You can measure it either in watt-hours or in ampere-hours. The first is
the true energy delivered, and would be the area under a voltage-vs-time
graph at constant discharge current. If current isn't constant, you
would have to measure the voltage and current at each time interval to
be rigorous, plot the product of V and I vs time, and integrate that
function.

However, capacity of NiCd and NiMH cells is just about always specified
in ampere-hours, or milliampere-hours, since the discharge voltage is
fairly constant anyway. That can be measured by simply discharging the
battery at constant current and multiplying by the discharge time. If
the current isn't constant during discharge and you wanted to be
accurate, you'd have to measure the current at various time intervals,
plot that against time, and integrate the result. Of course, a simple
rectangular or triangular integration would be simple to do even with a
spreadsheet, or a very simple program in the language of your choice,
and would be entirely adequate for the job.

But because a NiCd or NiMH cell voltage stays pretty constant between
1.2 and 1.25 volts during the majority of the discharge period, you
could also discharge it with a resistor, then estimate the average
current by assuming a voltage midway between those values, and simply
multiply by the discharge time. That would be close enough for most
purposes.

1.0 volts is the usually specified cutoff for NiCd and NiMH cells. When
the cell voltage reaches that value, there's very little energy left, so
the voltage falls very rapidly beyond that. There's actually very little
energy left at 1.1 volts with a normal cell, but one suffering from
voltage depression (the so-called "memory" effect that's cured by
discharge to 1.0 volt) can deliver quite a bit of energy at 1.1 volt.

Roy Lewallen, W7EL

Bruce W.1 wrote:
Say for a NiCad or NiMH battery, how is battery capacity calculated?
Say I put a resistor across the battery and measured the voltage
periodically. Is it the area of the curve above 1.1 volts, 0.9 volts,
or what?

Thanks for your help.