My BMW i3 -- Range Extender (REX)

I'm leasing an i3 that has a range extender, a 675-cc motorcycle engine in the rear of the car and next to the electric motor that runs an electric generator, providing power to the battery when the charge drops below 6% of its capacity.  In other words, the BMW i3 REX is really a series hybrid.

Because BMW designed the i3 to meet California zero-emissions guidelines, this range extender can only be run in a limited way because the motorcycle engine does emit.  This is why the gas tank for REX is only 1.9 gallons and why it can't run until the lithium-ion battery drops below 6%.  As I understand from what I've read, European versions of this car do allow drivers to run REX at any time to add charge to the battery, regardless of its charge content.

Here are two pictures of the exposed rear of the i3, one with only the electric motor and the other with the range extender installed next to it:

Because the gasoline motor needs to run at times, BMW actually has designed it to be automatically turned on approximately every seven weeks, even if the i3 has adequate charge.  However, that charge must be less than 75% for the maintenance run; if the car charge is more than that, the maintenance cycling will wait until it drops below 75%.  The driver is informed on the display when the maintenance run is occurring.

I decided that I wanted to see how the range extender works, so I let the charge on my i3 drop below 6%.  You can just barely hear the motor running, and the instrument panel changes to show that the REX is running.  The picture below shows that the lower-left gasoline gauge is illuminated when the range extender runs.  That gauge will be gray in normal operation.

I don't intend to use REX because my i3 will be used as an in-city car, but it's nice to know that extended range is possible.

The speed of the motor (and motor noise) will increase when the car goes faster, but if the demand for speed or power up a hill exceeds the capability of the 25 kW REX, the i3 will slow down.  Regenerative braking can recharge the battery, even when the REX is running, and the motor will be turned off when the charge exceeds 6%.  REX will also turn off when the car is stopped.

REX is rated by the EPA at 39 mpg (approximately a range of 74 miles), so a longer trip is possible with the car as long as you're able to stop for premium gasoline to refill the small tank.

But also remember that because the i3 uses regenerative braking and has three driving modes (Comfort, Eco Pro, and Eco Pro +)  range can be extended depending on which mode you use and how much regenerative braking you can provide.