Long before rechargeable nickel-cadmium (NiCad) and nickel-metal-hydride (NiMH) batteries were used for starting model airplane engines, we used standard primary type (non-rechargeable) 1.5-volt dry cells. In fact, the nominal 1.2-volt-per-cell potential of NiCad and NiMH batteries were barely high enough to sufficiently light the ignition coils in the glow heads and/or glow plugs, which were designed for 1.5 volts. Today's glow plugs work just fine on 1.2 V or 1.5 V.
I have written before about how as a kid on a very small modeling budget, I would often spend a long time flipping the propeller of my Cox .049 engines while using a single, worn-out D-cell battery (usually 'borrowed' from my father's only flashlight). One Christmas my parents got me a field kit that included a can of 25% nitro Cox fuel, a glow head clip, and a starting battery similar to the one shown here. This battery came from eBay along with a plastic model I bought from someone. It is now on display with the rest of the items in my collection of 'stuff' I used to have as a kid. Of course all of my original stuff was destroyed or lost over the years due to poor flying skills or neglect.
You can see in the photo that the Cox model 789-3 starting battery was comprised of three 'D' cells wired in parallel to produce three times the current capacity. If you are not familiar with how a glow plug works and why the glow element doesn't burn out when more current is available, here's why. The element wire it nichrome or some alloy thereof that is very sturdy and able to handle high temperatures. It has what is termed a positive thermal coefficient of resistance, meaning that as the temperature increases, so does the resistance. As long as the voltage supply is not elevated past the designed operating voltage, current draw is limited by the temperature of the element. If fuel on the element decreases its temperature, the resistance decreases and the current, if available, will increase until the element temperature is back to normal. That is why having a power supply with plenty of current capacity is important for getting an engine started even when it is partially flooded. In that way, the glow plug element acts sort of like a self-regulator to keep the voltage constant.
It is ultimately the current through the glow head element that will cause it to burn out (fuse), so increasing the voltage too much can cause enough current to flow to fry it. The more sophisticated field box power panels with built-in glow plug supplies use a pulsed current output to keep the element at a constant temperature.
Those three leaky D cells have been discarded, BTW, but the wires and glow plug clip attachment Fahnestock clips are taped in place for display purposes.
Posted January 7, 2017