Consumer Reports brought the problem to light when one of its drivers was behind the wheel of an i3 using the range-extender. When the driver attempted to pass another vehicle on a rolling, two-lane road, the BMW suddenly had no power to accelerate – a scary situation. CR started examining the car and found something pretty shocking: After driving at a constant speed for a while without any regenerative braking in range-extended mode, acceleration to 60 miles per hour plummeted from about 9 seconds normally to a staggering 27 to 40 seconds in their testing.
A BMW spokesperson told CR that it knows about the problem and has a fix coming next spring that also works on current models. The upgrade includes a state of charge indicator, a warning about loss of power and uses the car's navigation to boost the battery before driving on rolling terrain. It is not clear if the severe power deficiency will also eventually result in a recall.
The issue is bad news for the i3 that is already proving quite popular, with a reported six-month waiting list to get one. However, the range-extended model already had a small hiccup in the US when BMW had to hold them at port until the EPA certified the fuel economy for the variant. It was eventually rated at 117 mpge with an estimated total range of 150 miles – 72 miles on electric power and another 78 miles with the range-extender running.