2nd back Up Battery

Because I happened to have an extra Earth-x battery I Installed it behind the pilots seat in my Kitfox Super Sport (aircraft has a Rotax 912iS engine). It is not used for starting because of wire size, but can be used to power the computers. It is connected to the main 12v buss with a 20 amp switch type circuit breaker which is normally in the off position. After take off I turn the breaker on to" top off"  the backup battery and then turn the  breaker off. The voltage level of the battery shows up on my Advanced 5600 EFIS as voltage number two and is read about every 2 seconds. I repeat, the main reason I installed a back up battery is because I happened to have one sitting on the shelf unused. I feel the chances of having to use it are very low. Because of the 912iS dual generator system the main use of the back up battery would be to power the buss during an emergency landing resulting from a complete engine failure . I suspect the plane will be on the ground long before the main battery would have expired. In the event the B alternator failed the addition of the back up battery would increase the time which the 12 volt buss would have power for essential electronics while flying to the nearest suitable landing field. In addition the EFIS and one of the GPS units both have their own back up batteries.