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The voltage is only the 'pressure', it does not reflect the amount (amps) of electricity in the battery. Fair chance that the plates are pretty well sulphated. Have used one of these 'battery conditioners ' in the past and suffered exactly the same fate as yourself. A normal lead acid battery thrives on being discharged very slowly and then re-charged slowly, the to-and-fro effect keeps the plates quire clean. If there's anybody on here who's as ancient as me they might recall when your elderly aunt had a radio that ran on an accumulator which was just a lead acid cell. The radio used to discharge it very slowly, and when it was flat the man used to come round and swap it for a fully charged one. They lasted for donkeys years.