The black thing is probably just a box with a couple of resistors. It may have a switch to turn the lights on and off too (seems like a good idead instead of having to unplug and replug the battery to turn them off and on). The only thing you need to regulate power to the LEDs is a resistor (from the looks of it they have one for each pair of LEDs - you would have to have at least a 6 cell battery pack to pull that off and they would probably still be dim). If you have a resistor for each LED you can run off of far fewer volts. (I just posted a longer explanation in your other LED string).