07 August 2009

Battery temperature

I do not drive much on the freeway, but I had to yesterday. After charging the Mini E all day at work as usual, the battery temperature was 104 degrees F when I left. I have sun shades in the windshield and on one side window, but it is still fairly warm in the car even with the fan running while the batteries charge.

On the freeway I was driving rather aggressively, and saw the battery temperature peak at 109 degrees F about three quarters of the way through my 68 mile trip. Now this is fine, the car ran perfectly, the AC is very strong, range is great. But 109 degrees is close to what I recall as the limit for regenerative braking to function.

So I question the wisdom in the owners manual of recommending that the windows always be fully closed when the car is parked. If I did not have the car plugged in and charging, it would be much hotter in there without the fan. Has anyone else gotten into a high enough temperature that they lost regen? The only blog I have read about loss of regen was related to problem that was repaired.

So I leave the windows slightly open if there is no rain in the forecast. And if rain is expected, it is usually cloudy enough that the car stays cooler even with the windows closed fully.

But I would like to know from Mini if the fan always exhausts interior air to the outside to cool the batteries? Would it work better to reverse the fan when the inside of the car is hotter than the outside? Can the battery fan reverse in winter to heat the cabin when the batteries get hot? (Maybe the batteries don't get that hot in winter.)

It is rare for me to drive aggressively, so I don't think I'll see these high battery temperatures again. I suspect that Mini or BMW will go to a liquid cooled battery when they come out with a production electric car. But they seem to have done a very nice job for a prototype.

And I love the fact that the car stays as cool as it does parked in the sun when it is plugged in. Too bad the AC won't run when it is charging but it is hardly needed in my experience. Maybe I should get in interior thermometer and put some more numbers in this post...

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