Montana Cruiser
Adventurer
It's a bit more complex than 4Run is stating and the fact the other boxes are unknown is to him is testiment to that... and if you run down your primary batt to 10.8 volts, you will damage it especially if it's a starter batt.
It is a very excellent idea and you should buy one from me.
Maybe you could be a little more transparent about what one would be buying and how they would go about buying it