Bill, I use two 6 volt batteries And two switches, I use the switches with the charging jack built in with the switch. I use the smallest diode that radio shack has. the battery is plugged into the switch,now the switch is plugged into the recv. the red wire going to the recv is the one you cut and put the diode in. That keeps one battery pack if it shorts or gets a bad cell from drawing your second pack down . I have tried it without the diode and believe me if one battery goes bad it will draw the other pack down. The diode does use up a little voltage but not enough to worry about. Be sure to put the diode in the right direction,voltage will only pass thru in one direction.