You can still salvage the system as is.
Use 24V battery, but insert a 12V SMPS at each point that has the controller on.
Provided the point does not draw more than 1Amp you can use this device from Mantech - 7812 which is very efficient and has very little loss. I suggest you add a 10uF 25V capacitor on the input side and on the output side. Easy to connect, Volts In, Ground/0V, Volts Out. You will need a soldering iron.