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Simply measuring battery voltage is not a true indication of the state of a battery. A battery test has 2 aspects to it: There is the voltage of the battery (which is a rough indication if the battery is fully charged). Then, there is the ability of the battery to hold and deliver the power it has stored. I.e. the voltage could show the battery is fully charged but the battery could be badly sulphated and useless. You need to test the voltage of the battery under a heavy load condition.