Speculative : New PEV Prius only using half available LiIon capacity.

Considering the 14 mile stated range of the new plugin-prius I was little surprised by the total weight of the battery pack (330 lb). Did this strike anyone else as perhaps a little more than they expected for just 14 miles?

Heres some rough calculations, admittedly I don't have all the data so it's speculative. Say they only use two of the three packs for EV mode (I think thats what I've read), and also say that only 2/3 of each packs weight is actual battery (the remainder being housing etc). Well thats still 147 lb (67 kg) of LiIon battery. Now given that the energy density of modern LiIon batteries is supposedly 0.1 to 0.16 kW-hr/kg*, then taking even the lowest figure (0.1) that would still equate to about 6.7 kW-hr of EV mode.

Also, its been estimated that in EV mode the Prius needs about 0.15 kW-hr per km, which translates to about 4.2 miles per kW-h, and I think this is a reasonably accurate figure, maybe even a little conservative. So that would mean 6.7 kW-h should give a range of about 6.7 times 4.2 equals 28 miles, double the quoted value!

So the conclusion would be that either

1. I've got no idea what I'm talking about (which is always possible).

OR

2. Toyota have somewhat over Engineered the LiIon EV system, probably going for about 70% to 75% maximum charge and about 25% to 30% minimum charge, so as to give a long service life.

Do these numbers seem in the right ballpark?

* Lithium-ion battery - Wikipedia, the free encyclopedia

 

What you're talking about there is more related to volumetric density and pack/module construction density rather than mass density. Given the obvious importance of weight to the vehicles overall performance I don't see any reason why they wouldn't be using at least somewhere near current "state of the art" for the mass energy density. Also, I didn't assume anywhere near the highest. The quoted range was 0.1 to 0.16 KW-h/kg and I only chose 0.1, which is about 62% of the maximum.

 

Yeah but that's probably still well under 1000 total cycles right. A 14 mile PEV under heavy use could go 1000 cycles in under three years, and if that was enough to wear out the battery then (in the context of the sort of reliability people would expect from modern car) that would be pretty bad.

I don't think any of us know for certain just how good are the very latest LiIon batteries that the large PEV players are currently developing/testing, but there's lots of sources that suggest that with currently available LiIon technology that you could easily wear out a pack in under 1000 cycles if you mostly use the full capacity with deep discharge/charge cycles.

 

Yeah one reason might be the Prius's much smaller pack sizes, so there's relatively more of the weight taken up by the enclosure etc, compared with the Volts much larger pack.

But I also think that Toyota is probably being really conservative on the longevity side too. I'd say they are aiming for more than 10 years out of those packs under typical usage.