Motor: satellite down gearing, 350 watts, rated @36 volts.
Pack: 40.2 volt 10 series by 7 parallel.

Lately the last 3 times I go up steep hill the battery wires, controller, and motor are hot to the touch, not burning but very warm and a heat sensor kills the power. As I sit half way up a hill waiting for it to cool off I have to disconnect the battery for it to reset. I always assist the uphill climb by pedaling as much as I can on seat not standing. I often check for the correct tire pressure. And my ebike is 100% throttle controlled but it's not easy modulating a throttle when you're pedaling vigorously.

This has only been occurring since I fixed my battery pack by replacing a defective parallel row 18650 batteries in my 70 battery pack. Since I fixed the pack my ebike has never been so torquey. I get the same top speed but it really throws you back when you floor it from zero like never before. The main reason I got this ebike was mainly due to living at the top of a hill.

I can't figure out what would be the best solution to fix this problem.

Any ideas?

  • 1
    This is more a topic for electronics.stackexcange, but I have a strong suspicion that your 40.2 volt battery is overloading the motor that has been rated for 36 volts.
    – ojs
    Jul 29, 2020 at 13:55

1 Answer 1


As one commenter suggested, the issue is likely not that the voltage is incorrect, as a 36V battery means 10 typical Li-Ion cells in series, which will have a fully charged voltage of 42V.

It is possible that your battery pack had a BMS or some kind of current limiter or fuse system that prevented too much current from being drawn, or that some of the motor controller settings, such as current limiting were changed.

It is also possible that when you modified the battery pack you used a different type of 18650 cell that those originally present in the pack, or used a different type of conductor to add those cells to the pack, which has a different amount of resistance.

To debug I would first suggest using a current and voltage meter to measure the power output and current draw to ensure that your usage is below 350W, and then possibly adjusting the settings on your motor controller.

  • 1
    If OP has used different 18650s or a different mah from original then eventually his battery pack will fail anyway. It won’t evenly charge the pack and it will discharge uneven also. OP should at least at minimum check all cells are the same or at best make sure the mah / capacity matches across all cells in the pack.
    – Dan K
    Jul 30, 2020 at 7:49
  • @Dan K The pack has a BMS, that assures that all the batteries would be in the same voltage rage. I replaced the row with less capacity cells, it's all I could find that were already had a welded connectors strip welded on them. I'll let you know when that will fail. Jul 30, 2020 at 14:18
  • @Syntactic Garbage You make me think of something about adjusting the controller pack. It has a learning mode, maybe I should reset that mode, it might regulate the voltage better. Maybe the original pack had such a different power profile enough to require a reset. Jul 30, 2020 at 14:24
  • @Eric Huelin although the BMS is supposed to balance each parallel section of batteries to the correct voltage you can’t balance them as you used lower mah batteries on one section. Without them being balanced you will have issues further down the road. When you replaced the cells I take it you attached any balance wires to the appropriate cells for the BMS to detect voltage whilst charging?
    – Dan K
    Jul 30, 2020 at 15:04
  • @DanK since the entire parallel row was replaced, and the new cells were balanced on installation, as long as the BMS is wired properly it shouldn't matter too much for charging. What I would be more worried about is discharge--those cells will die first and I'm not sure how the BMS will handle that. Jul 30, 2020 at 20:16

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