OK, after about a month of driving without any charges, the IMA light came back on indicating deteriorated battery module again.
I had two modules in the garage, so next step was to find the weakest two and replace them. Most guys take the individual sticks out and test them separately, but that would take like a week for me and I don't want to down the car for that long. So how to find the weakest modules quickly?
So, I came up with this plan: Do a full equalizing charge first, then drain the pack and see which stick runs out first. The ones that run out first have the lowest capacity. Charging was easy since I had that already wired up. Dishcharging shouldn't be too hard either. Normally if I just let my wife drive the car, it comes back near zero. Well, I didn't want to wait for her either, so I drove the car up some hills on the freeway, then put it in neutral on the way down so it didn't regen. I repeated this until the SOC indicator was down to 1 bar. This should be nearly empty and done under actual operating conditions.
Next step was to pull the pack and open it up so I could probe the individual modules. It's alot of screws and bolts, but nothing really tricky or hard. Once I had the ends exposed, I wired up a 120 VAC recepticle to the pack ends, and placed a jumper across where the pack switch goes. Pack was reading around 156V at this point. I wanted to put a good load on it, but couldn't find my old 220V stove top element, so I tried using an old 1000W hair dryer. When I turned on the dryer, it made a snapping sound and some really stinky smoke came out of the handle. Guess it didn't like DC. Oh well, next I tried a 100W light bulb. I figured it might burn out pretty fast, but I had a pile of them I pulled out when changing to compact fluorescents. It lit up OK and was really bright. While this was discharging the pack, I constantly measured all the 20 cell modules. I figured when the first one hit 6.5V or so, it was done. The light ran for about 2 hours, so there were quite a few watthours left in the pack even though it was too dead to assist the car.
While waiting for the cells to die, I took apart the hairdryer for failure analysis. It seems all the guts were OK, but the switch arced and did something nasty inside. After togglig the switch a few times, it started working again. With the switch already on, I then plugged it into the pack. Wow, that thing was like on turbo overdrive. It really sped up the discharge process as well.
Finally, the winners (or losers) were becoming obvious and I concentrated the voltage measurements on those. Unfortuantely for me, it was a 3 way tie, with another close behind and I only had 2 new ones. Oh well. I replaced the lowest two, then reassembled the pack and reinstalled it in the car. If I get more modules, I already know which two are the next weakest. The weakest ones would were in positions that I figure had the highest temperatures on average. Not on the outside edge and closer to the top.
After installing the pack, I did another long equalizing charge to make sure all the cells were up to full and balanced. We'll test again and see how it goes...
Pack removed from car-
All the relays and sensors are on one end. This stuff all comes off with 10 bolts.
Looks like this underneath:
Ghetto test setup.
Removing the other end of the pack exposes the ends of the sticks and makes for easy replacement:
"One test is worth a thousand opinions"