On the same day that BMW released the results of a study conducted by University of California - Davis on driver's experiences with the Mini E, the New York Times takes another look at the disappointing cold weather performance experienced by many of the test drivers. The UC-Davis study talked to 57 of the 450 drivers and found that they were by and large satisfied with the performance of the prototype electric Mini.
The Times report, however, indicates that while drivers generally like the driving behavior of the Mini, range anxiety remains a problem especially when the mercury dips. Exacerbating the problem for Mini drivers is a less-than-accurate state of charge indicator. We experienced the same problem when we drove the Mini E back in November 2008 with the gauge dropping from 72 to 49 percent after less than 6 miles of driving. These results are exactly the types of lessons that BMW was hoping to learn from this program and the company deserves some credit for actually testing the Mini in the cold climate of the northeast rather than focusing on more temperate California like most automakers.
Estimating the state of charge of lithium ion or other advanced rechargeable batteries is more difficult than it might seem. We spoke to General Motors battery chief Denise Grey about this problem back in 2007. Unlike traditional alkaline batteries, the voltage doesn't drop gradually off as the battery is depleted. Instead, the engineers must model the battery and do more complex monitoring of the pack and cells in order to estimate its condition. This is just one of the elements of the development work being done at GM's battery lab in the runup to production of the Volt, and elsewhere. Every automaker planning to sell EVs will need to make sure the state of charge indication is as accurate as possible to help the driver estimate how far they can go. Without an accurate range estimate, anxiety really will be the result.