Friday, June 25, 2010
Hot in the city tonight
The cold weather seems to have a worse short term effect than hot temperatures do, since the range is dramatically reduced when temperatures drop below 30 degrees. However, I think the long term effects of overheating the batteries would be a concern if I actually owned the car and planned to keep it for 10 years of so. Supposedly, the life of these lithium ion batteries will be shortened if they were exposed to high temperatures (over 100 degrees) frequently.
Yesterday it was about 95 degrees outside with high humidity. I had a 50 mile drive that was mostly at highway speeds. I had the A/C on and wasn't paying much attention to the temperature gauge until I got the warning icon that has a cog with a temperature symbol in the middle(see above). I think it showed 112 degrees at the time. When this icon comes on it means that the battery temperature is getting too high and the car in going to reduce the amount of regenerative braking that the car uses. I saw this icon last summer a few times and noticed the regen was less aggressive. As the temperature continued to climb the regen was less and less apparent until the temperature climbed up to 116 degrees and then the regenerative braking completely disengaged. It was the first time that has ever happened to me and it was really strange driving the car without regenerative braking at all. Having the car coast like a regular car does when you lift off the accelerator was strange after driving it for almost 13 months now with the aggressive regen that the car usually has. The reason the regen lessens and then completely disengages when the battery temperature get too hot is because charging the batteries causes them to get even hotter. This is good in the winter when I'm trying to keep the batteries as warm as possible, but when they are overheating like they were yesterday you don't want to do anything to make them even hotter.
The good news is that BMW realizes that the thermal manage system is necessary and the ActiveE and the Megacity EV's that they are making will both have sophisticated thermal management systems like Tesla has to keep the batteries within a certain range of temperatures (something like 80 to 95 degrees). This will help keep the car's performance nearly the same in the cold as they are in the hot weather, while also prolonging the life span of the batteries.