Most aircraft battery systems now operate with a 24 volt rather than 12 volt battery. But with these larger battery systems, a slightly higher voltage ground power unit or portable battery pack can make a considerable difference in the efficiency and starting power of a system.
The charging system of a modern aircraft requires a slightly higher generator output to keep an aircraft battery working at full efficiency. Most 24 volt batteries require roughly 28.5 volts to be fed through the alternator and voltage regulator for full operation.
The starting operation of an aircraft can see the largest voltage drop during the operational phase, especially the initial ignition of the starter motor. A voltage drop of more than 15 percent can be experienced in the initial starting of an aircraft utilizing a 24 volt GPU or portable batter pack.
With a 28.5 volt GPU, such as those manufactured by Start Pac, the effect of the voltage drop is much less dramatic. Because of the larger voltage load in the GPU, a voltage reduction is less significant. In a 28.5 volt unit, the voltage drop rarely moves below 20 volts, even at an amperage level as high as 500 cold cranking amps. Because starter motors are currently designed to accept 30 volts at minimum, the use of a higher rated GPU should not cause damage to the starter motor.
In terms of battery life of a GPU, a larger voltage capacity can make a difference in the useful life of the GPU battery. The size of battery plates can make a difference in the number of cycles a GPU will have during its useful life.
The larger the battery plate, the less wear and tear on the plates during cycling operation. The more battery cycles, as will be experienced in smaller voltage unit, will deteriorate the system more quickly. Generally, a 28.5 volt unit is designed with larger plates and will provide a longer battery life than a 24 volt unit.
When considering a new starting unit, moving up to a 28.5 volt unit can make sense. Before purchasing such a unit, a review of the aircraft charging requirements and the load capacity of the starter motor should be determined beforehand.