If you really just want to carry an extra battery and want to make sure it's charged at all times then I would recommend one of those expensive light weight ones on KTM twins. Mount it in the underseat storage and run power cables from the main battery through a battery isolator to the battery under the seat. Then have jumper cables for that oh shit moment. You would only need to run two wires so setup would be a breeze.
Or make sure to run accessories like heated grips through a circuit that shuts off with the ignition.
Also load test your battery.
-Chrome don't getcha home, but carbon fiber sure is fancy
My real name is Chris to stop the cornfusion at the RTE's