Symptoms of Imminent Failure
To know how well a battery is performing, you need to know its testing method. Armed with a voltmeter, check out its voltage output, with and without applied load. If it drops substantially below 12 V without load, you can be assured that there is a dip in performance. A major reason for the drop in performance is sulfation, which is crystallization and deposition of lead sulfate inside the battery, in the course of operation.