Cell failure and replacement

It's been awhile since my last post - nothing of importance to note. Electro has been working great, although in the last couple of months I have driven the car pretty much zero time because of the really cold weather (no heater in the EV). So, Electro has been quietly hibernating in the garage, waiting for Spring. While working in the garage one day, I heard the BMS audible alarm go off. Thinking it was an open in one of the signal wires (remember, the cells are all wired in series back to the BMS, and an open in that chain signifies a problem from one of the cell modules.

So, this Saturday I went into debug mode, tracing down the cause of the open. I found it in one of the rear cells. Remember, the cell monitors show green when all is well with the cell voltage, red when the cell voltage is out of range, and nothing if the cell voltage is drastically low. Look at the middle cell in this image:

Something is up with that cell. So I measure the voltage across it:

1.3V? Nominal is 3.1-3.4V. The cell is clearly in trouble. When I picked up this collection of cells back in February, I happened to pick up an extra cell that has just been sitting in my garage cabinet collecting dust. I went to measure the voltage across it, and was pleasantly surprised that the cell is still at 3.1V! So I went about extracting and replacing the problem cell. Disconnecting the signal wiring and straps was a snap, and by partially inserting the lock down bolts, I could pull the offending cell out vertically, and drop in the replacement without too much fuss.

After putting in the replacement, the cell monitors installed all worked fine. I did a cursory check of the remaining bolt down connections, and buttoned the whole pack up. After resetting the BMS, the audible alarm stopped, which means all is well with all cell monitors.

It was handy having a replacement cell on hand for this exercise. I will be contacting the folks at EV Components (now at Current EV Tech) about sending this problem cell back (they probably would like it back) and about purchasing another spare or two.