Friday, December 11, 2009

Yeah it's THAT cold

The temperature gauge read 19 degrees when I left for work today. The battery temp was 49 degrees which is just about as cold as I've seen it so far. After about ten minutes of driving the battery temperature got up to 65 degrees which is still low, but not so low that my range will suffer too much. My garage is insulated and it is always much warmer than outside but it still gets pretty cold when the temperature is this low outside. I just bought a garage heater that I will install sometime in the next couple weeks. I always wanted to get a garage heater, but it was never a pressing issue until now. I'm worried that when the outside temperatures get down under 10 degrees the battery temp will drop under 40 degrees overnight and that will really diminish their strength and thus cut further into my range. I'm not even sure the car will start if the battery temperature drops below 40. I vaguely remember a BMW representative telling me that 40 was basically the limit and anything lower than that is uncharted waters and they don't know what the car will do if the batteries get that cold. I'll set the thermostat to about 45 -50 degrees so the garage will never get colder than that. That should keep the battery temp above 60 since it will be charging overnight and that raises the temp a little. I suspect this will help keep my range from taking such a beating in the winter. So far I have seen my range per charge drop from 100-110 miles to 75-85 miles in this extreme cold so I want to do whatever I can to help the batteries maintain as high a temperature as possible. Even taking the car out for a short drive during the day rather than letting it sit in the parking lot all day without use can make a difference. The trick is to try to not let the batteries get too cold because they will lose the ability to accept a charge and even a short drive can raise the battery temperature 10 to 15 degrees. Parking the car in a sunny place is also helpful because even in the winter when it's cold out the cars cabin will be noticeably warmer if it's parked in the sunlight. I knew this was coming, and all I can say is that it's going to be an interesting 3 months..


  1. Tom

    The car will still run if the batteries get below 40 degrees, but you may lose regenerative braking and the car may refuse to charge until the batteries warm up. But they actually "hold" charge better when cold, meaning that "self discharge" is less at low temperatures, more at high temps.

    Technically, the internal resistance of the batteries increase as temperature goes down. So power may be slightly limited at low temperatures, and the batteries will self heat more quickly when cold.

    I heat just the interior of the car when it is parked, mostly for my own comfort but it helps the batteries too. My batteries were at 55 degrees last night when I left work around 9 pm with outside temperature of 25. That is with a 900 watt forced air heater in the passenger foot well. The cabin was only 4 degrees warmer than the batteries at 59. But I leave the back hatch ajar slightly, so the cabin is colder than if it were closed tight.

  2. Hi Jim, Nice info. When I went to the NYC event I sopke to someone from BMW, I thing it was Richard Steinberg and I remember him telling me to do what I can to keep the batteries warmer than 40 degrees. He said in early testing their cars wouldn't even start up when the batteries got down to 38 degrees, but that they did some work and somehow improved that before they made the production models that we are driving. I don't want to tempt fate!