tiistai 8. heinäkuuta 2014

Ambiguity of energy consumption

Yesterday I took it upon myself to see just how far I could get with just the single set of 60Ah CALB SE cells that I currently have in the car. That part of the equation is rather clear. It's 25 kilometers or a little over 15 miles. How much energy was used, now that's a different question altogether.

I did measure the amount of energy used by the charger to charge the pack from quite empty to 87.6 volts or 3.5 volts per cell, with a CC/CV charge pattern. The result came in at 5.8 kWh. Now I don't know how accurate my cheap watt meter is, but if we assume it's anywhere near the thruth, the mileage from the wall was 232 Wh/km. Not bad, considering it was mixed city driving and no regen.

I suppose that's actually the most realistic way of measurement. After all, it determines how much you'll end up paying for your kilometers. That comes down to a little over 2 cents per km with our electricity prices, by the way. Or 2,32€ per 100km.

Nevertheless, it would be nice to know how much the actual driving uses. If we assume 90% efficiency in charging, we get 5220 Wh. Or about 209 Wh/km. Even better. The pack should however contain about 4800 Wh, or 60 Ah times 80 volts. That way the result is 192 Wh/km.

My Cycle Analyst however sees things quite differently. It measured 62.6 Ah into the pack and 87.8 Ah out while driving. It's obviously not counting the Coulombs quite correctly. I'll have to try and figure out why that is and make the numbers match. For now, I know I can drive the car quite a bit further than the Cycle Analyst numbers would indicate.

As for the range, the 25 km for a 25 cell pack of 60 Ah cells is pretty much dead on for what I had guesstimated beforehand. Doubling the cell count should double the range and give me the 50 km I had in mind originally. That's about 80 € per kilometer in battery cost with these cells.

2 kommenttia:

  1. I think the difference between the in and out Ah-readings are right. When you are charging the amps are lower and when you discharge the amps are higher.
    I would change from measuring the Ah to measuring the kWh instead. You would more accurate data of your energy consumption.
    I noticed this in my own project by draining my 100Ah-pack empty with JLD404 showing only 88Ah... Lucky me it was only 500m from my home.

    1. In an ideal world both Ah in and Ah out should match, but as it's not very simple to accurately measure frequently changing 0-600 amps out and a steady 20 amps in on the same shunt, you never quite get there. I did mamage to make it match a little better already by removing the extension I had made to the Cycle Analyst shunt cable and also moving the shunt right on the controller minus input, which may or may not make a difference. Unfortunately Cycle Analyst only counts Wh out of the pack, but not in, so I mainly end up using just the Ah display. I haven't yet tested how many Ah out it would now read after full discharge or how much the new tires have improved my Wh/km, which I'm quite sure they have.