A diode will look like a little black cylinder with a band printed on one end. Install the diode in series on the positive lead from the battery with the band towards the ignition module. For the A123 you have the option of installing TWO in series if you're worried. This will result in about 6.1 V at full charge. Cover the whole thing with shrink tubing. I just cut those big 1/4" dia diodes from old circuit boards. radio shack was good, but they're sadly gone. Try Digikey.com. Make sure they're rated for at least twice your voltage, both forward and reverse. Also they must handle the current and wattage. Don't try to cut it close, over-rate the crap out of it. It won't hurt anything. Even a 125V diode will work.