Hi,
Just to get the ball rolling, you might want to note that 8 amps at 12 volts is 96 watts, so your load will have to be able to handle that without a break down.
You also will want to note that you'll need a low voltage cutoff because you dont want to discharge it too far down.
One traditional way to do this is to with an op amp and transistor. To get to the 8 amps you'll need at least two transistors, one to drive the other. The bigger one has to be able to handle the current and the power and will need a heat sink.