most likely there is a replaceable battery inside the battery box. As Rivets said, it may or may not be of a standard size. The manual mentions a charger and there are some cables involved. I'd almost bet that even if you got it running with a new battery, and you have the charger, it would only be good to mow a small backyard in a townhome or something like that, and you'll be replacing the battery every two or three years. Just a guess.
The part number of the battery shows up on at least one site as no longer available so a battery store will be your best bet. It is probably a motorcycle battery or something like that. My old computer battery backups used small batteries like that back around 2000. That is what I base the life expectancy off.