I'm having the same problem. I've been Googling for the past 3 days and haven't really found any solution other than to buy new
batteries. I was able to open up one of my
batteries and manually charge it with a bench-top power supply to 12.6V (4.2V/cell) @ 0.125A (low current to reduce risk of fire/explosion). The
battery is holding a charge which is good indication the cells are not totally ruined from over-discharging (albeit, likely has degraded capacity). Despite returning the pack to a stable state, the charging hub and USB air craft charger refuse to charge the
battery. When inserted in the air craft, I can press once, press and hold for 2 seconds to turn the
battery on, but the air craft will not power up. I did some more digging and discovered the
battery management IC is a Texas Instruments part number (BQ9003). I couldn't find a datasheet anywhere for this IC. I believe the
battery management IC sets a persistent error flag when over-discharged which renders the
battery useless. To reset this flag would require an SMBus adapter and software, similar to what is shown in the two videos linked below. Cheapest I could find a TI EV2300 HPA002 pod was for $100 on eBay ($120 for two new
battery packs on Amazon...).
Battery Mavic Air Repair (Remove Permanent Fail):
Battery Phantom 4 Repair (BQ30Z55) - Romanian Lang:
(This one is in Romanian, but you can auto-translate captions to English)
Here is a good summary of the issue:
Few week ago, my brother-in-law passed me a DJI Spark to play. The unit has not been used for 2 over years so the natural thing to do is to charge all the
batteries up before exploring further. I left the drone
batteries charging while heading out for an errand but a quick test at night...
www.uncle.la
Disclaimer: I do NOT recommend anyone attempt to manually recharge an over-discharged LiPo
battery without first knowing what you are doing. There is a real risk of fire and/or explosion so precautions must be taken.