Ks14b over voltage

[Xoltri]

Hey everyone,

Having a problem with a friend's daughter's KS 14B.  It is not reporting the battery voltage correctly and thinks it is overcharged, causing it to not balance or rock back and forth.  I've tested the battery voltage with a multimeter and it is currently at 65.3v.  Each cell tests at 4.08v or so, which is fine.  However, through the app the wheel is reporting it as 67.6v. The wheel only has the problem when fully charged, and research online says that the behavior of the wheel rocking back and forth indicates that the wheel thinks it is overcharged.  My charger is confirmed to output 67.2v.

 

So I talked to Jason at ewheels and he says they no longer make parts for this wheel, so I am wondering what I need to change on the motherboard to fix the problem.  Anyone have any ideas?  I am competent at soldering but not in determining what to replace.

[Rywokast]

I would think that would be something on the bms which is reporting incorrectly to the board?

Edited July 25, 2020 by Rywokast

[Chriull]

On 7/24/2020 at 1:56 AM, Xoltri said:

Hey everyone,

Having a problem with a friend's daughter's KS 14B.  It is not reporting the battery voltage correctly and thinks it is overcharged, causing it to not balance or rock back and forth.  I've tested the battery voltage with a multimeter and it is currently at 65.3v.  Each cell tests at 4.08v or so, which is fine.  However, through the app the wheel is reporting it as 67.6v.

That's "just" an deviation of 4% - so 2% per measurement device. Nothing too uncommon.

What's the specs of your DMM? Although on board voltage measurements are more often reported to be off. Afair especially with the ks14d - maybe they had some "bad pbatches". 

On 7/24/2020 at 1:56 AM, Xoltri said:

My charger is confirmed to output 67.2v.

With the same DMM?

On 7/24/2020 at 1:56 AM, Xoltri said:

so I am wondering what I need to change on the motherboard to fix the problem.  Anyone have any ideas?  I am competent at soldering but not in determining what to replace.

There should be a voltage divider after the battery supply bringing the voltage down to the right range for the uC's (stm32...) ADC.

If you can identify this voltage divider on the board you could calibrate the voltage measured by the firmware by exchanging a resistor (or soldering a second one on top)...

Edited July 28, 2020 by Chriull

[Xoltri]

Well, based on the behavior of the wheel I don't think it's an issue with the calibration of the multimeter, and I have read that others have had this issue with this wheel also.  At 4.08v each cell in the pack is only 85% charged, but the wheel is seeing 67.6v (max is 67.2) hence the rocking back and forth or not balancing behavior.  Once I am able to get the wheel to balance and then carefully ride it around for a short while it starts behaving normally.

I will try and see if I can figure out the voltage divider, thanks for the recommendation. In addition to the incorrect sensing of over voltage on a full battery, I believe it has also caused a cutout as the wheel did not warn of a low battery and it caused the BMS to shut off the pack.