How To Check A Charging System – EricTheCarGuy

[q0987]

Hello all,

If I got it right, Eric said that if the voltage of battery is more than 12V with all loads on, then the alternator is good.

However, I heard different views here:

How to test voltage on alternator:

Step1> without car running, test the voltage between battery +/- terminal. The reading should be in the range of 12.5V ~ 12.8V

Step2> with car running and turning off all loads, test the voltage between battery +/- terminal. The reading should be in the range of 13.8V ~ 14.8V

Step3> with car running and turning on all loads, test the voltage between battery +/- terminal. The reading should be in the range of 13.8V ~ 14.8V

Here is my new testing result with my own multi-meter.

1> Run the car with all loads OFF
Reading: ~14.13V

2> Run the car with all loads ON
Reading: 12.38-12.51V

3> Run the car with all loads ON but turn OFF rear defrost
Reading: ~13.40V

Based on eric’s comment, then my alternator is fine. but it is not good based on the other comment.

Question> Which one is accurate?

Topic

[David Whipple]

I have found Although voltage/load testing a charging system is usually enough to make an accurate diagnosis. Occasionally I also have to use an inductive amp meter to see if the alt is capable of producing enough amps even though voltages remain in the good range. the amp ratting is usually stamped on the housing of the alt. Sometimes I find the cable coming from the alt isn’t capable of carrying the full load.

[EricTheCarGuy]

I think this video makes it more clear.

[Jonathan]

In my personal experience I always check for voltage above 12.5 from the alternator, I guess it does depend on the vehicle.

[EricTheCarGuy]

[quote=”shospeed” post=52281]In my personal experience I always check for voltage above 12.5 from the alternator, I guess it does depend on the vehicle.[/quote]

The battery is the alpha and omega of the automotive electrical system. If you check at the alternator you’re missing a bunch of wiring and connections between it and the battery. If you found low changing voltage at the battery then you might check directly at the alternator but the voltage at the battery should be checked first. Also, some alternators are hard to get to, most times it’s much easier to check at the battery anyway.

Thanks for the input.

[tugsy]

with the vehicle running disconnect the positive cable at the battery if the car turns off then the alternator is no good if it stays running while the cable is diconneted the alternator is good

[Nick]

I’d never run the car with no battery, the battery acts like a big capacitor when the car it running and keeps the voltage from the alternator stable and prevents voltage spikes and excess current from the alternator affecting your car’s electronics.

[Jameson]

the alternator output test should be preformed around 2000 rpm for an accurate test. For a general voltage reg test, you should see around 14V at the battery when car is running. You should do voltage drop tests at all connections b4 condemning alternator.

really the best way to test for a DIYer is to go to NAPA or some auto parts store that tests the alternator for free, it will show the amperage output and you can compare it to what stock output is.

a charging output test though is pretty easy at home with a voltmeter, verify that you are getting 13.5 to 14.6 volts from generator. do voltage drops tests to make sure all volts getting to battery

[EricTheCarGuy]

[quote=”TNT” post=63727]I’d never run the car with no battery, the battery acts like a big capacitor when the car it running and keeps the voltage from the alternator stable and prevents voltage spikes and excess current from the alternator affecting your car’s electronics.[/quote]

The truth is you should NEVER do this and it is NOT a valid test of the automotive charging system. You can actually damage some of the electronic components of the system by doing this especially on a newer vehicle with a lot of electronic controls.

[Rafael]

Hi there!

Based on Eric’s videos on how to diagnose your alternator/charging system, I posted an article on how to build a car battery monitor.

If you believe you may find it useful, go to:

http://www.instructables.com/id/Troubleshoot-your-car-battery-with-ATtiny/

Thanks for your attention.

rlarios

[EricTheCarGuy]

[quote=”rlarios” post=80514]Hi there!

Based on Eric’s videos on how to diagnose your alternator/charging system, I posted an article on how to build a car battery monitor.

If you believe you may find it useful, go to:

http://www.instructables.com/id/Troubleshoot-your-car-battery-with-ATtiny/

Thanks for your attention.

rlarios[/quote]

This is a great article. I recommend you check it out.

[Nick]

Personally I like a Voltmeter on the dash.

makes life simple. and you have an another indicator for potential problems every time you use the car.

when the car’s been stood the voltage should read above 12v, Ideally 12.6v with all lights off and no drain.

you can see how well the charging circuits are working on the move, if you have too many accessories (less than 13v with the car running) or a faulty charge circuit. or if it’s over charging ( much over 14.4V isn’t good ).

you can do the equivalent of a load test every time you start the car, you don’t want to see the voltage dip below 10v Ideally, if it goes below 9v your looking at a new battery and/or battery cables test, clean up or even in some cases replace.

IMO the best load test you can do on a battery is pull the fuel pump fuse or disconnect the ignition coil and crank the car over for 5-15 seconds and watch the volt meter.
It’s all fair and well checking a battery with a load tester against the battery spec, in my experience plenty of parts stores and even manufacturers supply batteries that are too small and only work while at the peak of their powers.
Sometimes within months they can pass that test on the battery testers at the store and not be enough to crank your car properly.
At least if you load test using your car, you know to ignore what they say can get bigger or better battery which will fit and should give you trouble free starting and power for your car.

for example, my car, many books and battery look up’s and VW recommend a type 063 battery which is too small for my car, you’d be lucky to get a years service out one a standard 063 in my car. however a 027, 075 are ideal or even a 096 fits perfectly and give many years of trouble free service. my current 096 battery has been going 5 years and still cranking fine.

[Author]

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