Assume you're talking chassis battery and not engine. Others have much more experience than I with this. But I would give it a full charge, disconnect, let sit for 24 hours, take a volt reading, compare to mfgs specs. Read again after a day, and again to see how much it was dropping daily. With my limited knowledge a continual drop that was too large (again against mfg specs) might indicate the battery's inability to hold a charge and suggest the need to replace.