[FONT=Arial, Helvetica, sans-serif] A good battery, after a full charge, should read approximately 12.8 volts (open circuit with nothing connected to battery) or pretty close. Float voltage @20C (voltage with the alternator charging the battery) should be about 13.6 volts. These voltages vary more than you would think with temperature and battery chemistry (the type of battery e.g. gel cell, flooded, AGM). A few tenths of a volt is the difference between a "good" battery and a "bad" battery. Low open circuit voltage on a fully charged battery = inability to deliver any substantial current. Use the manufacturer's tables and a GOOD voltmeter (millivolt resolution and accuracy). Manufacturers of high quality batteries will be able to provide the voltages, maximum charging current and charge times, With cheap batteries you will be pretty much on your own. Batteries of a given type, such as all flooded lead-acid batteries, will have roughly the same characteristics so you might be able to "fudge" by using the data from a different manufacturer.
Warning: cheap batteries are no bargain[/FONT]