rockford fosgate T-15001bd is the good one, but i dont think your budget will do, 12.5- 14.4 volts, but you already have it, a 2 parad capacitor will help to play, another battery will be an additional load to your alternator, so dont make it hard for it.. if you have the budget, replace your main battery by some high current one like powermaster, it would be better.