check your ground, black wire going to the engine or frame, if that is corroded or bad the charge is being lost that way. The battery may have a surface charge which would allow the battery charger to say it is good, however, if you load test it, ie try to start it, it is not good. New battery should only run about 60-90 dollars. I would buy one. Oh ya, things that sit for long periods, boats, tractors, atv's will end up rendering batteries unusable faster.