my motor but not sure.

My ebike controller, wires, and motor overheat when climbing uphill then shuts down, this occurs when the pack is mostly fully charged.

Disproven: ( When the pack goes down to exactly 36 volt from discharging I get no overheating goes up hill as it should. When charged the pack reaches 40.2 volts and it heats up with that much voltage.) @35.3 volts it still overheats.

So now I'm trying to find a way to reduce the voltage when the voltage is over 36v. Any suggestions on preventing overheating?

  • \$\begingroup\$ Do you think your BMS could be defective? If it came with the same battery pack that had 7 faulty cells, I would suspect that as the potential issue. \$\endgroup\$
    – Jay
    Commented Aug 12, 2020 at 1:36
  • \$\begingroup\$ How to avoid the electric system overheating? Simple: turn it off and pedal. Question on the usage and repair of consumer products are off topic here. \$\endgroup\$ Commented Aug 12, 2020 at 2:40
  • \$\begingroup\$ You overcharged them significantly \$\endgroup\$ Commented Aug 12, 2020 at 2:51
  • \$\begingroup\$ The pack is 10 in series not 7 \$\endgroup\$ Commented Aug 12, 2020 at 4:53
  • \$\begingroup\$ Overheating uphill ... reduce current. That means reduce torque. Any number of ways to do that. Here's a few : slow down, lose weight, use a lower gear, assist by pedalling. \$\endgroup\$
    – user16324
    Commented Aug 12, 2020 at 11:42

2 Answers 2


Overheating is caused by overloading. You are using too much throttle on hills that are too steep. Or maybe the rider has an over-eating problem causing too much weight.

Maybe your battery is a modern lower voltage Li-FePo4 one but your charger is for an older higher voltage battery? 10 series Li-FeP04 cells must not be charged higher than 36.5V.

  • 1
    \$\begingroup\$ 7 Li-Ion cells should never see more than 4.2V until below 5% CC so 29.4V max. 36 V will kill them fast. \$\endgroup\$ Commented Aug 12, 2020 at 2:33
  • \$\begingroup\$ The pack is 10 in series not 7 \$\endgroup\$ Commented Aug 12, 2020 at 4:52
  • \$\begingroup\$ @Audioguru So if I use a charger that shuts off at a lower voltage that should charge the batteries fully but at a lower voltage or will that charge the batteries but resulting in less ah? Or both? The reason I got an ebike is to carry uphill my grocery shopping. It's not me that's overweight but the whole cargo. \$\endgroup\$ Commented Aug 12, 2020 at 21:09
  • \$\begingroup\$ We (and probably you) do not know if your battery cells are the old Li-Cobalt type that have a fully charged voltage of 4.20V or if they are the new Li-Ferrous type that have a fully charged voltage of 3.65V. Since you said they heat up when the pack reaches 40.2V (4.02V per cell) then they cannot be the old Li-Cobalt type that are cool when they reach their max voltage of 42.0V (4.20V per cell). \$\endgroup\$
    – Audioguru
    Commented Aug 13, 2020 at 12:08
  • \$\begingroup\$ Since the battery pack had 7 defective cells since the first day and now they heat up when charging then the cells must have VERY poor quality (cheap No-Name-Brand). \$\endgroup\$
    – Audioguru
    Commented Aug 13, 2020 at 12:17

Sounds like you damaged the battery charging at too high a rate. Causes heat and the resistance shoots up.

Eg. Tesla - their supercharging stations (80-120kW) damage their batteries , namely the net capacity. UC Riversides EE sept did a study on the Tesla battery and the rapid charging. After 25 rapid charges, the net capacity dropped 25% for good and would continue to decay. Faster you charge, the higher the resistance and heat.

Get a new charger, one that has a max of 36v but 24v. Igor be safer. Charge tine will be a little longer but you won’t damage the battery.

  • \$\begingroup\$ I wonder who came up with that Universal std for Supercharger. I thought it was the car that regulates demand charge. \$\endgroup\$ Commented Aug 12, 2020 at 14:11
  • \$\begingroup\$ When you make statistical statements please back them up with reference articles, authors, charts, etc. Copy Tesla articles and put them in quotes. Always note who the source is and maybe provide a URL link. Then you have a chance at upvotes. \$\endgroup\$
    – user105652
    Commented Aug 13, 2020 at 2:36

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