Load the battery to 1/2 the cold cranking amp specification for 20 seconds, or find someone who can. With the load still applied, the terminal voltage should not drop below 9.5 volts. A good battery should crank the engine with the ignition disabled for 30 seconds without the terminal voltage dropping below 9.5 on a 12 volt battery, or 4.5V on a 6 volt. That's been the industry standard for over 50 years.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!