Touring Machine Company

Because the aircraft industry standardized on the nominal CHARGING voltage of 28
volts rather than the DISCHARGED voltage of 24 volts. 24-28?? Same animal with
a different nametag.

Now, why 24/28 volts? Because the aircraft needed to be lighter for military
performance reasons. Two 12 volt batteries in series comes nowhere near the
weight you can save in a fairly complex airplane (say, for example, a P-51) by
using a lighter weight copper wire for the same wattage load (double the voltage
= half the amperage for a given wattage). Remember, wire is sized by amperage,
not by voltage. INSULATION is sized by voltage.

So why was there 12 volts to begin with? Because Detroit started making cars
with a much higher compression ratio and to turn the starters over, the old 6
volt batteries weren’t cutting it. Bingo. Two 6 volters in series gives 12
volts and that was close enough for Detroit gummint work.

The REAL question is who decided on 6 volts (3 each 2 volt lead-acid cells in
series) to begin with.

And the inquisitive student might ask, if 24/28 was so good, why not go 3 in
series and get 36 volt systems…or like the phone company with 4 in series for
48 volts? Because, grasshopper, the calculation WAS made to find out the most
efficient combination of voltage/current/wire size and at the time (WWII) it
came out just shy of 30 volts. Rather than dick around with special 30 volt (15
cell) batteries, the decision was made to use off-the-shelf dual 12 or single 24
volt “industrial” batteries.

Having twice the amount of volts means literally that the current is half with equal power drawn, see volts, amps and ohms law. As a result wiring can be thinner thus you will save some weight.

Another advantage is that 28 volt systems have more reserve in cold weather where the 12 volt battery looses its power more quickly. Not too mention the fact that a 24 volt battery has a lot more cranking power for starting. Which is really helpful when starting small turbine engines.

The choice of engine usually dictates which kind of electrical system you will need. Rotax engines are sold with a 12/14 volt system (starter and alternator). Other engine manufacturers might have an option for either system. If you already have an engine: check its battery, alternator and starter motor to see which system it is.