Here’s the short version of what happened to my pack recently. I frankly had no idea this was possible, yet, from the response of a few friends with considerable lipo experience, it is painfully possible, if not common.
The bottom line. One bad cell can cause a small cascade failure of a pack. That failure can cause the larger cascade of any packs paralleled to it. If allowed to progress, it can bury your entire pack.
Here’s how my pack is wired.
I just happened to notice my pack voltage was extremely low one day, while the bike was sitting in the garage, and started charging it immediately. Three of the banks of paralleled packs responded normally. The fourth was kind of funny, and the fifth kept dumping voltage. It would charge, then immediately drop down to about 2V per cell (group). I pulled the pack apart, and charged each individual pack. All the packs were fine, took charges normally, and held their charge, except for the 4 packs in that one wonky bank. They were all toast.
What happened? It would seem that I had a cell failure in one of the packs in the wonky bank. Because paralleled cells want to balance normally, when that cell shorted, all the other cells started discharging into it. They then dropped below salvageable voltage, and the packs were toast. At that point, the rest of the pack started discharging into that bad bank, and I came close to losing the whole 74V 20Ah pack.
How to keep this from happening? Well, testing the packs, for one… running them through cycles, making sure they all work normally, and, I’d add a step I didn’t before, testing them and cycling them when they’re paralleled in banks. Also, test and match the internal resistance of each pack with it’s buddies in it’s bank.
Part two. Get a cell-level monitor. Either a fancy cell-log (only $30 or so from Hobby King) or a simple one (at $6 each), but something to keep an active eye on each bank of cells, and alarm you if you have a problem.
The end result?
I pulled the bad packs, in the bad bank, out. I rebuilt the pack using 3, rather than 4 packs in parallel, giving me a 15Ah 74V pack, which I’m dubbing the “MicroPack”. I’m adding cell loggers.
I can’t just add new 4s packs to build it back up to the same Ah rating. They won’t have the same internal resistance, and will be over-taxed as a result. I can either start over with all-new packs, or live with one smaller capacity pack with what I have left. I can’t build a second “MicroPack”, either, and put it in parallel with my first pack, again, because it won’t match.
The good news is, with the MicroPack the bike still is scalded-cat quick… with a little more voltage sag than I’d like, but still fun. …but a very expensive lesson learned.
Update: I’m not at all convinced that the bad bank caused the discharge of the entire pack. Through some discussion on ElMoto, I’m reminded that the controller was on, though the contactor was not… so there had to be some current draw. If that’s the case, the pack voltage dropped, probably causing the bad bank to fail. Stay tuned.