The Volt, batteries, and hype – an analysis

July 17, 2007

A couple months ago, GM announced its battery contracts for the Chevy Volt. Since then, there’s been a fair bit of buzz over exactly how -and when- those contracts are going to bear their electrified fruit. Some of that blog commentary has come from people within the EV industry itself – notably Martin Eberhard of Tesla Motors. In his rather scathing blog entry, Martin disparages the series hybrid concept as being flawed.

I have to disagree.

I’ll paraphrase his logic concisely as I can: Since a series hybrid has a smaller range (40 miles) due to its smaller battery pack, it will thus need to be recharged much more often than a pure EV, like those that Tesla is promising. Since batteries have a finite cycle life, the battery pack in a series hybrid will be subjected to much more frequent abuse than a pure EV. As an example, Martin says that a Volt equipped with cells rated for 500 cycles, and with a range of only 40 miles, this will result in a dead battery pack at only 20,000 miles! Compare this to the Tesla, which will allegedly get over 100,000 miles on its pack before needing replacement.

Martin is conceptually spot on, but he is seriously fudging the numbers, and for that he needs to be taken to task.

First off, he is comparing two very different vehicles with very different purposes (not to mention prices), so the entire comparison is invidious. But more importantly, the Tesla-spec 500 cycle cells he is referencing are not the ones that are going into the Volt. As anyone who has followed the battery debate will know, modern lithum-ion phosphate batteries have a significant lifecycle advantage over the more traditional oxide-based chemistries. These more sophisticated batteries are already in production from multiple companies like A123 and Valence, so there is little hype about them that needs to be dispelled. As an example, the A123 M1 cells (like those used in the KillaCycle) are rated for 1,000+cycles.

That little fact alone doubles Martin’s pessimistic assessment of a series hybrid. Furthermore, that cycle life is rated at 100% depth of discharge – recharging the battery before it reaches this low internal voltage, and keeping it cool can dramatically enhance the lifespan of the cell.

However, Martin’s argument- though fundamentally more of a business rip than a true scientific argument-raises an interesting question about the series hybrid configuration. Recharging the battery using the genset (whether it’s the Volt’s small piston engine or a Velozzi microturbine ) will place a big strain on the battery cells, no matter how great they are. Could and should there be a direct electrical link, then, between the genset and the electric motor? Could this help alleviate the stress on the cells? This could also be a useful feature in case of a catastrophic failure of all the battery system.

Only time and investment dollars will tell.


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