Originally Posted by Hillbilly_Hybrid
I would expect at least 10 years life in these batteries and over 250,000 miles.
I agree that Panasonic knows NiMh. But Toyota's strategy for long life (maintain the string of 28*6=168 NiMh cells in my 2004 Prius between 40% and 80% capacity) has a flaw that limits the long term life of the HV battery. There is no effort to maintain cell balance. This will cause the eventual reverse charge on a module which results in a battery failure.
To balance the cells in the string, the battery needs to be taken to full capacity and then trickle charged to bring the cells that are at lower SOC to full capacity. The already fully charged cells will just dump the excess energy as heat (use a low current and monitor temperature). My Supermate DC6 battery conditioner does this with multiple discharge/charge cycles, but can only handle one (6 cells) or two modules (12 cells) at a time. This requires disassembly of the battery unit (Warning!
Dangerous High Voltage) and is a long process if you don't have multiple battery conditioners (I only have one).