In article ,
says...
I guess a 12v automotive light bulb would be a readily available load?
Perhaps one of the electronic wizards here could suggest an
appropriate wattage for a meaningful, safe, fast test?

The smallest dome light or similar from an auto store ought to be
close enough to a glider load that isn't running a transponder or a
PDA with a backlight. Measure the amp drain, see how long it takes to
drop down the lowest voltage you can use, then (time x amps)/(actual
glider amps) will give a good indication of your expected battery
life.

For maximum battery life, fully recharge after every flight, or at
least no later than about 50% depleted.

Also, is it valid to do a 'fast' test when a glider load is hopefully
closer to 10 to 20 hrs? Can we presume that if the battery can cope
with a 'fast' discharge then it should be able to handle a slow
discharge rate?

It's "valid", but it will understate the amp hours you can get at the
lower load. The data sheet for the battery will give you the capacity
at different rates, so you can adjust your measurement for the lower
rate in the glider.
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Eric Greenwell
Richland, WA (USA)