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cells, which typically have a charge of
about 2 volts each when it comes to
aircraft, hence the need to combine
them in order to do anything useful.
Knowing how to do this is handy
when you are out in the field with a
discharged battery and you are trying
to connect two car batteries together
to start your aircraft.
The polarities are positive and negative,
marked as plus (+) or minus (-), or red
and black. If you join batteries in
series, that is, one after the other, with
the positive of one connected to the
negative of the next:
you will get a voltage which is the
sum of the cells, but with the
amperage rating of one cell. If you
join them in parallel, with the positive
and negative terminals connected to
each other:
you would get the voltage of one
cell, but the amperage of all of them
(see above).
Since a typical aircraft runs on a 24-
volt system, you would therefore
connect two car batteries in series. Be
aware, though, that the terminals are
different sizes to stop them being
confused with each other, so you
need to carry an adapter in your
navbag to connect them up in the
middle (try Halfords). Be wary of
jumper cables, as they may open up
and spark when a load is applied.
Ensure that batteries have a load on
them before completing a circuit.
A battery cell is made up of electrodes
surrounded by electrolyte. Different
materials are better or worse at this
job, so you might get more or less
voltage out of one type of battery or
another, but the most common is
zinc-carbon (there are also alkaline,
mercury, NiCad, Lead-Acid,
Lithium, etc.). Electrons flow from
the negative (-) electrode, through
whatever circuit the battery is
connected to, back to the positive (+).
Primary cells are not rechargeable,
but secondary ones are.
A flat battery has maximum internal
resistance, which will generate lots of
heat when the aircraft alternator or
generator attempts to charge it (on a
bench, only a very small current is
used). It is therefore not a good idea
to continue flight if your battery gets
discharged – in any case, it should be
replaced before the next flight.
The battery itself will be rated in
terms of Amp/Hours, meaning that
it’s supposed to provide a certain
116 JAR Private Pilot Studies
number of amps for a certain
number of hours when fully charged,
though it is never wise to rely on any
battery for more than about 20
minutes (officially, they should last
for at least 30 minutes). To get an
idea of your aircraft’s capabilities,
add up the number of devices that
use power (check the circuit
breakers) and divide them into the
amp/hour rating. So, if your devices
collectively use 45 amps (see below),
and your battery supplies 45
amps/hour, you should be able to
get an hour out of it. You could also
use half the devices and get two
hours. When faced with such an
emergency, it is usual to use the
navaids, for example, to get a
position fix, then turn them off until
you start feeling a little lost, then
turn them on again until you are
once more certain of your position.
The same with radios. This will get a
little extra time out of your battery.
There are two types of battery used
in aircraft, lead acid, as found in cars,
and NiCad, as found in portable
computers. People who use both will
already understand the difference,
but just in case, the lead acid’s
output tends to fall off with
discharge, whereas a NiCad can
pump out power at a constant rate
until it can do no more, as well as
recovering more quickly. The trouble
is that NiCads have short memories,
in that if you keep charging them up
when they have only discharged a
little way, they will begin to think
they have a lesser power rating, so to
stop them causing hot starts they
need regular deep cycling to keep them
awake. So, although it’s good
practice to start a helicopter, for
example, from a battery cart, to
preserve the ship's battery for better
reliability in remote places,
occasionally a battery start is good
for the system.
Another problem with NiCads is
that they can spontaneously combust
when too much current is drawn and
then replaced (actually called Thermal
Runaway). This is why some
helicopters have a Battery Temp
caution light on the warning panel
which means you must land
immediately, before the battery catches
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