The Cub Cadet comes installed with a 12-volt electrical system. The system requires at least 6 volts to actually get started, but the system is built to accept up to 12 volts total once the power moves through the system. Anything more than 12 volts can cause damage to the electrical system in your Cub Cadet.
The Cub Cadet is meant to accept a 12-volt battery because that's the biggest amount of power that the tractor can handle. However if you can fit a 6-volt battery into the tractor and secure it the same way you would a 12-volt battery the system will still start as long as your battery puts out at least 6 volts.
If you're not sure whether or not the battery you want to use in your Cub Cadet will work, check your owner's manual. The manual will have a list of parts that are recommended and acceptable for owner's that do their own maintenance. The basic battery that is recommended for a Cub Cadet is the 12-volt battery with the serial number 224401, but other parts may be listed as acceptable.
As with car batteries, your tractor battery will eventually run out of charge. Rather than replacing the battery right away, you can charge the battery. Hook up your battery to an appropriate charger according to the instructions for your particular model and then wait for a full charge to be restored to your battery.