Bonka Battery Rebuild (step by step replacing a bad cell)

[jrsimple (Jarom)]

A year ago a battery fell off the seat in my car and landed on the floorboard on a small pebble that punctured one cell. Battery Leak (puncture) - source for a new cell? I discharged that battery and stored it in a fireproof box until I needed it (actually I didn’t notice until after I flew with it).

A month ago while I was flying I lost one cell in another battery pack. I didn’t notice until I had landed that the packs were extra hot (compensating for the lost voltage). I decided to rebuild the pack using a spare cell from my prior pack. I took some pictures along the way to do this quick write up.

Warning: We all know that LiPo’s can be dangerous, be careful! Never short the tabs of the same cell (i.e. use plastic not metal tools when manipulating the tabs while soldering).

1. Open the pack from the bottom
   

   

2. Remove tape and foam
   

   

3. Unsolder wires
   

   

4. Cut welded tab connector from bad cell (as close as possible to the bad cell)
   

   

5. Peel cell off pack (a little heat from a heat gun softens the tape adhesive)
   

   

6. Get your new cell and use prior solder to resolder wires (I used a high heat soldering iron to quickly solder without allowing too much heat to move into the cells)
   

   

7. Depending on the cell you are replacing, you may be able to fold the excess tab over to solder the tabs between cells together (using the solder already present). I had to use aluminum flux to solder one pair of connectors together. Make sure you solder the cells in reverse polarity (i.e. in series, positive to negative).
   

   

8. Use polyimide adhesive tape to layer protection between the tabs of each cell and tape it all together with the foam.

9. Use battery pack heat shrink to wrap it all back up.
   

   
   
   
   

10. Check the pack and cell voltages, re-balance the pack.

I sourced all my materials from Amazon:

1. Aluminum solder and flux: https://www.amazon.com/gp/product/B010UMSB9Y
2. Battery shrink wrap: https://www.amazon.com/gp/product/B07FXL9K1F
3. Polyimide Adhesive Tape: Amazon.com
4. Soldering Iron (really it is too big, but made quick work of the solder): Amazon.com

[tarifachris]

Wow - good job. Not an easy task to do.

[GliderPilot (Paul Butler)]

Thanks for sharing! I have repaired my batteries before but didn’t know you could buy solder that would stick to the aluminum tabs.

[grejen711 (Greg Jensen)]

Thank you for sharing this and taking detailed pics. I was looking to see the pack construction at the cabled end (top I guess) because I’m trying out a new mounting system. My packs are now carried inverted on an aluminum shelf so their mass (multiplied by G) is supported on tabbed end of the cells. I see there is a really nice layer of foam and tape there. The weight is spread out pretty well on my shelf so it seems I’ll be alright for sure with resting the packs on that end. I’ve flown it this way twice now and all seems well. Sure is quick and easy to mount the packs as a single brick.

[Pdwhite (Paul Whitehead)]

Clean setup. I like it.

[grejen711 (Greg Jensen)]

thank you
Need to tweak the cable ties or cut the cables shorter and solder on new plugs. The intention is to allow a second bank/brick of batteries hanging off those aluminum tubes. All the cables in the centre.
Ordered four more shipped to Canada but have not yet had it confirmed.