(ET) re: battery testers

["Wombat" <Wombat RealNS com>]

First, a battery tester is basically a resistive load bank (usually what is
called a carbon pile) that draws a specific amount of current.  The
generally assumed correct test is to load the battery at some level until
its voltage drops to 10.5 volts.  The resultant time is its capacity.

You could build a simple tester by using a resistor (auto headlamps work
nicely in parallel) clamped across the battery and timing the run by
monitoring with an accurate voltmeter until the voltage drops to 10.5 VDC. 
Don't forget to also check specific gravity both before and after, a few
points difference can make a big change.

Neil