Atthough I mentioned a simplified method of battery test, once you
understand the procedure, you can measure the battery voltage under load and
after the initial few seconds, you can plot the discharge voltage vs time.
So, if in five minutes, the voltage drops .03 volts, we know the voltage
drop is linear, so in 60 minutes the voltage will drop 12 x .03 or .36
volts.

As Andy mentioned, it can be computerized.

Interestingly, the ability to hold a charge also means the resistance to
change. So, if you see a battery on the shelf and put a charger to it and
the voltage soars instantly to full voltage, it is bad.

Once you understand what is happening, you can usually tell if a battery is
good within a few seconds, and know the projected life in a minute or less -
all with minimum equipment. A lttle knowledge gets you a long way. In
fact, you can often hang a auto lamp across the battery with a digital
voltmeter and watch the voltage drop. The less drop, the better the
battery.

Colin