Figure 7 illustrates the correction for hydrometer
readings when the acid temperature is above or below
26.7oC (80oF). In example No. 1, in cold weather, a
partially discharged battery in a vehicle at 6.7°C (+ 20°F)
might read 1.250 indicating it was almost fully charged.
However, when the correction factor is applied, the true
value is only 1.226. Example No. 2 could be
encountered in a battery exposed to the sun In hot
weather. Also, electrolyte frequently reaches 43°C
(1000F) in service in warm weather. The 1.235 specific
gravity reading might indicate too low a state of charge to
install in a vehicle or that there is a problem in the
electrical system if the battery is in service. However,
the true reading of 1.246 may not be unreasonably low
depending on the length of storage of the battery or the
type of service which it has been experiencing in the
Figure 7 - Specific Gravity Temperature
SPECIFIC GRAVITY DETERMINES ELECTROLYTE
Specific gravity of the electrolyte determines the
temperature at which a battery will be harmed or
damaged by freezing. A fully charged battery can be
stored at sub-freezing temperatures without freezing the
electrolyte. However, as a battery approaches the
discharged condition, the electrolyte will freeze at higher
temperatures. Table 2 shows the approximate freezing
points of electrolyte at various specific gravities.
- 9.4°C (+15°F)
Batteries must be kept fully charged when batteries
or vehicles are stored in sub-freezing temperatures.
TEMPERATURE AFFECTS BATTERY OUTPUT
CAPACITY (CRANKING POWER)
Battery temperature affects the output capacity of a
battery with respect to cranking an engine. The cranking
power of the battery is reduced as battery temperature is
Table 3 lists the approximate percentage of output
capacity of a fully charged battery at various
Note that a fully charged battery at - 17.8°C (0°F) gives
only 40 percent of its cranking power at 26.7°C (800F).
Percentage of Battery
Capacity (Fully Charged)
Batteries at less than the fully charged state will provide
even less of their output capacity. This emphasizes the
importance of keeping vehicle batteries fully charged
when low temperatures will be encountered.
MAINTENANCE (IN VEHICLE)
The battery is a perishable item and requires attention.
With a reasonable amount of care the life of a battery
can be appreciably extended. Neglect and abuse will
invariably cause shorter battery service life.
The battery should be inspected at the time of chassis
lubrication or other periodic services.
Battery maintenance includes the following:
Inspect battery and mounting (Figure 8).
If corrosion is found on the terminal posts, remove
the cable terminals from the battery (ground cable
first) using the proper end wrench and a cable puller.
A terminal cleaning brush can be used to clean
tapered posts and the mating surfaces of the
CTS-4111B - Page 7