Charging lead-acid batteries in the airplane is
not recommended because of poor GPU out-
put regulation, but if absolutely necessary,
the GPU should be regulated to 28 ±0.5 volts,
all airplane electrical equipment should be
turned off, and both batteries and the GPU
should be monitored continuously.
Controls
One two-position ON–OFF battery switch
(Figure 2A-2) is provided to interconnect the
battery bus to the battery charging bus. Power
is routed through the battery relay for battery
charging from the generators or to supply the
battery or GPU power to the airplane systems.
The battery switch, when in ON, provides a
ground to the battery relay. Unless the battery
relay closes, the batteries cannot be connected
to the airplane electrical system and operate
equipment except those hot-wired to the bat-
tery buses. The batteries are always connected
in parallel.
Indicators
Electrical system gages are grouped in a clus-
ter on the instrument panel (Figure 2A-3). A
single DC voltmeter indicates the voltage on
the battery charging bus from the highest volt-
age input (batteries, generator, or GPU).
Airplane generators and GPUs normally pro-
duce a higher voltage than the batteries; there-
fore, when either of these is powering the
battery-charging bus, battery voltage can no
longer be read. Nickel-cadmium batteries are
equipped with an overheat warning light sys-
tem, and some airplanes have battery temper-
ature indicators. Two red warning lights
labeled “BAT 140” and “BAT 160” are in-
stalled on the instrument panel or glareshield.
The warning lights are controlled by thermal
switches within each battery.
The BAT 140 light indicates that one or both
batteries have reached 140ºF. To determine
which battery is hot, check the battery temper-
ature indicators, then turn off the battery switch
and land as soon as practical. The BAT 160
light indicates that one or both batteries have
reached 160ºF. To determine which battery is
hot, check the temperature indicators. Then
turn off the battery switch and land as soon as
possible.
2A-3
FOR TRAINING PURPOSES ONLY
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