Reviving a dead LiFePo4 cell (Hi-Power, 40Ah)
Well, the BMS issue was finally sorted (a different thread, turned out the main controller was dead). Have been charging for a while now, but we've got some cells that REFUSE to come up to voltage, and force a long balance. Without cooling, the BMS's cells report high temp from the resistors and cut the current, so the balance is abysmally slow (this'll be improved when we make the new box with integrated fan and heater).
I think, as a result of the slow balance, the BMS was repeating the balance cycle over and over. I've heard that it can be bad to hold the LiFePo4s at 3.65, but most sit at ~3.35, and none higher than 3.41 when not charging, so that sound good at least.
However, back when the BMS was being troublesome, all the modules ended up trying to balance the cells to 2V, and managed to drag three cells (each "cell" is actually three 40Ah cells in parallel) to 1.4, 1.7, and 1.6-ish volts. I removed these cells and individually charged them with an RC charger, unfortunately without a LiFePo setting, so using a LiPo setting. They got to about 3.3, and held it. However, while two of them might be alright, we're currently troubleshooting a super-low full throttle current (125A out of 500A), and while monitoring the cells, we've discovered the #3 cell (on the bottom layer, almost farthest up, of course) has decided to be a jerk. This leads the car to having a top speed of, oh, about 25MPH.
Basically, as soon as the draw from the pack exceeds 100A, the #3 cell drops so low, the cell module turns off and kills communication for the rest of the pack. This means it drops below 2 volts.
So, does a cell that holds 3.3v static, then falls when a load is applied mean it's truly, completely dead? Or is there another way it can be recovered? We can get three new identical cells, at a cost of ~$160, but if these can be revived, that's a much better choice.
Last edited by hydrochloric; 02-28-2012 at 09:14 PM.
|
Linear Mode
