The cheapest way to measure the battery health is to put a load similar to
what you will be using and time how long the voltage is adequate to operate
your equipment. you do not have to be a techie to do this - just a simple
understanding of ohms law.

Depending upon the size of the battery and the battery current you wish to
draw, an example that we will use would be a Radio Shack 20 watt, 8 ohm
resistor catalog number 271-120. Current draw is calculated by using ohms
law: I (current in amperes+ = E (voltage) / R (resistance). In our example,
I = 12 / 8 = 1.5 amps. Power consumed would be P = EI = 12 x 1.5 = 18
watts. That means the load resistor will get hot under test, so keep it
away from things that will melt. Make sure there is good circulation around
it.

Take a cheap digital voltmeter (expensive ones will work) and monitor the
voltage from time to time. Be sure to note the time you start the test.
Stop the test when the voltage gets down to the lowest voltage you can use
in your aircraft.

Let us say you start the test and 4 hours later the voltage gets down to
10.25 volts and you terminate the test. You will have drawn 6 amps out, so
you now have a 6 amp battery. Often batteries are rated to go down to a
lower voltage than you wish to operate them at, so the new amp rating may
not be as much as advertised.

If you are really cheap, you can use an automobile headlamp or taillamp and
hang it on the battery and just watch how long it stays bright. You will
have to know how much current is drawn, which you can sometimes look up in a
database, or measure with an ammeter.

When doing tests like this, a little math gets you a long ways. This is a
somewhat simplified explanation, since the actual current drawn depends upon
the voltage and as the voltage drops down, the current draw will decrease -
but that is the same thing that will happen in your aircraft.

Colin N12HS