MOTOR TRUCK SERVICE MANUAL
ELECTRICAL
Fig. 11 High Rate Discharge Tester
Fig. 10 Checking Battery Specific Gravity
RECHARGING
Hydrometer barrel and float must be kept clean with
soap and water so float will not stick to sides. Inspect
Fast Charge
float occasionally for cracks which would allow acid to
enter the airtight float and make reading unreliable. If
A convenient method for recharging a battery is by
paper scale inside float is wet, it is an indication that float
the Fast Charge Method, Fig. 8. This can be
leaks and should not be used.
accomplished in the vehicle and the battery does not
have to be removed. Fast charging, however, is only a
Temperature Correction for Hydrometer
booster charge and does not fully charge a battery, since
this takes time. Also, the electrolyte temperature must
No hydrometer reading is strictly correct until a
be watched closely (never allowed to exceed 125F) or
temperature correction has been applied. Floats are
damage can result. A sulphated battery should never be
calibrated to indicate correctly only at one fixed
fast charged. When using the battery charger always
temperature. The reason for this is that acid volume
follow equipment manufacturer's instructions. Most
expands when heated and shrinks when it cools. When
modern charging equipment can be set for either a fast
expanded it will not be as dense and it will not raise float
or slow charge.
as high, thereby causing reading to be low. Temperature
correction is, therefore, most important when making a
Slow Charge
specific gravity test. Add .004 for every 10F above 80F
and subtract .004 for every 10F below 80F.
In general the Slow Charge Method for battery
charging is as follows:
Cranking Test
1. With vent plugs in place wash top of battery using a
A cranking test should be made with high rate
solution of water and baking soda. Rinse with clear
discharge equipment, Fig. 11. Follow the test equipment
water.
manufacturer's instructions for the instrument used. If
such equipment is not available, similar results may be
2. Remove all vent plugs.
accomplished by cranking the engine with center high
tension wire of distributor removed. With a voltmeter
3. Fill cells with distilled water to indicator level in
connected across the battery terminals, crank the engine
battery cover. Never fill with acid. If distilled water is
for not more than 15 seconds. During this period a 6volt
not available, ordinary tap water which is colorless,
battery should read not less than 4.8 volts; a 12volt
tasteless and odorless may be used.
battery not less than 9.6 volts. This test is valid only if
battery's specific gravity is 1.225 or above at 80F.
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