Skip the auto shop - Our mechanics make house calls

Q: How Do I Know if the Problem Is the Alternator or the Battery

asked by on

How to know if the problem is the alternator or the car battery?

A: When your vehicle’s engine will not turn ov...

When your vehicle’s engine will not turn over or the Battery Light comes on in the dash, how do you know if it is caused by a defective battery or alternator? This article will help explain how each component works and how to know which one is the problem.

Most of today’s vehicles still use a 12 DC volt (Direct Current) battery that is made up of a plastic case with 6 individual lead acid cells separated by a plastic or other material plate. Each cell has lead plates with a negative and positive plate each separated so they will not touch with a permeable material. The plates are in an electrolyte solution to cause a chemical reaction between the negative and positive plates of each cell. Each cell is capable of producing about 2.1 DC volts. Since there are 6 cells in a battery connected in a series circuit the 6 cells are added together for a total of 12.6 DC volts when fully charged. Through the electrolysis process to produce voltage, the plates transfer some material from one plate to the opposite charged plate. As the charge is used, the battery voltage drops. The alternator is used to put a charge back into the battery. A battery will accept and release its voltage as long as the lead plates in the battery last or when they touch each other, causing short circuits. Once this happens, the voltage and power of a battery will drop till it can no longer produce or receive power.

The alternator in most vehicles is a belt-driven voltage generator. The alternator has a rotating rotor with an electromagnet built into it. The rotor rotates inside a field coil. As the rotor rotates inside the field coil it will cause a magnetic field that will produce an alternating current voltage (AC) of about 120 AC volts or more as the magnetic lines of force are broken. This is the same type of power your house uses. Your car cannot use this AC voltage since it can only use DC current voltage. The 120 AC volts is then rectified inside the alternator with a voltage rectifier to convert the AC into a DC voltage. The conversion process will make DC voltage that is regulated to be between 12.5 to 15 DC volts. The voltage levels may be different between manufacturers. The DC current is used to keep the battery at a stable constant voltage level of between 12.4 to 12.9 volts DC.

In order to test if an alternator is charging correctly, you first need to have a battery that is in good working order. To test a battery you will first need to look at the cold cranking amps or cranking amps rating on the battery. A battery size of the inside plate surfaces determine the cranking amps of a battery. You should then test the battery using a battery load tester or capacitance tester to see if the battery is able to handle a normal load and maintain a working voltage for the electronics and starting system in the vehicle. When load tested the battery voltage should not drop below 9.6 volts under load. Charge and retest it again if it falls below 9.6 volts. If it fails again then replace the battery.

Once a good battery is in place, then the alternator can be tested. An alternator is rated in amps. Different amperages for different vehicles depend on the electrical loads it needs to keep up with. To test the alternator a volt and amp meter is needed. Hook up the voltmeter to the battery and the amp meter around the large cable of the alternator. Run the engine at between 1500 rpm to 2000 rpm with all the lights and accessories like the AC compressor and blowers on max settings. You should have a voltage reading of between 12.9 and 13.5 volts and an amperage reading of over 85% of rated alternator maximum amps output. For example, a 100 amp alternator should put out between 12.9 and 13.5 volts and above 85 amps.

If you try and test an alternator with a bad battery then you will get incorrect voltage and amperages during the test and you may think the alternator is bad when the battery is the cause of the problem. Always retest the system after a battery is replaced since an alternator that is overcharging a battery can boil the electrolyte and shorten the life of the battery.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: My car needs to be jumped like every other day why?

This sounds like the battery is weak and cannot hold a charge very well. Have the battery tested to see if a replacement is needed. Then check charging system to make sure it is in good working order. Your problem...

Q: How Can I Tell If My Car Battery Needs Replacing?

A battery needs to be replaced when it can no longer support the electrical and starting system of the vehicle. How will you know this? The most common first signs that a battery is starting to go bad may not...

Q: 1992 Pontiac Firebird runs great but has trouble starting 1 in 3 times (it just clicks) .

Hey there. YourMechanic is most likely correct. The gears on your starter have to mesh and catch the gears on the engine in order to turn it over. If the teeth on the gear have been worn down, it's likely...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...