To test a phone charger with a multimeter, you measure the DC voltage output at the USB plug to verify the charger is delivering power. Modern USB-PD chargers may output 5V, 9V, 15V, or 20V depending on the charger's profile and the phone model it supports.
What a multimeter test can (and can't) tell you
A multimeter measures open-circuit voltage, the voltage present when no phone is connected. This confirms the charger is not completely dead and is producing some output. However, the test cannot assess performance under full load, nor can it verify data communication protocols like USB-PD negotiation. A charger that shows correct voltage at the plug may still fail to charge a phone if its internal circuitry cannot negotiate the proper power profile or deliver sufficient current.
Step 1: Prepare the multimeter
Before measuring, ensure your multimeter is ready. Check the battery, a low battery can produce inaccurate readings. Insert the red probe into the jack labeled VΩmA (or VΩA) and the black probe into the jack labeled COM. Perform a self-test by turning the dial to Ohm (Ω) or Diode setting and touching probe tips together; the multimeter should display a small resistance reading or a value close to zero.
Step 2: Set the multimeter to DC voltage mode
Turn the dial to the DC voltage setting, typically marked DCV or VDC. For phone chargers, it is recommended to start with the highest available voltage range, such as 200V, to avoid exceeding the maximum range. Select a range of 20V or higher to safely accommodate voltages up to 20V from fast chargers. If your multimeter does not have auto-ranging, choosing a range that exceeds the expected voltage prevents damage to the meter.
Step 3: Measure the charger's output voltage
- Plug the charger into a wall outlet.
- Touch the red probe to the inner USB pin (the positive contact) and the black probe to the outer metal shell (the negative contact). For a USB-C port, the inner pin is typically the one at the back of the connector.
- Read the voltage on the display. A standard charger will show approximately 5V. Fast chargers may show 9V, 15V, or 20V depending on their USB-PD profile. A reading of zero volts or an unstable, fluctuating value indicates a faulty charger.
Interpreting the result: Apple documents that iPhone 15 and later fast charge with a USB-C cable and a USB-C power adapter supporting USB-PD. For iPhone 17, iPhone 17 Pro, iPhone 17 Pro Max, and iPhone Air, fast charging requires at least 40W, with recommended profiles including 5VDC/3A, 9VDC/3A, and AVS ranges of 15-20VDC/2A or 9-15VDC/2.67A. iPhone 18 Pro and iPhone 18 Pro Max need a USB-C cable rated for 60W or more and a 60W-or-greater adapter with AVS. Samsung phones and tablets use USB-C PD charging, and some chargers support Super Fast Charging or Super Fast Charging 2.0 for supported Galaxy models. Google Pixel phones charge over USB-C, and Google recommends using the power adapter that came with the phone.
Understanding voltage readings for fast chargers
Apple adapters use specific voltage profiles: 5VDC/3A for standard charging, 9VDC/3A for fast charging on compatible iPhones, and AVS (Adjustable Voltage Supply) ranges up to 20V for iPhone 17 and later models. Samsung Super Fast Charging and Google Pixel fast charging also rely on USB-PD variable voltages. If your multimeter shows a voltage that matches one of these profiles, the charger is likely capable of negotiating that voltage with a compatible phone, but the multimeter test alone cannot confirm that negotiation will succeed.
Step 4: Test the USB port functionality (optional)
If the device is not recognized when connected to one USB port, try connecting it to another port on the computer to identify if the issue lies with the port itself or the device. Restart the device to resolve temporary software glitches or conflicts that may be causing USB connection issues.
When to replace your phone charger
Replace a charger if you observe any of these signs: zero volts at the USB output, fluctuating or unstable voltage readings, visible scorch marks on the adapter or cable, or a loose USB port that does not hold the plug firmly. A charger that shows correct open-circuit voltage but fails to charge a phone under load may still be faulty, its internal components might not deliver the required current or sustain the negotiated voltage. In such cases, the charger should be replaced even if the multimeter reading appears normal.
Frequently Asked Questions
Can I test a wireless charger with a multimeter?
No, a standard multimeter cannot measure the electromagnetic field used by wireless chargers. Wireless charging requires a compatible device to complete the circuit, and a multimeter probe cannot simulate that load.
What if my multimeter shows 0V but the charger works with my phone?
This usually means the probes are not making proper contact with the USB pins. Ensure the red probe touches the inner contact and the black probe touches the outer shell firmly. Some USB-C ports have recessed pins that require careful probe placement.
Can a multimeter test the amperage (current) of a phone charger?
Yes, but only under load. To measure current, you must connect the charger to a device or resistor that draws power, and the multimeter must be set to current mode (A) and wired in series. This is more complex than a voltage test and carries a risk of blowing the meter's fuse if done incorrectly.
Does a multimeter test work on all USB charger types?
It works on any charger with a USB-A or USB-C port by measuring DC voltage at the output pins. However, chargers with proprietary connectors (like some laptop bricks) may require a different approach, and the voltage reading may not reflect the charger's ability to negotiate with a device.
What do the terms "AA and AAA battery specs" have to do with charger testing?
Understanding AA and AAA battery specs helps you recognize that a multimeter measures DC voltage for both batteries and chargers. The same DC voltage setting used to check a 1.5V AA battery is used to test a charger's output, just at a higher range.
What can go wrong with a phone charger?
Frayed or damaged cables causing short circuits, loose or broken connector pins preventing proper contact, internal wiring issues or damaged components causing complete charging failure, overheating, and compatibility issues.
Can a faulty phone charger damage my phone?
Yes, using a charger with frayed cables, loose connector pins, or internal wiring issues can cause short circuits, and a charger with voltage output exceeding device specifications can lead to overcharging, overheating, and battery damage.
Is it safe to use a third-party phone charger?
It can be safe if from a reputable brand certified by safety organizations like UL, but poorly made or counterfeit chargers pose risks of overheating, short circuits, or electrical shocks.
