Besides checking the battery terminals to insure that they are free of any corrosion and checking the acid level in the battery, the entire system, including your pumps, float switches and basins should be inspected every 3-4 months to insure they are performing properly and are free and clean of any debris that may impede their operation. Battery acid is only added once, at the initial start-up. Thereafter, if the cells are showing a low level, distilled water will be added. These batteries generally last a good 3-4 years before needing replacement. You could always take it to a local battery supplier or auto parts store to have the battery checked. They can test it and verify if it is ok and will still hold a charge or if it needs replacement.
Only certain battery backup systems are capable of adding a second battery to double the run time as the system controller must be configured to handle this type of connection. The Basement Watchdog model 24EP6 is the smaller of the two basement Watchdog batteries and would not be the recommended battery of choice even if your system was capable. You would need to consider the model 30HDC140S for such an application.