Li-Ion 6S battery BMS blocked after deep discharge — how to recover?

Build, Repair & Mod
1

Hi all,

I have two Pilotix 12000mAh 6S Li-Ion batteries that I use for surf foil assist. During a session I accidentally only connected one battery instead of both in series, causing that single battery to power the entire system and become deeply discharged.

The battery shows 0V on the main XT60 connector, but when I measure via the balance connector I get approximately 4.03V per cell — so the cells themselves seem fine. I think the BMS has gone into protection mode.

My charger is an RC Plus Duo Cube 80, which has a Battery Meter Testing function (which confirmed the cell voltages) but doesn’t seem to have a recovery/boost mode.

Has anyone recovered a blocked BMS on a similar Li-Ion pack? Is there a way to reset it, or do I need a specific charger with a recovery function?

Any help appreciated!

2

I don’t think that those batteries would have a BMS in them. The xt60 would be a direct connection to the cells. Maybe an internal fuse in side the pack?

2 Likes
3

If you have balance output then there is unlikely to be BMS inside.
But then the fuse is the most likely problem - as without BMS there is usually some protection needed and a fuse is the most straightforward possibility.

But … I have checked the battery pack - there is no fuse in specs.
You should have straightforward connection - so some balance cabling should be connected directly to power lines. Have you checked which connection is missing ?
Zrzut ekranu z 2026-08-24 12-42-25

1 Like
4

This doesn’t really make sense. If the batteries are in series, not connecting one would result in no powering of the system at all.

More sounds like the batteries are in parallell?

1 Like
5

Thanks for the suggestions! I measured the individual cells via the balance connector and found the following:

  • Cells 1-5: ~4.0V each ✓
  • Cell 6 (pins 6-7): 0V ✗

So cell 6 is dead. I also noticed a very faint burning smell when I first opened the waterproof housing shortly after the incident, though it has since dissipated and the cell shows no visible swelling.

It seems the cell was damaged by deep discharge during the session. I’ll probably just replace the battery pack rather than attempt a cell replacement.

Does anyone have experience replacing a single 21700 cell in a pack like this? Is it worth attempting or should I just get a new pack?

Thanks!

6

As long as you have spot welder and soldering iron it is pretty simple. You will also need shrink tubes.
Usually disassembly is the trickiest part - in cases it was glued.

1 Like