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Old 10-03-2009, 04:43 PM   #111 (permalink)
The $500 Electric Car
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Originally Posted by mcmahon.craig View Post
No, it will be the LOWEST of all the batteries. In fact, it's the worst CELL.

Say you have two batteries connected in series. One can provide 20Ah to an EPV to 1.7Vpc, and the other only 15Ah to the same point.

When you discharge the batteries, the cell voltage will obviously drop. At 15Ah the voltage on the 15Ah battery will be 1.7Vpc (about 10V), but the 20Ah will be more like 11.5 volts.

At this point, if you discharge the batteries further, the lower capacity battery WILL be damaged. If you attempt to retrieve the last 5Ah out of the good battery, you will probably reverse a cell in the bad one and destroy it.

If you're going to use mismatched batteries, please at least use one of these to avoid exploding batteries. You could also perform a measured discharge (using a known load, like a headlight) to 10.5V to ensure your batteries are at least somewhat matched.
I think JasonG realized the weakest battery (or cell in that battery) is the Ah capacity of the pack; I was the one being optimistic that my weakest battery will deliver 75 Ah. Now, I was thinking about buying a load tester, but there is something wrong with that sentence: the word "buy". I've already spent a couple HUNDRED dollars on this, and I am WAY over budget, as NONE of the ICE parts have sold as I was expecting. My depression level on this project is peaking, but now I am out so much money, that I really can't afford to spend, so that I pretty much HAVE to finish. Anyway, my "load tester" is my daily commute car. I have been systematically using each battery in my pack to get to school every day since I got them. So far, so good. But I will say this: if I don't get 6 good batteries out of the eight, this project is finished....

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