24
Volt vs. 28.5 Volt Power Supply Tutorial
Why
are the two voltage values sometimes used simultaneously
creating some confusion when dealing with aircraft electrical
systems? To answer this question I will try to put forth
a simplified explanation with some exceptions but applicable
to most aircraft.
Almost
all aircraft produced today have a 24-volt electrical system.
The advantage of 24-volt versus 12-volt on older aircraft
is weight saving due to the requirements of smaller wire
size and decreased cost. Thus the aircraft battery now is
a 24-volt unit comprised of twelve 2-volt cells. To be a
little more exact, each 2 volts cells 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, one must apply a higher voltage
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 -. Thus when
the alternator/generator is in operation 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 one applies a load to the battery, the voltage will drop
as the load is increased. For starting purposes we would
like to keep this drop to a minimum.
Most
starter motors or motor/generators on turbine engines are
rated to accept 30 volts without damage. At the time of
first activating the starter, the highest amperage draw
is experienced. The amperage can be over 1000 amps on some
turbine engines. As the start progresses, the amperage requirement
becomes less throughout the start cycle. Due to the high
loads involved in a start, a battery must be in good condition
to minimize voltage drop. Of course at lower voltage levels
the battery will put out a less amperage. Some starter motors
will pull a good battery down to 15 volts during the first
phase of the start.
Now
most engine manufacturers (turbine) recommend a 28.5-volt
start if available to minimize voltage drop. This is normally
accomplished with some sort of external ground power unit
(GPU). In this scenario it is most advantageous for the
turbine engine in the fact that 20% or more energy is being
delivered to the starter motor, resulting in a faster, cooler
start thus increasing turbine life. Also from a maintenance
standpoint, using 28.5 volts various aircraft electrical
components under load can be checked out above 24 volts.
In
conclusion, one should always 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®
verses 24-Volt Start Pacs®. As one
can see, higher voltage is available throughout the start
cycle on the 28.5-Volt models.
Click
here to see the Graph
Comparing all 6 Models
Battery
Temperatures
All batteries suffer from cold temperature. Aircraft batteries
will not supply enough energy to start a turbine engine
when exposed to very cold temperatures. Cold temperatures
have less affect on the high tech lead acid batteries used
in Start Pac® than most aircraft batteries.
Even these high tech batteries will lose performance in
colder temperatures.
Battery
Life
Battery life is cycle dependent in most cases. The more
the plates in the battery are stressed or cycled the faster
they wear out. Worst case scenario is cycled from fully
charged to nearly depleted. In this usage, around 400 cycles
can be expected. 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 to prevent sulfation. This condition will greatly
reduce capacity and output of the Start Pac®.
Also avoid completely depleting a battery 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 replacement is required.
Battery
capacity verses Voltage Drop
One would think two batteries of the same voltage but different
capacity would have equal voltage drop under the same load.
Not always true. The battery with the larger capacity will
have less voltage drop due to having a larger plate surface
area. Since the load is essentially spread out over a larger
area, the battery chemistry is less affected. See the following
graph comparing all 24 volt Start Pac®
Models. If a user requires many repeated starts between
charges, always choose the unit with the higher capacity.
Click
here to Compare
the Model 1324-1QC, the Model 2300QC and the Model 3324
Thoughts
on Voltmeters
The voltmeter is an essential tool 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 example illustrates
that a voltmeter can be used to determine battery condition
and a low state of charge only when the battery is put under
load. If not under load even a depleted battery will read
almost full normal voltage. This is why Start Pac®
does not use a built in voltmeter, as it really is not an
indication of the state of charge and also adds to the cost.
There are some electronic indicators that can measure the
state of charge but are very expensive. The voltmeter belongs
in the cockpit, not on the GPU.
