Technical data

Document No. 5-0324 Rev C
Page 12 of 21
OPERATION
A. Applications: Aircraft batteries are used to start engines and auxiliary power
units (APUs), to provide emergency backup power for essential avionics
equipment and lighting systems, to assure no-break power for navigation units
and fly-by-wire computers, and to provide ground power capability for
maintenance and preflight checkouts. Many of these functions are critical to safe
operation of the aircraft, so the state of health of an aircraft battery is of utmost
importance. Aircraft batteries are certified to have a certain minimum capacity for
emergency operations in the event of an electrical generator system failure. If
the battery is used to satisfy essential or emergency power requirements, its
capacity must be tested periodically to assure airworthiness. See Servicing
Section for capacity test instructions.
B. Battery Charging: The aircraft’s electrical system automatically charges the
battery when the engine(s) are running. Most aircraft allow charging using
external power as well. The battery charging system in most aircraft is of the
constant voltage type. With constant voltage charging, the battery will accept
charging current inversely proportional to its state-of-charge (that is, the lower the
state of charge, the higher the charging current). When the battery reaches full
charge, the charging current tapers off to a very low value (typically 0.5% of the
C1 capacity), and remains at that level to keep the battery on a “float charge”.
Therefore, an ammeter reading of the charging current (if present) is useful in
determining the approximate state of charge of the battery.
C. Temperature Compensation: Battery service life can be prolonged by
compensating the charging voltage based on the battery temperature. For
aircraft that have an adjustable voltage regulator, the following table provides
recommended settings:
Table 1. Recommended Voltage Regulator Settings
Battery Temperature
Voltage Regulator Setting (Volts DC)
12V System
24V System
Below 0°C (32°F)
14.5 – 14.75
29.0 – 29.5
0 to 15°C (32 to 59°F)
14.25 – 14.5
28.5 – 29.0
16 to 30°C (60 to 86°F)
14.0 – 14.25
28.0 – 28.5
31 to 45°C (87 to 113°F)
13.75 – 14.0
27.5 – 28.0
Above 45°C (113°F)
13.5 – 13.75
27.0 – 27.5
D. Ground Power Units: When aircraft are powered by ground power units, be
sure that the DC output voltage is adjusted to the range shown in Table 1.
Through the years there have been many reports of overcharged batteries due to
ground power units being set too high. High charging voltages will shorten the
battery service life significantly.