11 August 2009

80% charging efficiency at highest rate, 60% at lowest

Today I turned off every circuit breaker except for the smoke detectors and the clock radio, then recorded the power company watt hour meter. I read 14% charge remaining when I plugged in the car, and took another reading when the charge rate slowed down at 98% and we really wanted to turn on the central AC again.

I read total power used as 29.3 kilowatt hours for adding 84% charge. By the way this took 2 hours 32 minutes at 50 amps. Sorry, I did not get a precise voltage reading. So:

29.3 / 0.84 = 34.88 KwHr needed to charge from 0% to 100% by extrapolation.

The Mini E spec sheet says usable battery capacity is about 28 KwHr. If we define charging efficiency as usable energy available after charging divided by energy input while charging, I get:

28 / 34.88 = 0.8 or 80% efficiency.

This is very good. I did a similar measurement long ago with the 120 volt charging cable when that was all I had and got a number closer to 60%, presumably due to the fans running just as hard but being a higher fraction of the available charging power.

The 2.9 hour charge time given for the highest charge rate seems to be quite accurate for 0% to 100% charge. However, the 23.6 hour charge time given for the low rate 120 volt charger seems to correspond to charging from 30% (when the first low battery alarm comes on) to 100% by my measurements. I extrapolate to about 37.5 hours for charging from 0% to 100% with the 120 volt box. Of course, this would depend on ambient temperature since that can affect fan speed. I'll meaasure again when it cools down in the autumn. The high rate charge time might be less affected by temperature between 40 and 100 F.

The real point of all this is trying to estimate how much of an effect terrain and start stop driving have on range versus the effect of speed. Assuming that regenerative braking is as or more efficient than the high rate charger, I am guessing that 80% efficiency goes a long way towards removing the effect of hills and start stop driving. High speed might be the bigger cost to range.

And by the way, I once figured the max regen rate works out to about 20 Kw, which is much higher than the 12 Kw max charge rate for the wall box. So efficiency might be above 80% for regenerative braking.

I had thought I should strive not to go below 30% battery charge since that is where the low battery warnings start. But one of the Mini engineers at a Mini E social event encouraged me to run it as low as I want, even into the negative area. He insisted the battery was fully protected and I would only lose speed at very low charge levels. I usually get home in the 20% vicinity. You can see the details in my trip logs. (Click on the title line of the oldest posting.) I have noticed limited acceleration when passing when I am below 20% but never any sense of a speed limit yet. Some day I'll get the nerve to get down to 'zero' charge or below. Not this week.

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