When a laptop will not charge or will not start, the charger is the first suspect and the easiest thing to rule out. A multimeter can check a charger's output in a minute — but the reading needs interpreting. Some chargers deliver their full voltage only to a laptop they recognise, USB-C chargers output nothing until they negotiate with a device, and a charger that reads perfectly with no load can still collapse under real use. This guide covers each case.
Safety first
- Measure only the low-voltage output — the plug at the end of the DC cable. Never open the charger's case; the mains side inside carries dangerous voltages, and capacitors can stay charged after unplugging.
- Use sharp, thin probes on the plug and avoid bridging the centre and the outer contact. A short across the output can damage the charger or trigger its protection.
- If the charger smells burnt, is cracked, or its cable is damaged, retire it.
Barrel plugs: which contact is which
Most barrel plugs have the positive contact inside and the negative contact on the outer sleeve. The polarity symbol on the charger's label confirms it: a circle with a dot, with the plus sign pointing to the centre, means centre-positive.
| Plug type | Contacts | Notes |
|---|---|---|
| Plain barrel | Inner (usually +), outer sleeve (−) | Two contacts |
| Barrel with centre pin | Inner sleeve (+), outer sleeve (−), thin centre pin (ID) | Common on some brands; the pin carries an identification signal |
| Rectangular slim tip | Several flat contacts, including an ID contact | The ID is often a resistor or a simple signal |
| Magnetic connector | Several spring contacts | Often outputs a low detection voltage until a laptop is recognised |
The ID signal tells the laptop what charger is connected and how much power it can supply. The concept is explained in the glossary entry for adapter id.
Measuring the output with no load
- Set the meter to DC voltage.
- Plug the charger into the mains.
- Put the black probe on the outer sleeve and the red probe inside the plug on the positive contact.
- Read the voltage and compare it with the label.
How to read the result:
- Close to the rated voltage — the charger produces its output. A small difference from the label is normal.
- Zero — the charger is not working, its cable is broken, or it is a type that outputs nothing until it detects a laptop.
- Much lower than rated, or pulsing — the charger may be in a protection or detection mode, or it may be faulty. Check its type before deciding.
- Fluctuating while you move the cable — a broken conductor in the cable, usually near the plug or where the cable enters the brick.
To check for an intermittent cable, use the meter's MIN/MAX function while gently flexing the cable along its length. A dip in the recorded minimum points at a break.
Why a good charger can read low or zero
Several types of charger do not output full voltage without a laptop:
- Magnetic connector chargers typically supply only a low detection voltage until they recognise a laptop.
- USB-C chargers output nothing at all on their power pins until a device is attached and signals through the CC pins; then they start at 5 V and only switch to higher voltages after negotiating with the laptop. A plain multimeter on the end of a USB-C cable therefore usually reads zero, even from a perfectly good charger.
For USB-C, use a USB-C power meter between the charger and the laptop, or a USB-C trigger board that requests a specific voltage. The negotiation itself is explained under usb power delivery, and diagnosing USB-C laptops that will not charge is covered in USB-C Laptop Not Charging: PD Controller Diagnosis.
Checking the ID pin
On chargers with a centre ID pin, the pin carries a signal that the laptop reads, not power. With the charger plugged in, the pin may show a small voltage relative to the negative sleeve, or a signal that a multimeter cannot interpret. If the laptop reports an unknown charger, charges slowly or refuses to charge while the main output reads correctly, the ID circuit — in the charger, its cable or the laptop's input — is the likely cause.
A broken ID wire is common: the centre pin is thin and the conductor inside the cable is fine. Continuity from the centre pin to the corresponding contact at the brick end can confirm it, but that requires access to the brick's connector, which many chargers do not provide.
Testing under load
A charger can read the correct voltage with no load and still fail when the laptop draws current. To test under load:
- Measure at the laptop. With the charger connected to the laptop, measure the input voltage on the board — at the DC jack's input capacitor or the input fuse — while the laptop starts and runs. The voltage should stay close to the rated value.
- Use an electronic load if you have one, set to a current within the charger's rating.
- Use a power resistor only if you are sure of its rating: it must handle the full power, and it gets very hot.
A voltage that drops significantly under load, or a charger that cuts out and restarts, points at a failing charger or a damaged cable. The laptop side of a charging fault is covered in Laptop Not Charging: Charging Circuit Diagnosis.
Interpreting results together
| Charger output (no load) | Output at the laptop | Likely cause |
|---|---|---|
| Correct | Correct, laptop does not charge | Laptop side: charger IC, battery, ID detection |
| Correct | Drops or cuts out | Charger or cable weak under load, or a short on the laptop input |
| Zero | — | Charger dead, cable broken, or detection-type charger |
| Fluctuates when cable moves | — | Broken conductor in the cable |
If the charger shuts off the moment it is connected to the laptop, the laptop's input rail may be shorted. Disconnect, measure the input rail to ground on the unpowered board, and see the troubleshooting sequence for a shorted input.
Example: charges only at certain cable angles
A laptop charges only when its charger cable is held in a particular position. With no load, the charger's output reads correctly, but with MIN/MAX enabled and the cable flexed near the plug, the minimum drops to zero. The conductors inside the strain relief are partly broken. A new charger solves the problem; the laptop itself was never at fault.
FAQ
Why does my USB-C charger read 0 V?
USB-C chargers do not output power until a device is attached and signals that it can accept it. Use a USB-C power meter or test it with the laptop.
My charger reads a little higher than the label. Is that bad?
A small difference above the rated voltage with no load is common and usually harmless. Under load the voltage settles closer to the rating.
Can a charger damage a laptop?
A faulty charger that outputs too much voltage, or one with a short between its output and its ID pin, can damage the laptop's input circuit. That is one reason to test a suspect charger before trusting it.
How do I test the charger's ID pin?
With a multimeter you can check for a small voltage or continuity in the cable. Whether the laptop interprets the ID correctly is best checked in the laptop's BIOS or system information.
Should I use a universal charger for testing?
A good universal charger with the correct voltage and tip can help. Be careful with cheap ones — wrong voltage, wrong polarity or a missing ID signal can confuse the test or damage the laptop.
Bottom line
Measure the charger's output with no load, then at the laptop under load. Remember that magnetic and USB-C chargers do not show their full voltage without a laptop, that ID pins carry signals, not power, and that a cable fault often shows only when you flex it. If the charger checks out, the fault is on the laptop side.