I’ve wondered before why the Prius and other hybrids didn’t allow you to “fill up” the batteries as well as the gas tank — surely, if all of the electricity didn’t come from the engine, that would increase fuel efficiency some, wouldn’t it? (Of course, I’m still paying for the electricity that’d go into it, but that’s beside the point.) Forbes explains why that’s not feasible:

The problem is batteries–they are why the dream of a purely electric car has yet to come to fruition. Today’s battery technology cannot give the necessary power and range to make batteries practical for the average consumer. They also add weight and are costly.

Hybrids are different from electric cars. Most hybrids use nickel-metal-hydride batteries, which last for many years–perhaps as long as the car. Toyota tells me there are Priuses that have gone more than 275,000 miles on the original batteries. The secret to such longevity? According to Toyota, the car never charges its batteries to more than 80% of their capacity nor drains them below 50% capacity.

The philosophy behind a plug-in hybrid is that it drains the battery to almost empty before the gas engine cuts in. That is how they make those 100-mpg claims. This requires a bigger battery pack and, preferably, a lithium-ion battery, which packs more power per pound than nickel metal hydride.