Saturday, August 6, 2011

Some Like it Hot - The MINI-E Does Not!

The outside temp read 107 and the MINI-E's batteries were a sweltering 114

It's not too often that it is so cold or so hot outside that the primitive thermal management system of the MINI-E really can't cope with it, but it has happened a couple of times in my two years with the car.

I should remind everyone that this is a prototype EV and it was designed for research, so don't think production electric cars will be this susceptible to weather extremes. Most all EV's that you can either buy now or will be able to in the near future have a complex thermal management system, capable of keeping the battery pack at optimal operating temperature regardless of the ambient temperature, so you won't have to worry about overheating problems like I do. The BMW ActiveE, for instance, the MINI-E's successor, will have such a system and it's one of the features I am anxious to try out.

Temperature warning icon
So getting back to the heat wave we recently experienced here on the East Coast, we had a solid week of high 90 degree and even a few days of 100+ degree temperatures. On one particular day, I was in a bit of a rush and had about 50 miles of highway driving to do. I had the air conditioning blasting as you might expect, and was tooling along at 70-75 mph when I noticed the battery temperature warning light came on. It's really not a big deal, but the car is telling you it's time to monitor the battery temperature because the modules are getting really hot. Just by slowing down to about 60 mph I kept the temperature from getting any higher but it wasn't going down either. If I had continued to drive at 75mph, I'm sure it would have continued to get hotter.

Once I finished my journey I returned to my restaurant and plugged in. I was pretty low on charge, around 10% and needed to get up to about 35% to get home later in the day, but since I had about four hours, that should be no problem. In normal conditions, I would be charged up to 35% from 10% in about 45 minutes, as I have a 50amp EVSE at my restaurant, capable of charging at 12kWh. However, this wasn't normal conditions, and when I checked the charge level about three hours later, it was only up to 20%! Now I was starting to worry because I really wanted to get home as we were having guests over for an evening BBQ and swim and I didn't want to be late. I reset the charge rate to 12 amps, thinking the car may accept the lower amperage and charge rate. The reason the car wasn't charging is because when you charge or discharge batteries, the chemical reaction causes them to get hotter and the car is monitoring the battery temperature and basically saying they batteries are too hot already, so it's not allowing it to charge, especially at such a high rate because that will only get them hotter.

Anyway, either the lower charge rate worked, or by sitting for a few hours the batteries cooled down enough to allow charging and the car charged up to 32% in the next hour and that was enough for me to make it home on time. In all my time with the car this was one of the few times that I ever wondered if it would let me down and prevent me from getting to where I needed to go. Once home I plugged in and left the car alone. The next morning it was fully charged, however I don't know how long it took or if it was charging slowly because of the temperature, though I suspect it was.

One of the major reasons you wouldn't want your EV's batteries to get this hot is that the excessive heat will prematurely degrade the cells and you will eventually have to replace the pack sooner than if the temperature was kept at optimum temperature, which is roughly 75 to 95 degrees Fahrenheit. If thermally managed to keep the batteries at these temperatures, the cars range will be optimized and the life of the batteries will be extended. However the life of the MINI-E's batteries aren't really an issue for us, as the MINI-E trial lease program will end soon, giving way to the BMW ActiveE program which will ultimately lead to the 2013 BMW i3. Personally I think BMW intentionally left the MINI-E without any sophisticated thermal management to test the effects of the weather extremes. It's all part of the learning process. These cars are such a departure from the major automakers comfort zones, they really need to comprehensively test every aspect of the electric power train before they even think about putting an electric car in their showroom for sale. 


  1. Tom, Your blog has been a gift to all of us MINI E drivers and lovers of the car. You remind me that we are driving a prototype E and we are partners with our wonderful MINI Es. This has been an amazing two years and it's getting to know people like you that has enriched the experience beyond driving the car. Thanks for your left brain and right brain comments along the way. It's made me a smarter driver and a even bigger fan... of the car and the other MINI E drivers! Here's hoping for perfect weather for the rest of the time we get to keep our wonderful cars. Sandra

  2. Makes me wonder about the LEAF and its lack of a thermal management system. Wonder how it would've done under the extreme conditions, and long highway driving, you describe here.

    Given current climate trends, I think a lot more people in more places than before are going to be seeing these types of extremes more and more often.

    Obviously, the Active E is going to be a more sophisticated EV than the LEAF. Of course, it's also going to be a lot more expensive...

    BTW, how did the heat affect your home solar production?