Q&A with GM's hybrid chief Robert Kruse
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On the longevity of lithium-ion batteries:Skip to next paragraph
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RK: Your cell phone is powered by a lithium battery – and that battery's capacity decays over time.... I've chosen a "life of the vehicle battery" and my terms of "life of the vehicle" are eight to 10 years. Our promise of 40 miles of electric range is an end of life requirement.... So how do I do that? I have more battery capacity than I'm using. There's a sweet spot in there. I won't over charge the battery, or over discharge the battery. The maximum state of charge, the minimum state of charge, I consider to be highly regarded intellectual property. I also understand the temperature range that I need to use this battery at so I don't reduce its life. I won't charge or discharge my battery outside a temperature window. That's also something I consider highly prized intellectual property. So by never overdischarging, never overcharging, never allowing the battery to charge or discharge out of a temperature range all unique to the battery chemistry I've selected, it allows me to have confidence in the longevity of my package.
On whether, if drivers persistently ignore warning lights about a battery problem, the car might be programmed not to allow itself to be started:
RK: GM has more production electric vehicle experience than anyone else, having executed the EV-1 program. One of the things we learned in EV-1 is that electric vehicle customers suffered from 'range anxiety.' Where am I when my battery goes dead? My development engineers in that time frame ... went out and bought motorcycle engines, coupled them to generators, put them on trailers, and towed them behind EV-1s to allow them longer periods of electric operation.... So as we looked at concepting the Volt, we basically took that development engineer's solution and integrated that into the Volt.... So, if there is an issue, the customer will always have his vehicle available at his disposal through the extended range capability built into the vehicle.
As it relates to individual cell reliability, having proven capability to manufacture cells with pharmaceutical-level cell quality so that each cell is identical is absolutely critical to my success formula. If you can imagine having hundreds of these cells fit together, and one is slightly out [of] spec from the others, you're going to work that cell harder than you do the rest of the cells. If you work it harder, it's going to wear out sooner. Then, in a battery where these cells are strung together in series to get voltage, the chain is only as strong as the weakest link analogy begins, and you lose a cell.... I do have a level of redundancy built into my pack in the way I have my cells configured both in parallel, and in series, to be able to protect that.
The other thing you have to watch out for is that as these cells begin to charge and discharge, that no matter how identical you manufacture them, they will start to deviate from each other... part of what I have designed into the pack is a very sophisticated cell balancing capability, so that as the pack ages, the individual cells age. I will adapt the cells to the pack, constantly rebalancing my pack.... The software and hardware, the algorithm that does that, and how we do that, is also highly prized intellectual property.