A functional range extender would consist of:
A small displacement (200-600 CC) ICE generator, run at highest-efficiency rpm, to recharge the battery pack. Generator output would not be sufficient to drive the vehicle, just enough to extend the battery pack range to the next convenient recharge location.
It would not run on gasoline, but a less polluting, and more stable fuel, such as propane (easier refueling) or CNG (lower cost). 5 gallons of Propane, for example, would probably offer about 200 miles of range extension for a LEAF-sized BEV.
The fuel would also be available to a combustion cabin heater, the one use for which battery energy storage is particularly inefficient.
I think this could be integrated into the design of BEVs (and maybe even as a portable unit, and available for rent, as many have fantasized) at lower cost, and lower weight, than the huge battery packs some BEV manufactures seem to think are advisable.
So, say you are a San Francisco Bay Area resident. You usually keep the heater set to propane by default in the winter, extending the range by about 10% and reducing battery cycling accordingly, without even using the ICE feature. You refill the 5 gallon propane tank once a month or so, just to supply the heater.
When you want to take the BEV on the occasional longer drive, say to Tahoe for a weekend of skiing, instead of making 3 or 4 stops (with a 20-30 available kWh battery pack) for DC charges, you just turn on the ICE generator during your trip, as soon as your battery capacity drops to a level to efficiently accept charge, while you and your passengers are kept toasty warm by the propane heater. You stop for one 30 minute 80% DC charge at Auburn (120 miles in 2 hours of driving, about 20 kWh consumed from the battery pack, and 16 kWh used from the generator) and top-off the propane tank (you only used a few gallons) at the adjacent minimart. This is just enough generator-assisted charge to get you the last 80 miles over Donner Summit to your destination, but you never get “range anxiety" (or BEV "freeze anxiety" about road closures or delays, due to weather) as you know that if you get the “very low battery” warning, you can just pull off the road, and if there is no charge station (or only a L2) nearby, you can always find a place to stop for a short break, while you self-recharge for the last few miles, using your generator. And if you get stuck behind a semi that jackknifed in a snowstorm, closing the road, you can watch the generator add bars to your battery, as the propane heater keeps you and your passenger comfortable, while you wait for the road to be cleared...
I believe BMW may be the only manufacture currently contemplating this true ICE “range extender” option, for its BEVs...
