The issue was discovered by Consumer Reports as a driver attempted to pass a vehicle on a hilly two-lane highway after depleting the main battery. The i3 began to lose power, “subjecting the driver to more exposure in the oncoming lane” before recovering after coasting.
“But losing power just when you need it most is disconcerting to say the least,” CR wrote in a blog post.
Later tests showed that continued use of the throttle without regenerating the battery, either by coasting or applying the brakes, causes the acceleration to stretch from an all-electric best of 7.5 seconds to a maximum of 40 seconds.
BMW is reportedly working on a fix for the issue, though it is unexpected to arrive until spring 2015. The system will be revised to warn drivers of the low-charge condition, while the gasoline-powered range extender will kick in early to charge the batteries in certain scenarios.
It is unclear if the revised software will be issued through a formal safety recall, but BMW promises it will be available as an upgrade for existing vehicles.