Batteries are mysterious objects, and cause a multitude of troubles. The only true method of testing, to see the state of condition of a battery, is to charge it properly and then LOAD test it. Voltage tests and specific gravity tests only can only tell you that it is junk, not how good it is. in other words a battery can still pass these test and still not have the amps to start a machine.
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A LOAD test consists of drawing a set amount of amps(equal to what a good starter would draw) out of the battery and then monitoring the amount of presure (volts) the battery is able to maintain for a period of time(cranking time).

Most stores that sell batteries will preform this test free of charge, if you bring in a properly charged battery.

Dirty or poor conections can also act like a bad battery.

A bad starter can also act like a bad battery by drawing too much amperage or too little amperage.

Hope this helps and good luck