While older aircrafts generally used a 12-volt power supply, the aircraft being built today use a 24-volt battery. These aircraft batteries are actually comprised of a dozen 2 volt cells, which means a lower cost and smaller wire size. The latter is particularly useful, as one can transfer more power through smaller wires with less loss via heat or resistance when you use the higher voltage. Generally, if one wants to charge this type of battery, you will need a GPU or other external power source that is rated at 28.5-volts. This might seem confusing to the layman that doesn’t have experience with this type of system, but the starter motors or generators on turbine generators are generally rated to accept as much as 30 volts without being damaged, which is why the voltage rating on a portable power unit might not always be exactly what one would theoretically expect if they were just playing the numbers game..
Another issue that must be kept in mind with an aircraft engine or military tank starter is the potential voltage drop that occurs as the load applied to the battery increases. When the starter is first activated, this is when the maximum amperage draw is achieved. The amperage requirement drops during the start cycle, but the battery needs to be in good condition if you want to minimize voltage drop. A 28.5-volt start is also recommended if you want to reduce voltage drop.
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