Wednesday, July 22, 2009

As bad as it gets, or is it?

Preface:

A few weeks I was driving to work and looked down at the TS BMS system for kWh. The reading was FAR lower than normal for being halfway to work. My first though was, "crap it didn't charge and I'm halfway between home and work". I decided to keep going and keep light on the throttle. I made it to work with power to spare. It turned out the pack was completely charged when I left home, but the BMS failed to recognize most of the energy that was charged into the pack.

The Post:
It's been about a week ago, and I was just pulling out from work to head home. I noticed the computer again didn't register that it had charged. I assumed it was an error and headed home. Sadly this time it wasn't a false alarm. I drove about 32 of my 35 mile trip home when the BMS started yelling about a low cell. Even more sadly, this didn't alarm me since the computer is always giving me false positives. Regardless I started watching the cells and noticed all of them were a bit low (2.8v on average) with the lowest one being 2.5v or so. As I continued to drive they all started dropping, but one more so than others. I could see it going 2.5, maybe 5 seconds goes by, 2.4v, etc. It didn't take any time to reach zero volts under load. At this point I figured I've lost a cell, but the other were still in the safe zone.

I got home and after turning my charger amps down to 2 began charging. It took a few hours until the cell was up to 2.8v and I cranked it back up to 30 amps and let it charge.

The next day I was hesitant to even try to make it to work, but I figured this is half the fun of owning an EV. I was surprised to find out that not only did I make it to work but that cell was in the top 5 or so under load meaning the internal resistance was not changed and it's still behaving pre-incident.

How much life, if any has been taken away from this cell? No clue and only time will tell if fails before the others. However, and most importantly, this shows these cells are able to take a lot more abuse than I figured.

3 comments:

Gavin said...

Hmmm. Interesting (and worrying). Best of luck in the future with that BMS mate.
-Gav

Solar Saturn SG..sin gas said...

Brian,

If you were to do it over again,What would you use for a BMS coupled with your Voltblocker.

Terry

Brian said...

My current plan at the moment is to remove the TS BMS. Install a PakTracker which will give me the cell voltages, amps, total voltage that the old BMS did. It will also add history tracking through the serial output. I think this will be good to track a trip to work, and then check it every month or so and see if I see any trends around cell life. I'll then use my new version of the VoltBlocher to trigger high voltage and backoff my charger. I'm still debating if I'll replace this huge TS charger or not.