USB-C charger keeps renegotiating power? What to test

Your device starts charging quickly, then slows down or briefly stops and starts again. That can happen when a USB-C charger and device adjust their power, but a loose cable, hot device, or faulty port can cause similar symptoms.

Use the Charger & Cable Test to compare charging rates with different chargers or cables. It reports battery change as a percentage per minute, which can help you spot a repeatable difference. It does not show the USB-C power contract, voltage, or wattage, so it cannot directly confirm that power renegotiation occurred.

Compare the charger and cable

Open the Charger & Cable Test on the device you want to check. The test appears only if your browser reports battery information for that device.
Connect the charger and cable that show the changing charge behavior. Keep the device on a stable surface and note whether its charging state changes or the connection repeatedly starts and stops.
Let the test measure the charging rate, shown as a percentage per minute. Record the result along with the battery level and whether the device feels warm.
Repeat with one change at a time. Try a different USB-C cable with the same charger, then a different compatible charger with the same cable. Keep the device, outlet, and test conditions as similar as you can.
Compare results at similar battery levels. If one cable or charger repeatedly causes interruptions or a much lower rate, check that part more closely.

A percentage-per-minute reading is an estimate of how quickly the battery level changes, not a measurement of electrical power. It can vary as the battery fills, the device warms up, or the device uses more energy. A short reading by itself does not prove that a charger or cable is defective.

What the result means

The rate changes, but charging stays connected

This can be normal. A device may request less power as its battery gets fuller or warmer, or when it is doing demanding work. USB Power Delivery can also adjust the power supplied to match what the device can safely accept. A changing rate does not automatically mean the charger is repeatedly disconnecting.

The rate is consistently lower with one cable

The cable may be damaged, poorly seated, or unable to support the charging level your device and charger can use together. Some USB-C cables support less power than others. Repeat the comparison with a known-good cable that is rated for your device's charging needs.

Charging repeatedly starts and stops

Frequent connection changes are different from a gradual slowdown. Check the plugs and ports for looseness, debris, or damage, then test a different cable and charger separately. If the interruption follows one component, that component is the likely cause.

Both setups slow down at a similar battery level

The device may be reducing charging as the battery approaches full, or changing its rate because of temperature or a charging-management feature. Compare again when the device is cooler and at a lower, similar battery level. A browser test cannot read the charger's negotiated voltage or confirm which charging protocol is active.

The test does not show a charging rate

Your browser may not provide battery information, or the device may not report its charging state to the page. Battery Meter and Charger & Cable Test are available only on devices whose browser reports a battery. Use the device's charging indicator and repeat the physical checks below instead.

How to fix unstable charging

Reseat both ends. Unplug the cable from the device and charger, then reconnect it firmly. If the plug feels loose or moves easily in the port, avoid pressing or bending it to make contact.
Check for heat. Let a hot phone, tablet, or laptop cool in a shaded, ventilated place before testing again. Remove a thick case if it traps heat. Stop using a charger or cable that is unusually hot, smells burnt, or shows visible damage.
Try another compatible cable. Keep the charger and outlet the same so you change only one part. Use a cable suitable for the device's charging needs, and do not assume every USB-C cable supports the same power.
Try another compatible charger and outlet. Keep the cable and device the same. Use a charger that supports the device's required charging standard and power level. A different result can help narrow down whether the charger is involved.
Check the device's charging settings and status. On Windows 11, open Settings › System › Power & battery. On a Mac, open System Settings › Battery. On an iPhone, check Settings › Battery; on Android, open Settings › Battery, though names vary by manufacturer. Battery protection or optimized charging can affect when charging slows, so check the device's own explanation before changing a setting.
Inspect the port safely. Look for lint, bent contacts, or corrosion with a light. Do not insert metal tools or pour liquid into the port. If the port is damaged or a plug will not stay seated, arrange service rather than forcing the connection.
Restart the device and repeat the comparison. If charging still cuts out with more than one known-good charger and cable, a device port, battery, or charging circuit may need attention.

When the charger, cable, or device needs repair

If the problem follows one cable across compatible chargers, stop using that cable and replace it. If it follows one charger with different cables, replace the charger or contact its manufacturer. Do not keep testing equipment that has a cracked casing, exposed wire, scorch marks, or an unusual smell.

If several known-good charging setups all disconnect on the same device, the device's USB-C port or internal charging hardware may be faulty. Back up important data if practical, then contact the manufacturer or a repair shop. A browser test can help compare charging behavior, but it cannot certify electrical safety or diagnose a damaged charging circuit.

Why power can change without a disconnect

USB-C is the connector shape, not a guarantee of one charging speed. A compatible charger and device negotiate what power to use, and some systems can adjust that amount while charging. The battery's charge level, temperature, cable capability, and device workload can all affect the rate. The Charger & Cable Test measures the resulting battery change, not the negotiation messages themselves.

For a useful comparison, test at a similar battery level, keep the screen and workload steady, and change only one part of the charging setup at a time.