What is the average traction battery mode of failure

Jim,

You can usually continue to drive a Prius with a failing battery for a few weeks. At some point the battery will not have enough strength to restart the ICE and you will need a tow.

To more fully answer your question there are several HV battery failure modes in our Prii:

• The cells (6 in each module - 28 modules in my Gen2 Prius) will, over time become unbalanced (have differing States of Charge or SOC). If the module pairs (2 modules in series) show more than a 0.3 volt difference, the ECU will flag a problem. In some cases, you can put a charger (available in the PC store at about $400) on the battery and by slowly overcharging it, get the cells back into better balance. Note that doing this "balancing" periodically can usually delay this failure mode (and the next one in the list) beyond the useful life of the car.
• In my 2004 Prius one of the cells had reversed due to imbalance at 7.5 years and 195k miles. This permanently damaged that module and a more invasive repair was required (note that the other 27 modules were still in good condition). I installed a well done, refurbished battery from ReInVolt (still working in my Prius at 285k miles and 5 years of use).
• Some batteries fail with virtually all of the modules losing much of their capacity due to heat damage. This usually occurs in warmer climates and requires replacing the whole assembly or at least all 28 modules.
• The remaining failure mode is a leaking module which will be detected as a ground fault in the hybrid system. This was common in the Gen1 Prii. It requires at least replacing that module and cleaning up the whole assembly to remove the electrical leakage path.

JeffD