you have to pic a current that u think you need as the comparitor will output if its inputs are either higher or lower than the referance exactly.
maybe use lightbulbs for now, a 100w bulb is approx 10 ohms (easy calculations ) use 2 , 4 bulbs whatever, this way u can calculate the current you are drawing.
put a sensing resistor say 0.1 ohms or so in series and measure the voltage across it, or mathimatically calculate it( perhaps both to see how close things are.
use a resistor dividing network, or a zener diode (better) as your voltage refference.
get it as close to the volts across the dropping resistor as possible.
if the comparitor has an inverted output you can use this to drive the gate of a pass mosfet, as it will always be high when the comparitors input is not matched and the fet will conduct.
add and subtract a few lightbulbs to adjust the current to prove your circuit before hooking it to the h-bridge. Bingo .... it should work .... good luck
i did the same type of thing in my 1988 dodge minivan and saved buying the $600 SMEC (single modular electronic controller).(current to the altermator)