Advice on Proposed Battery "Modification"

[mikeross]

Hi Everyone. I have a 48v 10ah LiFePO4 battery which, like many others who have posted here, did not last me to infinity (or beyond). This pack has powered my bike for the past five-seven years, and has been a good, reliable pack up until now. The pic below will show the problem - a cell that has puffed up (not sure what these cells are). After looking over the pricing on a new battery, the idea of converting this thing to a 36v battery came to me. My thought was to remove the top four cells (and the BMS) for a simple and inexpensive fix. Does anyone have any advice or a better idea? Would a new 12 cell BMS be a good idea, or can the one on the pack also be modified? Not sure why the images rotated on me...Thanks! Mike

[Hillhater]

Have you checked each individual cell to see if they are out of balance or which ones are dead ?
You are asuming only the top cells are the problem, but some of the others look like they may be swelling slightly also.
Even if your fix works, i doubt there is much life in the rest of the cells after 5-7 years use !
PS.. I believe those are "psylion" cells.

[amberwolf]

After doing the test above to be sure only those end cells are problematic, a "quick" way to test your idea is just disconnect those four balance wires, then move the main discharge wire down to the 12th cell, then run the bike, see how it goes.

[mikeross]

Hillhater and Amberwolf thanks for the great advice! The pack tripped the BMS on my last ride and the cell groups now test pretty close, which surprised me. This pack charges up to 58.x volts, but rapidly drops to 53.x volts, and gives me 2-3 ah before it trips the BMS. I'm going to charge the pack today while I can keep an eye on it and test the cells after it goes to 53.x. That should point out the bad cell(s). I did not know you could just disconnect the balance wires on individual cells and still have the BMS work - that is great!

The point about this pack just having been worn out is a good one (similar to its owner, haha). If I put about 2 cycles per week on the pack for say 6 years, that's about 600 cycles. Still, it might go a bit further...now it does not get nearly as much use.

Thanks again for the advice! Mike

[amberwolf]

I did not know you could just disconnect the balance wires on individual cells and still have the BMS work - that is great!

It doesnt' always...but it won't hurt to try at this point.

[mikeross]

Update. Been working on my "mod" today. Although one cell is obviously shot (see photo), the pack and individual cell voltages were surprising. Here is what I found:
Right after charging (not a full charge): pack voltage 56.9v, individual cells ranged from 3.54v to 3.44v, with the bad cell at 3.35v.
One hour after charge: pack voltage 53.7v, individual cells ranged from 3.35v to 3.37v, with the bad cell at 3.31v.

I expected the bad cell to be much worse than that.

The top four cells were then removed from the 48v pack (one cell definitely bad, maybe three spares). Then the positive and negative pack leads were moved to the new top level cells. Per amberwolf's advice, I removed the balance wires and just taped them up. The BMS appears to be working just fine. When this was done, all 12 remaining cells measured 3.35v. I cobbled the pack together for a short 3 mile test ride.

Per my cycle anaylst:
Start: 40.1v, 0ah
End: 39.5v, 1.2ah, 2.89 miles

Having experienced no flames or loud pops, I hooked the cobbled 36v pack to a 6 amp, 36v charger I had left over from another adventure. It is in my garage charging right now. I'll include some pics below.

Next step is to see how many ah I can get from the pack now. Hopefully, more than 2-3.

Mike

[dogman dan]

I expected the good cells to be much worse than that. But they are pretty bad. Screw around with it the rest of this year, but next spring, treat yourself to a new battery.

[mikeross]

Hi dogman dan. Ya, the pack is nearing the end. My whole bike took a beating on a fishing trip to Montana, and that certainly had some negative impact on this battery pack. It has been a really good pack, no complaints at all. But the bike is no longer used to get me to and from work every day. It is used for errands and "fun" trips on an irregular basis. This makes it harder to fork over a large chunk of money for a new pack. Guess I better start saving for one, though.

I'm glad to see you are still on the forum. When building my bike you were working on conquering a big hill near your house. Your posts were very helpful and entertaining. You know, it still makes me smile to ride the electric bike - it is just fun!

Mike