Re: Charge controller 24V to 12V
You should be able to use any 12/24 volt PWM charge controller. It will have the input voltage rating to work with "24 volt" panels (really Vmp~35 volts) and will regulate to 12 volts (if programmed correctly) for your battery bank.
There is really no "voltage conversion" going on... The battery bank simply "clamps" the array voltage to ~14.5 volts or so while charging. Then the charge controller turns off and the array voltage goes back to Voc-array.
The Xantex C40 is rated to 125 VDC maximum input voltage (and 12/24/48 volt battery banks).
20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.