Pack lifetime, NiMH vs. Lithium?

Have the OEM lithium packs been around long enough to compare pack lifetime against the OEM NiMH ones? This would be the mean (or median) time until pack failure for the two types. (Other differences may be significant but are not the point of this thread.)

I believe the 4th generation is the first where this comparison is possible. Our car is a gen2, and there was no OEM lithium option. The aftermarket lithium option was not as long lived as the OEM NiMH, but neither were most of the aftermarket NiMH, so that isn't much of a predictor for the OEM lithium.

 

Very implementation dependent. Consider the first generation Leaf. Those packs were not fluid cooled and they did not hold up well at all. The Gen4 Prius can swap NiMH (for cold climates) and Lithium (elsewhere), so I'm assuming that both are cooled the same way. Air or fluid, presumably air. Air cooling is adequate for NiMH (based on historical pack lifetimes), it might not be for Lithium. It might be that NiMH packs would last longer if they were fluid cooled. Hard to do the experiment as neither the modules nor the pack are designed for it (at least for Gen2, Gen3, and I assume Gen4, since those modules fit the earlier packs.)

 

BEV packs can certainly move more total current than one on a Prius, and have a larger volume. There is a point where air cooling just cannot get heat out of a box fast enough so fluid cooling is used. Although, in the majority of these instances the heat is eventually carried off by air passing over a radiator. I'm guessing that both a BEV and a PIP have about the same maximum current/maximum short term heat generation per cell, which is set by the chemistry and packaging. The Tesla packs are serial/parallel, with M cells in parallel organized into N sets in series. The Prius Lithium packs are said to be purely serial. The volume of the packs may be the most important factor. Prius packs are small, BEV packs are large. It may also be that the cells in a BEV are packed tighter together or the geometry is such that it reduces the available surface area, and so makes the heat capacity of the coolant more critical.