A little late with an answer but the battery is probably sulphated. Replacing the battery will make it all better but the new battery will eventually fail as well. They make battery desulphators (available on Ebay) for around $25 that are supposed to forestall sulphation and even reverse mild cases of sulphation. I am currently testing this on a badly sulphated battery to see if there is any improvement.
The main thing is to NOT discharge the battery below 50% capacity, i.e. never run with the battery indicator in the red zone and best to stop as soon as it enters the yellow zone. Recharge fully after use and do not overcharge. With my B&D CMM1000 I would plug in the charger for a day or two at the most after mowing and during the off season for a day or so once every month or six weeks. I have had the CMM1000 for over fifteen years and never replaced the battery.