BMW's i3 is an interesting car for a lot of reasons. It's BMW's first rear-engine mass production vehicle since the 700, it's the smallest BMW-badged vehicle we've seen here in a while, and it's a great lesson on why batteries still kind of suck today.
MIT Technology Review plunges into this issue, and investigates the car is really a "plug-in hybrid." Those are their words, and I actually don't think they're accurate. The i3 isn't a hybrid as such, since the optional gas motor never drives the wheels. It's really more of a range-extended electric car.
Still, that's basically a semantic quibble, as the MIT article is totally valid otherwise. The article points out that if you look at the i3's design, it was initally designed with the gas motor as a range extender in mind from the start, and the pure-electric version came afterwards.
You can see that clearly here — at the rear of the car, the 660cc gas motor sits right next to the electric motor, and on the pure electric there's just an empty hole with a support brace. If the car was designed as a pure electric from the get-go, that space wouldn't have been wasted, it'd be used for batteries or cargo or whatever.
This means that going into the project, BMW knew battery power alone wasn't going to cut it. And, if you look at the numbers, it makes total sense:
A gas tank about the size of a brief case can nearly double the range of BMW’s i3 from about 145 kilometers (90 miles) to 270 kilometers (168 miles). The gas engine also adds about $3,650 to the price of the car, boosting it, according to reports, from $41,350 to $45,000 in the U.S. Although the engine adds weight and cost, affecting performance, doubling the size of a battery pack would be more expensive and add more weight.
What would the tradeoffs have been of doubling the size of the battery pack to get a similar range? Automakers generally keep quiet about the cost of batteries, but current estimates suggest it would cost about $11,000 to double the size of the i3’s battery pack, far more than the price increase for the gas generator. It would also add another 230 kilograms of weight, nearly twice the weight of the range extender system.
So, for power-to-weight/-cost ratios, batteries still can't match good dirty old gasoline. That's not really news, as such, just a slightly depressing reminder of what we already know.
There's other unanswered questions here as well. Converting energy from one form to another is inherently wasteful. That means if you're going to burn gasoline anyway, you may as well just put that energy right to the wheels instead of making electricity for a motor. That's how the plug-in Prius and most actual hybrids work, because, generally, it is more efficient.
There's schools of thought that an optimally sized ICE engine running at precisely the right speed would be more efficient making electricity, and that's the approach Fisker was trying, though not to too much success. BMW may hit closer with their tiny 33HP 660cc engine making the power, but there will still be some losses.
There's also some encouraging news here as well, though. The fundamental flexibility of an electric drivetrain is well demonstrated here, in that a module to make power is able to be simply plugged in or out, depending on the model you buy. That could mean that future advances in fuel cells or radioisotope thermoelectric generators or captive black holes or whatever could be (potentially) easily fitted to cars, and that's an engaging idea.
Plus, the i3 design I think is actually pretty great for a small city car. Stylish, great looking interior, suicide doors — I think I'd really like this car, overall.
Also, if I'm honest, what would really be exciting to me is the i3, stripped of its batteries and electric motor replaced with a small, say 1- 1.5L 3 or 4 cylinder motor at the rear. Something with around 80-100 HP could be plenty fun in a car like this. Hell, maybe one of BMW's motorcycle flat twins in there, and then we'd really have a modern 700.
Actually, let's do that while the battery nerds work on those captive black holes or whatever.