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Hugh, You may be interested in this article "A critical review of using the Peukert equation for determining the remaining capacity of lead-acid and lithium-ion batteries" by Dennis Doerffel and Suleiman Abu Sharkh from the School of Engineering Sciences, University of Southampton. It is available from http://www.sciencedirect.com/science for a small fee. If the link does not work, just search for the lead author's last name. The authors describe testing lead acid batteries at high rates of discharge from fully discharged down to the point that the terminal voltage is 10.0 volts. After letting the batteries rest, they continued to discharge further at a lower rate until the terminal voltage was again 10.0 volts. Results were compared to discharging at the slow rate only. The total amphours delivered when a low discharge rate follows a high discharge were less by 5 to 10%. With 10% associated with a C2 and C/20 discharge of a 17 AH battery and 5% associated with a C/1.2 and C/13 discharge of a 65 ah battery. In short, the capacity loss indicated by Peukert only applies to a continuous discharge rate. When a slow discharge follows a rapid discharge, the total number of amphours delivered is almost the same (just 5 to 10% less) as if the discharge happened at entirely at the slow rate. If you were estimating how far your electric car would travel, that 5 or 10% may be critical. For the rates of discharge and depths of discharge normally used for off-grid homes the "lost" capacity is probably even less. Kent Osterberg Blue Mountain Solar, Inc. |
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