Mercane 8.8Ah pack repair

This all started back at this post. However I think that this time I've nailed it, mainly because unlike last times I've identified a very significant physical factor: a rusted through connection bus strip across the postives of one parallel bundle of cells (yes, the one that was down on that last BMS test).

I found it (and as you can see it was buried beneath the paper where the BMS sat) while trying to identify which set of physical cells that wire loom (just to the right of the picture) connected to, as I'd been making my voltage measurements off that. Its hidden under the paper behind the BMS as seen in this picture:

(featuring a Reddit Approved Kitchen Utensil as a tool).

Anyway, the voltage measurements at the pack didn't marry up with what the BMS was seeing, so I dug in to see. When I peeled it back I saw a big scab of rust across the cells, which basically just crumbled away when I hit it with the vacuum cleaner.

So essentially this leaves the last three cells unconnected to the BMS and so only one cell was getting charged and supplying power.

I cleaned it up with the wire wheel on my Dremel tool ...

which showed me that I just needed to bridge that set of cells and I'll be right.

Now lacking a spot welder I pondered how to best do this, and decided that because it was the positive ends I could prepare that lone cap with some solder if I worked fast, because the top of that cap is not directly connected to the electrode. So if I worked the right way I could solder with minimal damage. The cells left and right of it I could solder onto the nickel (hah ... fucking nickel my arse) strip.

First however I gave them a balance charge with my iMax and then soldered two strands of copper ~1mm in diameter across those three cells.

I charged it till it was only pulling 0.1A and took it for a 5km ride to test it.  All went well, so over night I charged it again. This morning when I checked the pack it was at 54.6V, with 0.00Amps going in to it. I thought that was very encouraging and ideal at about 4.2V per cell (bundle).

So maiden voyage on the repaired pack was one of my standard test routes.

which is an excellent result and compares very well to what I got before I put in the new BMS and just balance charged the pack by hand (hoping that would fix the issue, but it didn't):

Comparing those two results in more specific details:

shows that the pack has a bit more pep in places but what is really significant is the voltages were way up when I got back, suggesting I'm going to have increased range (as well as I'm sure the cell balances will be better with the new BMS).

This is the voltage upon return of that trip (and an additional little 2km victory lap I did). Further as I found in my pull down and mentioned in that post, the balance charging is doing much better with the new BMS than the old one:

the blue columns are the result of a "full charge"with the old BMS, which clearly wasn't balancing anywhere near as consistently as the new one. Probably equally important is that my testing to BMS shutdown yesterday (which prompted this new pull down) gave this result:

V discharged
4.03
4.04
4.03
4.03
4.03
4.03
4.03
4.03
4.03
4.04
4.2
3.66
4.03
4.02
0.12
50.7

which:

1. shows how bloody well balanced the cells were after initial "full charge" levels are shaved off by a few km riding and
2. is what led me to to this pull down to investigate why the BMS shutdown was happening and why that cell was low.

Discussion and Conclusion

I think that this time I've got it nailed, and unless there are other cells in this condition the only reason I'll need to pull the pack down again is if the solder joints I put on need repairing (which will only happen if the solder melts because the copper heats up under power: meaning my calculations on the required diameter are wrong).

Its pretty clear that this pack has failed (dead pack walking) by catching it early I've minimised cell harm and saved money. because of improper assembly and cheaping out on materials. I'd speculate that in the Chinese Sweat Shop where this was assembled the worker involved dripped some sweat right there on that part of the bus connector and started this rust process right then and there, before the pack was even sealed. I've seen the impact of a single drop of sweat on a circuit board before (the salt in sweat just helps electrolysis and oxidation reactions go nuts). Had they used decent nickel this wouldn't have happened.

I don't believe that there are other cells with suspect (rusty) joints because I can see all those other joints and this one was the only one hidden by the paper under the BMS.

This highlights the importance of pack construction being at least as important as cell choice. For even if these were LG cells and even if said LG cells were better, it would still fail because the assembly of the pack was flawed.

I'll do some range testing and perhaps do another pull down to check again.

Basically for

• my time, and 
• 2c worth of solder and scrap copper wire

I've saved a $250 pack which I anticipate will go on to last another year (its already lasted nearly one).

I hope this is also helpful to someone else.