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How to test any laptop charger easily (MacBook or Windows)
Short answer: a multimeter only works on old or very simple laptop chargers. Most modern chargers (Apple, Dell, HP and others) have a sense pin in the middle that talks to the laptop, and if there is no communication you will see no voltage. To really test them, plug them into a laptop that you know works, test USB-C chargers with your phone, or use an adjustable power supply with interchangeable tips to rule out the charger.
Many YouTube videos tell you to grab a multimeter, put the probes on the connector and read the voltage. The truth is that you will be able to test some laptops that way, but most laptops you will not. In this video I show you what you need to test every laptop and find out if the charger or the laptop is the problem.
Why a multimeter doesn't work on most laptop chargers
When you look at the connector of a modern charger you see several pins. It is not as simple as putting the multimeter probes on it and getting a reading. Usually the pin in the middle senses whether the motherboard is working correctly. If the laptop is not there, or the motherboard is not working properly, the charger does not send voltage, so there is nothing to measure.
I show a schematic of a random MacBook Pro: on the side you have the polarity that charges the laptop, and in the middle you have the MagSafe feedback line. The laptop needs to talk with the charger, and if they cannot communicate with each other, the voltage will not be seen by the multimeter. The same thing happens with Dell and other laptops that have this type of sensor. You may get no reading at all, or just a very low reading.
When a multimeter is enough: old and simple chargers
Regular chargers on older laptops with very primitive technology, without that sensing system, can be tested with a multimeter. Set the multimeter to voltage (DC; some multimeters read AC and DC in one setting, others have separate positions) and check the label on the back of the charger. It will say the output, for example 19 or 19.5 volts.
- On the typical barrel plug the middle is positive and the outside is negative.
- Put one probe in the middle and the other on the outside.
- Do not touch the walls on the side with the middle probe and do not short circuit it.
In the video I test a no-brand laptop that looks like a MacBook Pro. Its charger should give 12 volts but it only showed 7, and that was the problem with that laptop. I bought a regular charger with an adapter, plugged it in, and there we go: 12 volts as usual.
USB-C chargers: test them with your phone
USB-C chargers are the easiest ones to test. If you have an Apple product with USB-C, just plug the cable into the charger and connect your phone on the other end. If the phone charges, your charger is working.
Use a cheap laptop as a tester
Tricking a smart charger into giving power is complicated, so it is better to just buy a laptop for testing. For older MacBook chargers you can get a very old laptop, around 2006, for about $40. It does not matter that the computer is obsolete; you only use it to test chargers. For the newer connectors (MacBooks have the old type and the new type) you can buy a laptop from around 2015 for about $150 to $200 and use it just for that.
Adjustable power supply with interchangeable tips
For Windows laptops you can get a universal adjustable supply. The one I show has a potentiometer to change the output from 3 to 36 volts, 4 amps, 144 watts, and it comes with tips for almost all laptops. It cost about $26. It also includes a polarity changer (middle negative, outside positive). That piece is not an extension, so don't confuse it, otherwise you may fry your computer.
A lab power supply from Amazon for less than $50 is even better. You can set the voltage and amperage, and you can see how many amps the laptop is drawing. If it draws a lot of amps, something is going on with the computer. I also show a battery tester for about $400 that measures the rate of depletion of laptop batteries.
Frequently asked questions
Can I test a laptop charger with a multimeter?
Only old or simple chargers. Most modern chargers have a sense pin in the middle that communicates with the laptop, and without that communication they show no voltage on a multimeter.
Which pin is positive on a laptop barrel plug?
On the typical plug the middle is positive and the outside is negative. Check the label on the charger and be careful not to short circuit it.
How do I test a USB-C laptop charger?
Plug the cable into the charger and connect your phone on the other end. If the phone charges, the charger is working.
What is the easiest way to test chargers for many laptops?
Use a cheap laptop that you know works as a tester, or an adjustable power supply with interchangeable tips so you can rule out the charger and see how many amps the laptop draws.