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UPDATE: Martin Eberhard full of halibut, part 2

September 2, 2007

I wrote an earlier post concerning Martin Eberhard’s critique of series hybrids, in particular the lifespan of the Chevy Volt’s battery pack. In that post, I noted that Martin incorrectly assumes that all lithium ion cells are made equal, and also incorrectly assumes that a 40 mile range correlates to 100% depth of discharge of the pack, and thus a full (and brutal) charge/recharge cycle.

GM-Volt.com confirmed recently that the 40 mile range can be achieved with only 8 kWh of the pack’s available 16kWh, and that to maximize lifespan, the generator will kick at this point of 50% charge, and stop at 80% charge. This optimized charging cycle, combined with the innate durability of lithium-iron-phosphate chemistry, will help the battery pack last for years.

Some caveats though:

1) Whether this optimization scheme will result in the ridiculous 7000+ cycle life that A123 is quoting for its proprietary LiFePO4 chemistry remains to be seen. The M1 cells that A123 makes (of which I own several) are rated at only 1000+ cycles rated at 100% DOD.

This supposedly increased lifespan might be due in part to the newer automotive-grade electrode. More likely, it might be calculated using the optimum temperature and charge conditions.

2)This optimized charging scheme assumes that there is gas in the car. Most people will, of course, but there is bound to be the person who leaves gas out to save weight, and bleeds the car battery nearly dry on a regular basis. How significantly will this reduce battery life?

Also, how will recharging the car from grid while at various states of charge affect life? For example, how will charging beyond 80% of rated capacity (which could be liberally considered “overcharging”) be bad for the cell?

The riddle for the best battery pack isn’t solved yet, but it’s becoming increasingly clear that that there is something motivating the naysayers’ rhetoric other than cold hard data. For example, both Martin and Toyota Motor Corporation are shackled to using cobalt lithium ion cells, the merits of which are increasingly being called into question. With the case of Toyota, it’s highly likely that they realize they are behind in the plug-in hybrid race, and are trying to deflect public attention from this fact. With the case of Martin/Tesla, they have spent a lot of time, sweat, and tears making a high end system to tame the cobalt cells, and they aren’t about to readily admit that this system could soon be rendered obsolete. The fact that they continue to focus on cell-level specific energy, and not on gross battery pack specific energy, also suggests that they know they need to be on the defensive.

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2 comments

  1. i am an electronic engineer and have looked at electric vehicles for some time. there was an electric car show i saw several years ago that had a clever, simple idea: this series electric car (which was commissioned by an Arizona electric company) had 2 rechargeable battery packs, one was a battery that had high amp-hours, but could not put out hi current, so was used mostly while cruising on flat ground. the 2nd battery was heavier but had hi current capability, which was added to the first battery for when the car hit hills or headwinds. the ICE was able to charge both when the car was traveling on level ground. what i think a new series electric vehicle could add is a capacitor pack as a second battery. (they didn’t have today’s capacitors in those days) this could address many problems weight, life cycle, and range. once inertia is overcome, the power requirements to sustain velocity are not great, so a small ICE should be possible. PLEASE E-MAIL ME WITH YOUR OPINION ON THIS APPROACH. THANKS


  2. “what i think a new series electric vehicle could add is a capacitor pack as a second battery. (they didn’t have today’s capacitors in those days) this could address many problems weight, life cycle, and range.”

    There are actually several designs out there right now that do use capacitors to store and discharge energy, in addition to the main battery pack. The MiniQED concept that PMLflightlink put out was a series hybrid that used capacitors to smooth current flow and give quick bursts of power. As far as I can tell this feature was added mainly as a performance feature.

    A contrasting example is Maxwell Technologies’ use of ultracapacitors to absorb and discharge the energy gained from regenerative braking. As far as I can tell this is planned for use in city buses. Since buses are constantly starting and stopping, the capacitor’s high life cycle would help buffer the main battery pack against constant charging and discharging.

    As neat as this sounds, I sincerely doubt that a practical ultracap should hold any more energy than would go in and out during regenerative braking. For one, ultracaps are expensive, plus their specific energy is low. Most importantly, it’s hard to properly regulate the voltage coming out of a capacitor, since the energy stored=1/2 CV-squared. Thus, the more you discharge, the more the voltage drops exponentially.

    However, you bring up an interesting point, which is the possible subdivision of the battery pack. Two ways to approach this:

    First off, that power company’s EV sounds like it was using something like a Zebra battery (high capacity, low power) as the main energy source, while using a low capacity chemistry with a high discharge rate ( e.g. NiCads) to actually deliver the energy. This is clever, but a lot of the modern lithium chemistries have the best of both worlds – capacity and power, so at this point I don’t see an impetus to design a “hybrid” hybrid.

    Where the subdivision of the battery pack might also come into play, though, is whether or not the cells should be permanently wired in series. If one cell – or module of cells – fails for some reason, then the entire chain is broken. However, if it’s possible to automatically reroute the current so all is not lost, then driveability would be maintained.



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