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.
Hi Sparkyws,
From 0v upto around 1V just used a lab power supply set at constant current. Started at 5mA then upped it to 100mA. During this phase I kept cycling supply to see the effects of charge on a 0V lipo…yep never charge a 0V lipo. In this initial phase the voltage was decaying away rapidly with no load…points to damaged cell as everyone rightly points out. However my counter argument (to myself) is the voltage decays because the cell has no charge…its just behaving like a parallel plate capacitor. Also…what damaged the cell? Damage occurs during high charge or discharge where heat causes issues and can lead to thermal runaway. The batteries I’m playing with have had no thermal stress…they just self discharged at the 5% per month.
Next phase was to get some real current into them…but again not too much 500mA seems a sweet spot between charging and my impatience! Once all cells are balanced, and battery management reset all looks good voltage wise.
I have no accurate way of measuring internal resistance as all charging via the smallest balance cable DJI could have fitted. Battery performed OK in a low hover(kept an eye on temps and cell voltages) Tried a stress test of holding drone and applying full power…cell voltages dropped and DJI alarmed…not sure if a proper battery does that. Did an outdoor hover and est time was 25 mins and tracked cell voltages down…they all discharged at same rate…normal for a lipo.
Next step is to do a comparison between know and “repaired” flight time under hover on a calm day. Also need to know what maximum current draw a mavic pro pulls under full load…so I can bench test full load performance.
But your original statement is correct…a lipo that gets discharged to 0V is dead…but internet very sketchy about self discharging lipos when new.
I have revived a few dead dji batts that did not light up due to long time storage. One I got recently was a brand new Mavic pro one that was never charged that was dead. The 3 cells were like 0.01 volts each , so pretty much dead. It seems the DJI batts and the ATL company that makes them must use really high quality cells. I think the key to having any chance reviving these is to charge each cell at the slowest rate you can at the start. I normally charge them at 100ma with the nicad settings and if the voltage starts to slowly climb, its normally a good sign. I wait until the cell reaches 3 volts and then carry on charging on the lipo setting until it reaches storage setting. I do that on all 3 cells and then carry on charging the cells with a balance lead at 1amp to full and test. This latest batt works just like new with full flight time and cells even volatge at the end and showed up as 0 cycles, so was defiantly unused. For sure as others have said if batt is swollen, then cells are toast. Same as if when you start charging and batt does not start climbing, cell is gone. You can get new cells from Aliexpress when they offer shipping to uk, but the cells they cell are not of same quality as the DJI ATL originals. Also on a side note when you press the batt button to check storage, bear in mind it might say 3 leds, but once you connect it to charge the leds will drop and many times I take a batt with 3 flashing led’s that i have in storage and once charging starts it drops to 1 or 2 led flashes. So I top up charge all my dji batts every 2 or 3 months. As Mike rightly pointed out you will also need to reset the battery fail flags or you wont be able to normally charge and turn on battery. This does not work on every dead batt though, had many where 1 or more cells are long gone and will need new cells.
