Hello, glad that I found this thread - Im trying to save my Dyson.
I have V8. It stopped working - dead battery after 5y of use.
I took it apart and one cell vas 3,8V instead of 4,2V. Ok then lets change the cell. MF! Dyson use some 20700 cells 3000mAh 10C (as far I could find info),
I had some new Sony VCT6 18650 cell so I soldered it as a replacement of the 20700 with little bit of 3M tape help. Dirty job but whatever.
However... It does not work suprisingly. The same problem. After 1 minute runtime indicator shows a flat battery. It seems to charge it somehow.
Im not sure how to continue.
I will try to replace the firmware in BMS. Its unusable so I can not brick it more