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Hello Andrew:

The capacity of a battery is typically defined as the time for the
battery to discharge to a specified voltage when loaded such that the
current is the nominal Amp Hour capacity divided by 20. For example a
100 AH battery would be discharged with a 5 amp load.  If the starting
voltage was 13.6 Volts and a 2.7 Ohm was used the battery would be at
12.0 volts after 20 hours.  Rather than wait 20 hours you could apply
the 2.7 Ohm load a make very precise measurements of the voltage and
plot the slope of the line.  Where the line intersects 12.0 volts gives
the time to discharge.

Note that if the load is changed from AH/20 to some other value, the
capacity of the battery will change.
For example if you discharge the 100 AH battery at a 20 amp rate it will
be a 12.0 volts in 2 hours, not the 5 you would expect.  If this effect
is characterized for a given battery type, then you could use a load
other than AH/20 and compensate the extrapolated time to the AH/20 time.

Have Fun,

Brooke Clarke, N6GCE
http://www.pacificsites.com/~brooke/batt.shtml

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I would recommend using a constant current load. Using a simple resistor, as
the terminal voltage falls, the current also falls.

Regards

David Huisman

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