Car Repair Advice by Professional Automotive Experts
How Do I Repair My Car? /
How Do I Check a Car
Battery?
How Do I Check My Car Battery?
Checking Battery and Charging System:Wear protective eye wear and clothing and remove all jewelry when checking
your battery and charging system. Jewelry is a good conductor of electricity and
is not recommended. Most batteries wear out every 3 to 5 years and need to be
replaced. Always replace your battery with an equal replacement battery to
assure proper operation. Automotive batteries have a +positive terminal (red), - negative terminal
(black). The battery in this illustration has a protective cover over the
positive terminal to prevent short circuit in case of an accident. Electricity
is stored in the battery and then supplied to the vehicle when the engine is not
running. While the engine is running the vehicles alternator charges the battery
for future use. (Note: never disconnect the battery while the engine is running.
If the battery cable is disconnected from the battery a spark can be generated
which can cause the battery to explode or a major electrical malfunction to
occur.)
Step 1 - Checking Battery
To check a battery surface voltage, remove the positive terminal protective
cover. Connect the +positive side meter lead (red) to the positive side battery
terminal. Connect the - negative (black) side meter lead to the negative battery
terminal. With the vehicle not running and the car sitting over night the
battery voltage should be between 12.5 and 12.8 volts.
Step 2 - Identify Alternator
The alternator is rotated by a drive belt driven by the vehicles engine while
it is running. Electrical voltage and amperage are generated to recharge the
battery and supply voltage to the electrical system of the car. The alternator
is held in place with mounting bolts. There is a main electrical wire on the
rear of the alternator that supplies voltage to a main voltage junction box. If
your alternator is not charging properly, your battery will slowly drain down
from operating all the electrical systems in your car and stop the car from
running.
Step 3 - Checking Alternator Output
Connect the voltage meter lead the same way you would in a battery static
voltage check, Start engine (do not drive) at engine idle the voltage should be
between 13.6 to 14.3 volts. If not the alternator may need replacing.
Step 4 - Checking Alternator Amperage Output
Testing the amperage output of the alternator is good for measuring the
amount (not the level) of voltage the alternator can produce. This test can be
tricky because if the alternator is weak it can still show it as producing
amperage. Which is good, but if the voltage is low, it will still allow the
battery to go dead. To check the amperage output of an alternator an amp meter is
needed. Once the meter is connected start the engine. Next turn on all
electrical accessories and raise the engine idle to about 1200 RPM. The
alternator should output the max amperage it was designed to produce. Example: a
90 amp alternator should output about 88 amps. Note: An alternator cannot
sustain maximum output for long periods of time. If the alternator is forced to
operate at maximum output it will overheat and fail. An alternator is designed
to operate at max amperage output only for a reasonable amount of time.
Common Problems:
1. Alternator stops producing electrical power and the car stops because the battery dies.
2. Battery fails do to age or defect.
3. Battery cables become loose causing the electrical system to fail.
WARNING!
Always wear protective eyewear, gloves and necessary clothing before inspection
or work begins. Never crank an engine over when you are near the battery or
engine. A battery is filled with
explosive gases that can ignite when extreme heat or a spark is present. A
battery could have residual acid on the outside casing so use caution when
handling a battery and the surrounding area. Use baking soda to neutralize any
suspected acid residue before work begins.
How Do I Repair My Car? /
How Do I Check a Car
Battery?
Additional Resources
Additional "How to" Car Repair or Services
Find What You Are Looking For? Check Our:
ASK A CAR REPAIR QUESTION
| 
