Knowledge is Power

When the temperature drops, so does the power capacity of a battery. Anybody who has left a cell phone or ipod in their car on a freezing night knows this, and all of us MINI E drivers who hail from the colder climes have seen this in the reduced charge-o-meter readings we’ve been seeing.

But as I talked about in my last post, I’m starting to think that the 15-20% hit in the battery capacity that many of us have seen has more to do with the algorithm used to calculate the remaining charge, and the temperature of the batteries, than with an actual loss of capacity due to cold temperatures. And a test I ran last week seems to back this up.

First, here’s the theory – as long as the High Voltage battery pack temperature doesn’t fall below 40°, then I don’t think the battery looses much capacity, if any.

To put this to the test, I decided to pioneer-up last week and follow my warm-weather routine – which is to not plug in #217 after my usual 45-mile morning commute. In the past this would mean I’d show about 52% battery left when I parked in the AM, 60% showing when I’d start my evening commute home, and  anywhere from  5-15% of battery range left when I pulled in at home.  If my theory held water, I’d be fine – though running very low on battery power when I returned home 92 miles later. If however the 15-20% “cold deficit” was in fact accurate, then I’d be sitting on the side of I-287 waiting for a tow truck.

So last Wednesday morning I pulled into Morristown after a modest (no faster than 60mph) and heater-free trip, showing 48% of battery power left. The high that day was 38° , and by 4pm I was reading a 55% charge – but the battery never got colder than 42°.  I drove back home (again very conservatively, 65mph max), and also without the heater. The charge-o-meter showed 0% with about 8 miles to go, but as happened in the summer and fall I just kept going without a hiccup. After getting off the highway, the meter did the usual recalculation and I gained 5% more by the time I got home. The battery temp when I pulled in was 72°.

After 2 hours of sitting in an unheated but insulated garage, the meter read 11% charge, with the battery at a fairly cozy 68°. Which is right around what I was getting in the fall.

Ironically though, my ipod  – which was 50% full when I parked in the municipal garage in the morning and left behind in the car – did drain pretty quickly, and powered off about 10 minutes into my evening drive, even thought it’s powered by a Li-ion battery just like the MINI E. Sadly the iPod doesn’t tell me its battery temp, but I’m guessing that without insulation (as the MINI E has) it must have been around the mid-30°s.

So the conclusions that I draw are:

1) The magic number to stay above – battery temp wise – is 40°. Keep that temperature, and at most you’ll be shorted 5% of the battery.

2) The Charge-O-Meter needs a new calibration, so that we don’t any get unnecessary range anxiety.

Knowledge really is power – about 15-20% more power when you’re driving an electric car in the winter!


3 responses to “Knowledge is Power

  1. Gordon,
    There is definitely truth to your findings. I do think the two real key to extended range in these cold conditions is 1) keeping the battery temp as high as possible and 2)using the heater as little as possible. I noticed you didn’t use the heater in your experiment. I would like to see your findings if the same thing as you did here a few times with the heater on the entire trip and then a few times with the heater off the entire trip. I’m finding that the heater use alone is cutting about 10% to 15% off my range. I did an experiment this week where I preheated the cabin of the car with a ceramic heater while it was charging and then didn’t use the heater all day. The battery temp was 99 degrees when I finished charging. I was able to drive almost 100 miles and still had a few percent left when I was done.

    • No doubt that the heater has a big impact on the batteries Tom, and I’m convinced I wouldn’t have made the 92 miles if I ran the heater much. As it is, I’m sure that the reduced range I get is almost all attributable to the heater and not the cold. But if I get another chance to experiment (as in nothing to do that night) I will try it and see how far i get.

  2. Pingback: A Heated Commute « The Electric Ride: MINI E #217 Road Test

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