Cogging and unstable battery voltage measurement

Hello,

 

I bought a Flipsky FSESC 6.6 60A more than a year ago, but I began to use it on an eBike this week (with a 1500W wheelhub motor).
 

The motor has a lot of cogging even with the auto calibration, and first of all I noticed that the battery voltage measurement is not stable at all, even when stopped the voltage can oscillate very quickly between 47 and 51V (12s 20Ah 12C Li-po battery).

 

You can see the problem here : https://youtu.be/trGr6Sw6v0E

How can I solve this problem, what could be the cause?

 

Thanks a lot for your help