Car Electrical Systems Explained - How To Diagnose A Failing Alternator vs. A Failing Battery

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Is your battery warning light illuminated on your dashboard? Are you experiencing a lower than ideal car battery voltage or car battery amps? If that's the case, it may be time to look into servicing your vehicle's charging system—but how do you do that?

In this video, Nate Vincent, our Motorsports Director, takes you on a deep dive on how to diagnose a failing alternator vs a failing car battery. From testing the voltage and amperage using a multimeter, to going over the internal components of the alternator itself, everything you need to know to find the source of your problem is covered here.

00:00 - Electrical Issues Intro
00:38 - Why Is My Battery Light On?
01:00 - Inspecting Your Drive Belt
02:15 - Using a Multimeter to Test Your Car Battery
05:36- How To Check Your Vehicle's Battery Voltage with a Scan Tool
08:40 - How a Decoupler Pulley Works 
09:45 - Inspecting a Decoupler Pulley
10:45 - What is a Voltage Regulator?
12:18 - Testing the Voltage Regulator
12:30 - Is my Alternator Bearing bad?
12:52 - What is inside the Alternator? - Stator and Rotor and Rectifier Explained

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[RedondoBeach]

Clear, concise, easy to understand usable information. No assumptions about a DIY mechanic's knowledge. The benchmark for repair videos. Great job. Please make a video on troubleshooting cooling fans and associated electrical system components on a 2004 VW GTI VR6 2.8L.

[MySSS]

One of the best, best best video to understand without moving the camera where it goes everywhere. Thank you

[johngreathead]

Be aware of your multimeter's cables when working around a running engine. They could become entangled in the drive belt, likely destroying the meter and possibly injuring you.

[salmothymus]

first time immediately. Kudos to the young presenter...very clear and just about right amount of technical for my level of knowledge (liked that pulley comparison with bicycle crank) 👍

[renostubbs]

For a guy man you know your stuff, glad that I came across this channel. Thank you for sharing your knowledge and expertise

[gwgrote]

Nate, I know this "how to" is 2 yrs old but it is excellent and has helped me. I am the second owner of a Lexus. Lexus has all of the records of what was done to the car prior to me, which was frequent oil changes and detailing. Now that I'm approaching 100K miles, I realize that nothing was done on it but the few things that needed replacing during my sensor and rear brakes. I do not fix my own cars, but I will definitely buy a multimeter tester because I'm seeing an alternator in my future. Great job! 😎

[stephenmorrissey]

I had no idea alternators could adjust voltage depending on demand. Must be why these things are so expensive.
Thank you for the in-depth article!

[Davidmc]

You and your team did an excellent job on this video. Thank you, it's been very helpful.

[happycat]

If the battery is failing then use a manual load tester on the battery. If no load tester is available then use a multi-meter and look for a reading for around 12.2 ~ 12.3 volts with the car off. With the car running the readings will be around 14.0 ~ 14.4 volts. You can use a 12 volt cigarette socket voltage output tester as well as they are cheap and work really really well. If you are really careful you can disconnect the battery while the car is running. If the car dies when the battery is disconnected then the problem is with alternator. The 14.0 volts ~14.4 volts indicates that the alternator is working properly. You can always boost the car to get it running and then drive it around for 20 minutes. If you try to start the car again and you get nothing, the problem is the battery as the battery should be charged enough to restart the car at this point.

[ronaldbryant]

This is the best site and video I have seen on this red battery light issue, thank you for up the very simple explanation, with clear and concise instructions. Thank you!!!

[jaymorris]

Thank you for showing where the multimeter dial and leads go to on the multimeter, awesome, a lot leave that bit out.

[andrewdoherty]

A great video Nate. It is no wonder I like old vehicles - 2x '80's MB.
1/ I always carry a spare belt, but make sure it is the right one. I got caught.
On systems with twin belts, eg 380, make sure they are the same actual, not nominal, length.
2/ I start with the battery. If it is not 12.7 volts something is wrong. I then turn on all the lights and re-read it. If the voltage has collapsed the battery is shot.
3/ If the battery is not holding charge check the standing current drain. The drain should be about 10 mAmps.
Setting to Amps DC connect one meter end to the positive wire and the other to the positive post. Then, without loosing continuity remove the positive wire from the post. Two people are handy for this manoeuvre.
Warning. On a modern car if you loose continuity you may have to reset all the control systems. See Facts Over Feelings92.
4/ If the standing current is fine but the battery is not holding charge look for irregular discharges. My wireless standby capacitor periodically turned into a 4 ohm resistor. Not enough to blow the fuse but enough to flatten an occasional use car.
5/ If the alternator is over 14 volts it has lost battery contact. Either a dirty battery post or the control unit is faulty.

[HillBilly-]

You forgot a very important part of the charging system. Same problem as this AFTER replacing BOTH alt and battery. Replacing the starter cured the problem. Hot wire from battery and alt both go to the starter solenoid. Bad solenoid can burn the hot wires going to alt and battery.

[kdao]

Thank you for the in depth lesson. Was great, and thanks for making these amazing videos for DIY

[t]

Learnt a lot thank you ! My car was charging - but on a long journey stops charging on the way back! - everything is new - i am completely stumped - new connectors new wires, now wont charge at all! Focus mk1

[jolenemoyanchildkiller]

1999 ford taurus se. We had similar issues u explained in the video, After replacing alternator and battery..Battery light won't go out. after couple months. car died. dash lights, gauges, blinker lights, stereo. and battery wont charge. check megafuse before looking in the alternator?? your video very good.

[moffatk]

Very insightful video on charging system, covers checks on battery, alternator, obd scanner, explain alternator how it works even a DIY person can understand 👌🏻

[cesarpalmero]

Quick and simple. Keep it that way. Many thanks.

[mannymanano]

I have a 2010 Nissan Frontier, 6 cylinder. 4.0 engine. The charging voltage fluctuates, it starts at 14.1 on start up, and with in 3 minutes charging rate always starts dropping down eventually to as low as 12.7, then usually goes back to 12.8 or 12.9, then will hang around 13.0 to 13.1 volts. When I turn on the AC, and/or radio, and/or lights. It drops to 12.8 volts. Then it may or may not go back to 13.0 or 13.1. Mechanic said it was the alternator and put in a brand new alternator. It acts the same way.

Went to another mechanic he supposedly did a diagnostics and said the alternator is doing its job, but the battery tested only at 44 percent. He put in a premium battery and the battery is now holding a full charge for the last 3 days. Although, the way the charging system seems to be somewhat sufficient; to me there seems to be to be something wrong in the charging system for all these fluctuations, because when AC, etc. charging rate should go up, not down. I m concerned that eventually it will get worse.

[ginalolo]

I needed a few jumpstarts over the past few weeks, I was told the blower and the resistor for my Q5 need to be replaced, my battery is good, the diagnosis recommended humidity sensor too, are all these working together to cool the car? I'm more concerned about the car not wanting to start now than the AC blowing hot air. So, I'm thinking of maybe replacing the alternator first. I will very much appreciate any opinions you may have on that. Thanks much for your informative video!