When you say testing, are you referring to determinining the
current state of the battery or the capacity of the battery. Determining
the current state of charge can be done by disconnecting the battery
from all loads, letting it sit for 24 hours and then testing the voltage.
For a liquid lead-acid battery, a full charge will be 12.8v, half
charge is 12.2v. For gels 12.6v and 12.0 are reasonable figures.
As Larry says below, checking the S.G. with a hydrometer is
standard practice if you have liquid batteries. Needless to say that
this can't be done with gels and AGM.

If you are trying to determine the capacity of the battery then a
slow discharge is needed. The manufacturer usually rates the
capacity based upon the 20-hour rate. So it you have a 200AH
battery then discharging it at 10 amps will discharge it in 20 hours.
(discharge is defined as 10.5v in this case). No real need to go that
far, but taking it down to 12.2v would be half capacity and should
take around 10hours. This will give a pretty good indication of the
health of the battery. I usually just turn on enough lights to get up
to the current I need for the test. Of course you have to deal with
people constantly telling you that you have left your lights on.

Good Luck!
s/v Callista

"Rick & Linda Bernard" wrote in message
...
Anyone have a good way of testing 12V deep cycle batteries?

What I was figuring on doing was using a 50watt 12V lightbulb and
measuring
voltage with time. The only problem is that this may take a few hours.