Monday, January 12, 2009

Battery voltage vs. charge

Battery voltage FAQ

This FAQ has a nice table equating average battery voltage with percentage charge remaining.
According to the table (and extrapolating for 96V):

100% = 2.12V/cell = 101.76V
80% = 2.07V/cell = 99.36V
50% = 2.01V/cell = 96.48V
20% = 1.93V/cell = 92.64V
10% = 1.89V/cell = 90.72V
0% = 1.75V/cell = 84V

This is not to be confused with low voltage under load, which is during current draw. In those cases, voltage may (and does) dip temporarily to 72V or lower (on hills or when accelerating).

Today during the 8.6 KWh test drive, I periodically took readings of average battery pack voltage. If I use the above table and my recorded voltage measurements:

5.1 mi => 80% charge left
13.3 mi => 50% charge left
18.4 mi => 20% charge left
24.7 mi => 10% charge left

Still reconciling this conclusion with the theoretical data. At one hour of use, my US125s are capable of 145AHr, and this equates to 96V*145Ahr*0.80 = 11.1 KWhr should be my actual capacity. So it looks like with fast driving, I get about 24 miles on a charge.

More testing later this week.

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