You check your cells with an acid meter. A little squeeze bulb with an acid meter which measures the acidity of each cell. Right on the meter it tells you how many floating balls equals the acid level of each cell. It may look like the battery is sealed, but if you investigate further, most are not. If the acid is below any of the plates in the cell, there is a great chance the battery is toast. I have seen many a battery with four of six cells great, it only takes one bad one to screw up the battery.
PS Meter = about 5 bucks at any auto store, good luck.
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