i've always wondered this aswell. it is possible, my brother did it when i was a lot younger and the bass and everything was sweet man, you have to manually wire everything up though.
if i had to guess i would say find the live for the HU and connect it to the +ve of bat and the ground to the -ve. everthing else after that should be the same.
i have a friend at the moment who is at the moment trying make some sort of a solar powered recharging system for a car bat, might be something that you could look into aswell
