OK…strap in as its a long post!
So I was in process of getting my Mavic Pro ready for sale and 2 batteries were dead…nothing…no lights…dead! Hardly used them so did a lot of Googling and went down the rabbit hole!
After cracking case open and checking cell voltages discovered it had discharged to 0V. Time to throw away…but…and here is the issue…they were practically unused…less than 1 cycle. So after a very slow charge on each cell got them upto 2V…disconnect and down they go again. Google time…everything points to bad cells, My thinking is there is no charge in them yet…so up I went to 3V…stayed stable. So now cell is taking a charge. Got pack upto 11ish volts and light up.
So I have a battery that can light up…but won’t turn on…Google time…you need to reset the permanent flag on battery controller…raspberry pi connected to battery and bit of software to reset the flag. Flag cleared and pack starts. Fixed!
Now the moan. DJI has set the pack to hibernate, but it will still discharge a couple of % per month. So that battery you bought new from DJI and left on the shelf for a couple of years is now Probably dead and out of warranty. Hunted ebay and found 3 like that…all repaired. Then found one that had very low cycles but dead. Similar problem with permanent flag, but this time triggered by PTC sensor…that DJI don’t use! Evidence of a single drop of water caused this, could have been condensation…but basically rendered a battery dead.
Disclaimer…do not even think about doing this unless you know what the hell you are doing. Also if any sign of swollen cells…just toss it. But there are batteries out there that can be fixed…from 0V! Think about it…the cells have 0V during manufacture. They are then activated…so as long as they are not worked hard they appear to go back to a deactivated state.
Sorry for long post…but batteries for Pro are few and far between.