Long before rechargeable nickel-cadmium (NiCad) and
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.
The cardboard box size is 4-0" high by 2-1/2" wide by 1-3/8" deep. External battery connection
terminals are not included.
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
are taped in place for display purposes.
Website visitor George A. wrote to ask for the dimensions of the Cox starting battery
box so that he can create one using his printer. He also needed high resolution images of
all sides. The photos below show both sides of the flattened box, along with a ruler for scale.
The scans have not been edited except to move the terminal clips closer to the box to keep
file size down, so edit color and sharpness as you deem fit.
Posted January 7, 2017