I received a phone call recently from a person who needed help in converting his aircraft electical system from 24v to 12v. He wanted to do this, he said, to save weight. Its not the first time I have heard this. Its rather odd though, because if you research the reasons that Piper and Beech changed from 12v to 24v, they cite the desire to save weight!
To understand the problem, there are a few simple facts we need to understand:
To start an engine, it takes some amount of mechanical power. I have no idea what that amount is, but let's take a guess. Let's say that for some particular engine it takes about 2 HP to spin it up to startng speed on a cold morning. How much electrical power does it take to do that?
Turns out that is pretty easy to figure out. The equivalent of 1 HP is 746 watts, which we can approximate to 750. So 2 HP is equivalent to about 1,500 watts
Electrical power in watts is calculated by multiplying the voltage by the current. (Please don't use "amperage", it drives engineers crazy.) From this it is easy to see that (if we assume evverything is 100% efficient at least) a 12 v battery will need to supply 1500/12 or 125 amps. A 24 volt battery on the other hand will only need to supply 1500/24, or 62.5 amps to provide the same starting force.
Here then is the key. When you have a 24v system your battery needs to supply only half the current, and so can be of half the current rating, half the weight , and often half the cost. The mistake people make is to assume that if it takes a 200 amp battery to start an engine with a 12v system, it also takes 200 amps in a 24v system. Not so!
There is another benefit to the higer voltage too. The amount of energy lost to heat in the wiring and cables is proportional to the SQUARE of the current. With a 24v system the current is half that of a 12v system, so only 1/4 as much power is wasted and, the size of the wires may be reduced by a similar factor saving further weight.
Choose your battery and wire sizes carefully and a 24v system will be lighter every time.
Why do we sometimes see aircraft referred to as 28v? It turns out that to properly charge a 12v battery takes about 14.2v. To properly charge a 24v battery takes about 28.4v - For some odd reason lost in the mists of time, we always quote the battery voltage in 12v aircraft, but sometimes the charging voltage when a 24v battery is installed.