On Tuesday, February 3, 2015 at 1:26:11 PM UTC-6, Stephen G. Giannoni wrote
The failure mechanism is the electrolyte dries out. The terminal seals of t
he 38 battery modules have a rubber "O" ring to seal in the small amount of
gas that forms in normal charging. But when the battery gets hot, the "O"
rings let a little of H and O out and over time, this dries out the e
lectrolyte. Hot climates with little shade and hilly terrain driven at high
speeds heats the battery modules to higher temperatures. So some will fail
sooner than others, usually the ones in the middle of the pack where it is
most difficult to keep them cool.
The next generation battery modules have improved terminal seals and more i
nternal, cell-to-cell connections to reduce resistance. The 2004-09 modules
were a substantial improvement and the 2010-current even better.
If you have a Prius-aware scanner, one that can read the battery module-pai
r voltages, you can get an idea of how well the pack is balanced. There is
also a procedure described in Volume I of the shop manuals about how to tes
t the pack.
When modules in the pack are ready to fail, they lose their amp-hour capaci
ty. So the pack appears to charge very fast and discharge very fast. Eventu
ally a cell within a module will short out and then you'll get an error lig
ht and a Prius-aware scanner can identify the failed module. This is anywhe
re from a week to a month before the pack fails where the car no longer run
We've written fairly detailed descriptions about this over in the PriusChat
forum as well as the Yahoo Group, "Prius Technical Stuff." Both have much
better archives with PriusChat dating from 2004 and "Prius Technical Stuff"
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