Battery capacity checker

Get a 10 ohm power resistor (25W) and a digital voltmeter. If you
don't have a digital voltmeter - BUY ONE; they are a must have tool!
Connect the resistor across the battery and start recording the
battery voltage. The discharge current is V/R (Ohm's law), and is
about 1.2 amp. Your typical battery is 6 A-hr, so your test will be
over in about 5 hours (less if your battery is tired). In any case
stop the test when the battery voltage drops below 11V. Your
incremental value per reading is:
(Time interval [min] / 60) * (Voltage / Resistance)
Add these incremental values together to get the A-Hr capacity of the
battery.

If you don't want to spend $100 on the ELK battery tester (or have 6V
batteries like I do) buy a 2nd resistor. Attach the first resistor and
measure the battery voltage. Do the same thing for the second
resistor. You will have to compute the following:

I1 = V1/R1, I2 = V2/R2

deltaI = I2 - I1
deltaV = V2 - V1

mhos = deltaI / deltaV

Mhos are the inverse of ohms, so more is better as far as a battery is
concerned (more than 100). Check a new battery for a base-line value.
The resistors should load the battery close to its operating point.
The resistors could be a single power FET with a variable voltage
control. Suggested resistor values: 10 and 20 ohm (10W min):

http://www.digikey.com/scripts/us/dk...534&Row=282775
http://www.digikey.com/scripts/us/dk...090&Row=279318

You can also parallel 10 100 ohm 1W resistors to get a 10 ohm 10W
resistor.

Tom