FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 11. Aircraft Electrical Systems | 2. Storage Batteries | 11-17. Battery Freezing
9/8/98
SECTION 2. STORAGE BATTERIES
AC 43.13-1B
11-15. GENERAL. Aircraft batteries may
be used for many functions, e.g., ground
power, emergency power, improving DC bus
stability, and fault-clearing. Most small pri-
vate aircraft use lead-acid batteries. Most
commercial and military aircraft use NiCad
batteries. However, other types are becoming
available such as gel cell and sealed lead-acid
batteries. The battery best suited for a par-
ticular application will depend on the relative
importance of several characteristics, such as
weight, cost, volume, service or shelf life, dis-
charge rate, maintenance, and charging rate.
Any change of battery type may be considered
a major alteration.
a. Storage batteries are usually identified
by the material used for the plates. All battery
types possess different characteristics and,
therefore, must be maintained in accordance
with the manufacturer’s recommendations..
WARNING: It is extremely danger-
ous to store or service lead-acid and
NiCad batteries in the same area. In-
troduction of acid electrolytes into al-
kaline electrolyte will destroy the Ni-
Cad and vice-versa.
11-16. BATTERY CHARGING. Operation
of storage batteries beyond their ambient tem-
perature or charging voltage limits can result in
excessive cell temperatures leading to electro-
lyte boiling, rapid deterioration of the cells,
and battery failure. The relationship between
maximum charging voltage and the number of
cells in the battery is also significant. This will
determine (for a given ambient temperature
and state of charge) the rate at which energy is
absorbed as heat within the battery. For lead-
acid batteries, the voltage per cell must not ex-
ceed 2.35 volts. In the case of NiCad batteries,
the charging voltage limit varies with design
and construction.
Values of
1.4 and 1.5 volts per cell are generally used. In
all cases, follow the recommendations of the
battery manufacturer.
11-17. BATTERY FREEZING. Discharged
lead-acid batteries exposed to cold tempera-
tures are subject to plate damage due to freez-
ing of the electrolyte. To prevent freezing
damage, maintain each cell’s specific gravity
at 1.275, or for sealed lead-acid batteries check
“open” circuit voltage. (See table 11-1.) Ni-
Cad battery electrolyte is not as susceptible to
freezing because no appreciable chemical
change takes place between the charged and
discharged states. However, the electrolyte
will freeze at approximately minus 75 °F.
NOTE: Only a load check will deter-
mine overall battery condition.
TABLE
points.
Specific
Gravity
1.300
1.275
1.250
1.225
1.200
1.175
1.150
1.125
1.100
11-1. Lead-acid battery electrolyte freezing
Freeze point
C.
F.
-70 -95
-62 -80
-52 -62
-37 -35
-26 -16
-20
-4
-15 +5
-10 +13
-8 +19
State of Charge (SOC) for sealed
lead-acid batteries at 70°
SOC
12 volt 24 volt
100%
12.9
25.8
75%
12.7
25.4
50%
12.4
24.8
25%
12.0
24.0
11-18. TEMPERATURE CORRECTION.
U.S. manufactured lead-acid batteries are con-
sidered fully charged when the specific gravity
reading is between 1.275 and 1.300. A
1/3 discharged battery reads about 1.240 and a
2/3 discharged battery will show a specific
gravity reading of about 1.200, when tested by
a hydrometer and the electrolyte temperature is
80 �F. However, to determine precise specific
gravity readings, a temperature correction (see
table 11-2) should be applied to the
Par 11-15
Page 11-5