Take Charge!
The easiest way to
determine a battery’s
capacity is to fully charge
it, then discharge it at its
mAh rating until the cell
voltage equals 1.0 volts.
The mAh rating = (Average
Current) (Time it takes to
reach a cell voltage of 1.0
volts). If you don’t know
the battery’s mAh rating,
then just use an estimate
for the discharge current.
Most power tool battery
packs range from 1,200
mAh to 2,000 mAh. I used
a load that would average
approximately one amp.
The actual rate used
would only be critical if
you were manufacturing
batteries. For our
purposes, one amp will
work just fine. For AA, C,
and D cell rechargeables,
however, I would
recommend 0.25 to 0.4
amps. Keep in mind that
your load resistors will be
dissipating a lot of power
— more than 20 watts in
some cases — so you will
need to heatsink them or
use a fan to provide
cooling. I used eight Dale
50W two ohm aluminum
housing power resistors (in
series to achieve 16 or 12
ohms). I mounted them on
a large aluminum plate
using heatsink compound
between the resistor and
the plate.
Here’s an mAh
calculation example:
Estimated Average Current
(I) = [(Initial Battery
Voltage + Ending Battery
Voltage) / 2] / Load Resistance. If IBV = 18, EBV = 15,
Load Resistance = 16 ohms, and Time = 1.5 hours,
then the Average Current (I) is 1.03125 amps and the
mAh capacity is 1,547 (Average Current (I) Time).
The actual average current will be slightly different due
FIGURE 2. Excel plots of battery discharge curves.
FIGURE 3. Actual data in Excel used to create discharge curve plots.
FIGURE 4. Battery discharge curves showing comparison
between NiCad and NiMH batteries.
discharge (see curves shown in the figures). For our
purposes, this calculation will be sufficient to get a
good estimation of the battery’s mAh capacity.
In the enhanced test setup, I measure the actual
current using an additional A/D (Analog-to-Digital)
channel. You really don’t need to do this as the extra
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