Rick's Free Auto Repair Advice

Is it the battery or alternator

How to know if it’s the battery or alternator

Do I have a bad battery or alternator?

I see this question all the time on auto forums—“Is it the battery or alternator?”. But without additional information, there’s no way to answer the question. But there are tests you can perform to determine the root cause yourself to discover whether you have a bad battery or alternator. Here’s how.

Understand how an alternator works

Before you can determine whether the problem is a bad battery or alternator you first have to understand how an alternator works. Unlike older generator technology, an alternator needs power in order to generate power. Remember from high school science class that you generate electricity by passing a magnet across a wire? Well, in an alternator, that magnet is an ELECTOmagnet, meaning it needs power to turn a steel shaft wrapped in wire coils into an electromagnet. If the car battery is dead, the alternator can’t possibly generate power. So it comes down to a question of which came first; the chicken or the egg? In this case, did the battery die first, preventing the alternator from working or did the alternator die first preventing the battery from being recharged? Determining what happened first is your job.

For more information on how an alternator works, see this post

battery or alternator

Start by diagnosing the battery

Since the alternator can’t work without a good battery, you ALWAYS start by diagnosing the condition of the battery. If the battery is dead: ie, less than 12-volts, you must charge it in order to test it. But be warned; you can’t charge a dead battery by jump starting it and driving it. Why? Because the alternator needs power to generate power. So you’ll have to use a battery charger. Or, remove the battery and have it tested at any auto parts store.

How to diagnose a bad battery–


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

Start with the basics

  • Check the car battery voltage with the engine off. A discharged
    checking battery voltage

    Check battery voltage with a multimeter

    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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