"Testing" the battery means making sure it is fully charged, then
discharging it using a known load current, timing how long it takes
the voltage to reach 11.5V, and then computing the capacity by
multiplying the time to reach 11.5V by the load current.

According to Gill & the FAA, your battery should be up to delivering
a load current representing all the stuff it takes to get safely on the
ground for 30 min: this should include engine instruments, fuel gauges,
electric gyros, radios, transponder and some lighting.

A guess for the essential electrical load on your aircraft would
be about 25A. To test the battery, connect a 12V/25A=0.48 Ohm,
12V*25A=300W resistor to the battery, and time how long it takes
the battery to reach 11.5V. If your battery wont hold up this
load for 30 min, then it should be replaced...

Cranking is a separate test. An automotive service-station type of
carbon-pile battery tester can be used to put a load of ~225A on
the battery for 3 or so minutes. If the battery peters out short of
3 min, then replace it.

MikeM

O. Sami Saydjari wrote:
I have a Gill G25 battery in my Piper Arrow III (PA28R-201T). I
recently had a situation where I ran out of juice after 4 attempted
starts. While I am sure that I can improve my starting technique (I am
a newbie for this particular plane), I would like to have a bit more
power in my battery, especially in very cold conditions such as we have
up here in central Wisconsin.

An A&P told me recently that I am "stuck" with my G25 battery. He said
that he tested it and that it was "fine."

Based on a recent article in "Aviation Consumer", I learned that the G25
has only about 225 cold cranking amps. I noted that the G35S has 250
cold cranking amps.

(1) Is it true that I am stuck with a G25 battery and that NO OTHER
battery in the world can go in my plane? I find that a little hard to
believe.

(2) If there are other possibilities, can someone recommend one that has
better cold cranking amps that the G25?

Thanks in advance.

-Sami (N2057M, Piper Turbo Arrow III owner)