most important questions - how old is the battery and have you had it from new. Batteries can be lead-acid (sealed) or the gel type. If the former, and also if the battery has the facility to unscrew the 6 screw-plugs (assuming it's a 12volt) it's then possible to check the specific gravity of the acid. If you don't have one, Halfords sell Hydrometers (with instructions) to measure the SG. If the gel-type, this test is not possible. The battery should show the Ampere-hour capacity. Say, 30 Amp/hr, this would deliver 5 amps of current for about 6 hours when new. In a few months this capacity reduces and after 24 months can be down to about half the capacity. This is why the manufacturer usually gives only 2yrs.warranty. Not trying to be too technical, hope this info helps.The charger will show "charged" as the battery has taken "all it can" not all "it could" when it was new.