Awhile back I had a 12V die on me shortly after putting the car into Ig-On. This is out of a non-SKS car. I thought I'd pull the caps off and take a look. After seeing the dead cell, I got out the chisel and took the vent out completely. It is absolutely clear that one of the cells shorted out and went bad. It is such a shame, to have a car where virtually all components are over-engineered and highly robust, except this one.
Thanks for the rare chance to look inside a shot 12V battery. I see this thread as a unique learning opportunity. Can you give us some history on the battery: age, typical operating temps, times fully discharged if any, use of trickle charger(?), speculation on the cause of the short, early signs of impending failure, etc.
This was off a 2007, salvage title, 100K miles, daily driver for a high school student, 3 miles each way for the past several months, fleet vehicle before that. 2007 is long in the tooth anyway, but I speculate that the battery was drained and recharged at least once by the body shop that did the rebuild about 9 months ago.
I'm inclined to guess that the Prius Gen II 12V battery charging scheme is not particularly kind to the battery. After all, despite the small size, the lack of needing to do the massive task of spinning up the engine as required on other cars suggests that the working load is remarkably benign. So what is left to account for the rather poor lifetimes many of us observe save environment and charging? And the rear trunk position surely gives it a nicer life for temperature and some other insults than so many cars under the hood. I, personally got about ten years out of a Sears DieHard in a 1987 BMW 325i. This despite the fact that I ran the battery down well below the "need a jump to start" level at least six times. The problem was that turning off the ignition with the headlights on took the lighting down to parking lights, which was too much drain for the car battery to last the (long) day of work at Intel, but too little light for anyone to let you know. Not dead flat, I admit, but low enough for zero engine turn, often not even a relay click, and only a little instrument panel lighting. So why ten years there and maybe four years here? Does anyone here have deep knowledge of the variation in battery charging care by car, and comment on how the Prius Gen II measures up?
The charging algorithm is flawed because the max charging voltage is 14.0V. AGM batteries in particular need to have an equalization charge, defined by Optima as constant current to 14.7V, then constant voltage (14.7V) until current is 1A, then 2A for 1 hour. It is not a stretch to think that the bad cell in the photo was caused by reversal, due to lack of equalization between the cells.