Well thats not really true, as the total coil turn ratio is probably around 1:4000 to actually turn 5V DC into AC and step it up to 20KV. (its actually made by first stepping up the 5V DC into a much higher potential AC by means of a small inverter in the ignition unit then stepping up that AC potential up to 20KV).

10KV can jump thru a couple of centimeters creating an arc discharge. there would be not much of a difference between using a 6V pack or 3.7V pack. both will create a suitable arc if they were stepped up directly without loss. The main reason the ignition unit wouldnt work below 3-2V DC is the inverter side electronics are actually cant work with such low voltages. If the transistors were chosen to work with those low voltages, we wouldnt have any problems using a single nicd cell. This is my theory. I actually never opened up a chinese ignition unit.