Go look at the ratings on carbon pile lead battery testers. Below 11v on a few 
hundred amp test and the battery's marginal. In the shop, the old test was to 
crank the engine (even the biggest ones) and watch the voltage. If it dropped 
below 11v, it may be time for a new battery. Even then, it should pop back up 
to 12.5v. If not, bring out the replacements.

Batteries for smaller gensets and mowers are usually undersized and deep 
cycle/starting, so they can tolerate lower voltages without necessarily being 
bad. It's not just state of charge, but internal resistance/plate condition. A 
new battery shouldn't go much at all below 12v at starting if it's sized right 
and under a minute.
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