Battery instantly drops from 41v to 14v when plugged into ESC

DIY Builds Battery Builds
#1

I was out riding this weekend when my board spontaneously shut off around 50% battery. I took the board back to my shop and attempted to diagnose.

The battery reads 41v but the minute I put a load on the ESC, the voltage drops to around 14v and stays there until I unplug the battery from the ESC, at which point the voltage jumps back up to 41v.

Here’s a quick video that shows the problem in more detail: https://drive.google.com/file/d/1_MYZ8M_kedkkDDFPSTJ5Mihzvjc_wT_9/view?usp=sharing

This battery pack uses NCR18650B cells which have a max discharge of 6.8A.

Is the BMS limiting the output voltage of my battery?

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30 mph cutout to superman
#2

What’s the voltage on all the p groups? Seems like there are some dead or unbalanced cells in there.

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#3

Probably not. This smells like you need a new battery

#5

I’m measuring 3.5V across all P groups.

#6

What about when connected to the ESC showing 14v total?

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#7

That’s most definitely bms surge protection triggered. (personal experience)

or are those NCR18650B internally protected? They dun look like button-top but are you sure those cells are not protected?

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#8

Are you using the stock evolve bms ? Because that would be the reason why

#9

Sounds like a bad solder joint to me. Something in your setup is bottlenecking the current draw. Can you meassure the voltage as close to the battery as you can when you plug it in into the ESC? (like before the BMS)

If voltage stays steady you got a problem somewhere along your power input. I sudgest inspecting solder and connector joints to find the culprit. Might be in the BMS like someone stated if its not a charge only BMS.

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#10

Is there a partially disconnected XT-90S somewhere in the mix?

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#11

Where are your multimeter leads connected in this video? Are they measuring the voltage before or after the BMS?

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#12

Multimeter leads are connected to the battery’s charge wires, which route through the BMS.

I stripped the battery down and inspected all nickel strip, solder joints, etc. and could not find any damage. I agree with @soketsu this looks like it’s caused by the BMS.

If it is the BMS, do I have to get a new one? Or is there another solution?

#13

Just bypass the bms for powering your board. Only use that bms for charging.

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