24 Volt vs. 28.5 Volt Power Supply Tutorial
A question that is often asked is why are the two voltage values sometimes used simultaneously? Having both means that some confusion is created when dealing with aircraft electrical systems. What follows is a simplified explanation that – with some exceptions – is applicable to most aircraft.
Almost all aircraft produced today have a 24-volt electrical system. The advantage of having a 24-volt power supply (as opposed to the 12-volt of older aircraft) is weight saving and decreased cost due to the requirements of a smaller wire size. Thus, aircraft batteries now are 24-volt units comprised of twelve 2-volt cells. To be a little more exact, each 2-volt cell actually reads about 2.125 +volts with no load or 25.5 volts for the complete 24-volt battery.
To charge a 24-volt battery, a higher voltage must be applied in order to force electrical energy into the battery. This is accomplished with the ships’ alternator/generator working through the voltage regulator to produce 28.5 volts + or -. When the alternator/generator is in operation therefore, the aircraft’s electrical system is operating at a voltage higher than 24 volts. In this situation, the electrical system is maintained above 24 volts even under normal load.
GPU and Portable Battery Packs
As a load is applied to the battery, the voltage will drop as that load is increased. For turbine starting purposes, it is ideal if this drop is kept to a minimum.
Most starter motors or motor/generators on turbine engines are rated to accept 30 volts without damage. When first activating the starter, the highest amperage draw is experienced. This amperage can be over 1000 amps on some gas turbine start up engines. As the start progresses, the amperage requirement decreases throughout the start cycle. As a result of the high loads involved in a start, a battery must be in good condition so that voltage drop is minimized. Of course, at lower voltage levels, the battery will put out less amperage. Some starter motors will pull a good battery down to 15 volts during the first phase of the start.
Most engine manufacturers (turbine) now recommend, if available, a 28.5-volt start in order to minimize voltage drop. Normally, this is accomplished with some sort of external ground power unit (GPU). In this scenario, it is best for the turbine engine if 20 percent (or more) energy is being delivered to the starter motor, resulting in a faster, cooler start, thus increasing turbine life. Additionally, from a maintenance point of view, by using 28.5 volts, various aircraft electrical components under load can be checked out above 24 volts.
In conclusion, it’s fair to say that it’s always a good idea to opt for a 28.5-volt start, if available. Start Pac® offers two true 28.5-volt battery packs, utilizing 14 cells.
28.5 VS 24 Voltage Drop
See the following graph comparing the 28.5-volt START PACs® versus the 24-Volt START PACs®. As is clear, higher voltage is available throughout the start cycle on the 28.5-Volt models.
All batteries suffer from being in cold temperatures. When exposed to extreme cold, aircraft batteries will not supply enough energy to start a turbine engine. Cold temperatures have less affect on the high-tech lead acid batteries used in START PAC® units than on most aircraft batteries. However, even these high tech batteries will perform less well in colder temperatures.
In most cases, battery life is cycle dependent. The more the plates in the battery are stressed or cycled, the faster they wear out. The worst case scenario would be if they were cycled from fully charged to nearly depleted; with this kind of usage, around 400 cycles can be optimistically expected. However, if only a small percentage of the capacity is used at a time, up to 1200 cycles can be expected.
It is most important to recharge the battery as soon as possible after use in order to prevent sulfation; this condition will greatly reduce the capacity and output of the START PAC®. Also, it is important to avoid depleting a battery completely as sometimes it will go into a state of reversal and will be damaged beyond repair.
If properly maintained and operated, START PAC® batteries will last for years before requiring replacement.
Battery capacity verses Voltage Drop
It’s a common misconception that two batteries of the same voltage but different capacity would have an equal voltage drop under the same load: not always true. In fact, the battery with the larger capacity will have less voltage drop due to its larger plate surface area. Since the load is essentially spread out over a larger area, the battery chemistry is less affected. Please refer to the following graph comparing all 24 volt START PAC® Models. If you require many repeated starts between charges, it always pays to choose the unit with the higher capacity.
Thoughts on Voltmeters
The voltmeter is an essential tool used to aid the pilot when starting turbine engines. Any battery or GPU will have a normal voltage drop when initiating a start. If the pilot sees an abnormal voltage drop, they will abort the start so as to avoid a “hot” start. This illustrates how a voltmeter can be used to determine battery condition and a low state of charge only once the battery is put under load. If not under load, even a depleted battery will read as being at almost full normal voltage. For this reason, START PAC® does not use a built in voltmeter; it is not an indication of the state of charge and it also adds to the cost. There are some electronic indicators that can measure the state of charge but these are very expensive. The voltmeter belongs in the cockpit, not on the GPU.