Rick's Free Auto Repair Advice

Is it the battery or alternator

How to know if it’s the battery or alternator

How to diagnose a bad battery–

How to diagnose a bad alternator and battery

You’ll need a digital multimeter (DVOM) to check your car’s charging system

If you’re wondering if it’s the battery or alternator, this is the article for you/ Here are some tests you can perform to diagnose the battery and alternator and get the answer to the question “is it the battery or alternator?”

Using a digital multimeter (DVOM) to check your car’s charging system

Buy a DVOM and use it to check your car’s charging system


Start with the basics


  • Check the car battery voltage with the engine off. A discharged battery will read 12.2 volts or less.


  • Check for corrosion on the battery terminals. If you find any, remove the terminals, clean with battery terminal brush, battery terminal cleaner, and battery terminal protector spray. Then reinstall. If your battery terminals require replacement, read this post to learn the best way to do the job.


  • Check for proper belt tension on the alternator belt. Don’t know how? Read this post to diagnose a serpentine belt tensioner. Check for battery voltage at the back of the alternator. Connect your DVOM to the stud on the top or back of the alternatorAlternators may have many wires, but the largest (thickest) wire attaches to the top or back of the alternator. That’s the wire that carries the power back to the battery. Set your DVOM to DC volts (20 or less). Touch the black lead to any metal part of the engine and the red lead to the stud holding the large wire (engine off). You should see the same battery voltage reading you saw when you tested the battery.

Then test alternator while running

Start the engine and attach the black and red leads to the battery to read charging voltage. If your battery voltage was below 12.5 volts or less (but above 12-volts) with the engine off, it should now read a minimum of 13-volts with the engine running. If yours doesn’t there could be two reasons: the alternator is dead or not receiving proper communications from the PCM, or the PCM isn’t commanding charging. However, if your battery voltage was 12.5 or higher, and you have a late model vehicle, the PCM may have determined it doesn’t need a charge. In that case, turn on your headlights, blower motor to high, heated seats, and rear window defogger. Then raise the RPMs to around 2,000 and check again.

With all those electrical accessories running, the alternator should be outputting at least 13.5 volts. It should maintain that minimum amount as you switch on each additional electrical load. Hold the RPMs stead and watch your DVOM. If you see the voltage slowly dropping below 13.5, it’s time for a new alternator.

If you don’t see an increase

If the voltage at the battery doesn’t change when you start the engine and add electrical accessories, you could have either a bad alternator or bad PCM. The PCM in many late model cars performs the voltage regulation function. However, in other late model cars the voltage regulator is built into the alternator and the PCM only tells the alternator what mode to run.

Late model GM vehicles, for example, have a battery current sensor mounted on one of the battery terminals. Based on how much current the PCM sees, it determines what charge mode to set in the alternator. Here are the charging modes:

  • Battery sulfation mode– The control module enters this mode if it sees battery voltage is less than 13.2 volts for 45 minutes. The control module sets a targeted voltage of 13.9 to 15.5 volts for 5 minutes. This mode is designed to break up sulfation on the plates.
  • Battery charging mode- This is the normal mode of charging to bring the battery back up to normal charge. The system will slowly ramp the charging rate from 13.4 volts to a maximum of 15.5 volts depending on the mode. The system enters charging mode any time the system senses:

Wide open throttle (greater than 90%)

Headlights on

Wipers on for more than 8-seconds

Radiator fans are on high speed

Rear defogger is on

Battery state of charge is less than 80%

• Fuel economy mode- This mode provides only enough charging to power the electrical loads without drawing too much power from the battery. In fact, this mode may shut off charge completely if the PCM determines the battery doesn’t need charging. In this mode, the generator charges at only 13 volts. The system enters fuel economy mode any time the system senses:

Air temp is above 32°F

Battery current draw is between -8 to 15-amps

Battery state of charge is above 80%

Generator duty cycle is less than 99%

• Headlight mode- Commands just enough charging to run the headlights.

• Startup mode- After starting the control module sets a targeted charging voltage of 14.5 volts for 30-seconds to replace the power lost from cranking and startup

• Voltage reduction mode- The generator charges at 12.9-volts. The system enters voltage reduction mode any time the system senses:

Air temp is above 32°F

Battery current draw is less than 2-amps and greater than -7-amps

Generator duty cycle is less than 99%

Understanding new GM generators systems

Older charging systems used an alternator with an integral voltage regulator and a temperature sensor in the alternator to determine charging rates. When the alternator is cold, the voltage regulator raises the voltage output max limit. When the alternator is hot, it lowers the output set point. Unfortunately, this older system tends to overcharge the battery when you drive the vehicle on a long trip and undercharges the battery on short trips. These olders systems tried to maintain a voltage charging rate of a constant 14-volts. The newer systems vary the amount of charging voltage from 12 to 14 volts.

Newer charging systems like the GM regulated voltage control (RVC) system regulates output to improve fuel economy, extend battery life, extend bulb life, and extend switch life. The alternator is now called a generator. GM uses two types of voltage regulated charging systems: integrated (RVC) and stand-alone (SARVC).

  • RVC systems—Using a battery current sensor mounted on the negative battery terminal the body control module (BCM) reads communicates how much current is drawn from the battery. The BCM also monitors voltage from the positive battery terminal and ignition circuits. It communicates those values to the PCM. Since the PCM knows engine RPM and vehicle speed and commands a pre-determined charging rate to the generator.
  • SARVC systems—These stand alone systems don’t involve the BCM. They have a control module mounted on the negative battery terminal to interpret battery current, voltage, and battery temperature. The control module connects directly to the L-terminal on the generator.

Both types maintain the battery at 80% or higher state of charge. If the systems can’t maintain that level of charge, they will automatically shut down electrical accessories in order of priority to protect the battery. This is called load shedding. The system can also boost engine RPM to increase charging rates.

©, 2015 Rick Muscoplat

Posted on by Rick Muscoplat

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