Saturday’s New York Times described a relatively new but intriguing development: the desire of hybrid car owners to be able to recharge them by plugging them in, not just through normal driving, as intended. Americans love to tinker with their cars, and as cars become increasingly electronic, owners are apt to see them as coming under the “open source” revolution sweeping the software world. As the article suggests, some owners are either paying to have their cars modified for plug-in, or doing it themselves.
This practice raises a number of concerns, affecting carmakers, electric utilities, and consumers. Determining whether it achieves hybrid owners’ goal of improving the environmental performance of these vehicles is hardly straightforward.
First, in terms of the cars themselves, their battery packs and power management hardware and software have been optimized around the assumption that they will be recharged in only two ways: by capturing energy when the car brakes, and by tapping the engine as a generator when the charge level gets low. The batteries are chosen for this type of service, which is normally a shallow-discharge, frequent recharge mode, rather than the deep-discharge mode a pure EV needs. In practice, the metal hydride batteries in most of these cars ought to do both reasonably well, but I suspect the carmakers will see plug-in modification as a warranty-voiding event. That’s a big deal, because replacing these batteries isn’t cheap.
The larger issue is whether plugging in a hybrid really helps the environment (or energy security, for the Geo-Greens out there.) To answer this you need to know two things: what is the incremental generating source on your local power grid, and to what degree does fully-charging a hybrid reduce its driving-cycle benefit? E.g., if the batteries always start out full, where does the energy recovered from braking go? I don't have enough information on the latter, other than knowing it must reduce the perceived plug-in benefit to some degree.
Answering the power grid question takes you into the world of dispatch curves and Clear Skies legislation. If you assume most of these cars will be recharged at night, and that the base-load power for most local grids will be either coal-fired or nuclear (because these plants are hardest to ramp up and down), plug-hybrid driving could be either truly zero emissions (nuclear), or create more emissions than running on gasoline (in the case of coal.) Either way, it would save oil, because so little power is generated using oil.
If I sound lukewarm on the idea, it’s because I think it’s not quite ready for prime time. A hybrid built from scratch around the plug-in concept would be terrific, particularly for consumers with short urban commutes, but the kind of aftermarket modifications described in the Times seem likelier to tarnish the image of all hybrids by making them much more prone to operating problems. That would result in fewer consumers buying hybrids, negating the tremendous benefits even normal hybrids can provide. Anyone wanting a manufactured plug-in hybrid shouldn’t have to wait more than a few years, though.