I found my answer:
Q. Do you need a Basement Watchdog battery?
A. You can use any deep cycle marine battery, as long as it is not maintenance-free. However, there are several advantages to using a Basement Watchdog battery. The Watchdog battery accommodates the system’s fluid sensor which will sound an alarm when the battery needs water…a very important feature, since a dry battery will not perform. In addition, Basement Watchdog batteries run longer per charge, and last years longer. A typical marine battery will last 2-4 years, while the Watchdog battery will last approximately 5-7 years. Basement Watchdog batteries are shipped dry (without acid) so they stay fresh on the shelf indefinitely, while wet batteries have a short shelf life.
The Emergency battery is designed only for the Emergency sump pump system. It does not have enough energy to run the larger systems very long. The 7.5 hour battery can be used on all three systems. This bigger battery will run the Emergency system 12 hours continuously, since the system contains a smaller pump. It will run the two other systems for 7.5 hours continuously, intermittently for days.