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.
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Ted, why would the rest of the pack discharge into the bad bank?
Also, why can’t you replace the 4 packs that went bad (as a parallel set in series with the rest of the pack)? The packs in series don’t need to be matched for Ri, just packs in parallel (or so I thought).
I really don’t know why the entire pack discharged into the bad bank. I’m still trying to figure that out.
As far as the replacement goes, that was the advice given me by some very experienced lipo guys. The feeling was that the new bank would basically “feed” the entire pack, since the IR would be less.
I won’t go up against lipo experts…but I’m guessing they mean you don’t want to add one parallel cell to each set. But if you go back to the original configuration and add one more 4s4p set (4x4s packs in parallel) it should be fine.
Lower Ri cells in parallel get over stressed because they provide more current than the higher Ri cells. But in series, all the packs have the exact same current.