Rick's Free Auto Repair Advice

Is it the battery or the alternator — Step by step guide to diagnosing the problem

How to diagnose a battery or alternator problem

DIYers are often confused about how to determine whether it’s a battery or alternator problem

To diagnose whether the problem is the battery or alternator you always have to start by checking the battery.

Step 1: Eliminate resistance so you can get good test results

High resistance at the battery terminals, alternator terminals and ground terminals can really screw up your diagnosis. Even if your battery terminals look clean, remove the terminals and clean the terminals and posts. Then clean the connections at the alternator and all grounds.

Step 2: Test the battery

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

Start with the basics

1) Check the car battery voltage with the engine off. A discharged battery will read 12.2 volts or less. If your battery is discharged place it on a battery charger and recharge it. You can’t perform a reliable test on your alternator with a dead battery.

checking battery voltage

Check battery voltage with a multimeter

2) Once the battery is fully charge and your meter reads 12.6 volts, conduct a simulated load test and a conductance test. This will test for internal resistance.

If you don’t own a conductance battery tester, take the battery to an auto parts store and have them test it for you.

Step 3: Check the belt drive system

Check for condition of the alternator drive belt and its tension. Don’t know how? Read this post to diagnose a serpentine belt tensioner.

Step 4: Test the alternator while running

With a fully charged battery, start the engine and attach the black and red leads to the battery to read charging voltage. Your meter 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

• your vehicle is equipped with a power management system that only charges the battery when it needs it. Most late model vehicles have smart charging systems and power management systems that won’t allow the alternator to charge a fully charged battery.

To force the system to charge, turn the blower motor to high, turn on the rear window defogger, activate heated seats and turn on your headlights. Then rev the engine to 2,000 RPM and read battery voltage. It should not read at least 13-volts.

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 in voltage at the battery after turning on electrical loads

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 a bad PCM. The PCM in many late model cars performs the voltage regulation function.

Check for battery voltage at the back of the alternator. Connect your DVOM to the stud on the top or back of the alternator. Alternators 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.

Step 5: Check for parasitic battery drain

You can have a good battery and a good alternator and still find yourself with a dead battery in the morning. In that case, it’s possible you have a parasitic battery drain. A parasitic drain can be caused by a computer that doesn’t go into sleep mode and continues to draw high power.

Or, you can have an alternator diode that’s shorted to ground. Either way, you must conduct a parasitic battery drain test to locate the problem.

Understand the purpose of your battery

Your car battery’s main job is to power the starter motor and ignition system until the engine is up and running. After that, your alternator should provide all the power to run accessories like lights and heat. While powering those accessories, the alternator also recharges the battery for the power lost during cranking.

If your alternator fails all the power will come from your battery and that will cause it to discharge and fail.

Can you recharge a battery by idling?

If you think you can recharge a dead battery by idling, think again. It takes at least 4-hours of idling to recharge a dead battery. And, it takes at least 30-mins of steady highway driving to recharge a dead battery. If you idle less than that or don’t drive at highway speeds for that length of time, you’re not recharging your battery.

How an alternator works

Unlike older generator technology, an alternator needs power in order to generate power. 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


Understanding new GM generators systems

Older charging systems used an alternator with an integrated 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 older 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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