Why a device may reject an incompatible fast charger

Your phone says it is charging, but it is charging slowly. Or it stops charging when you connect a fast charger. This does not always mean the charger is broken. The device and charger may not support the same fast-charging method, or the cable may be limiting the power they can use.

Here is how to tell whether the charger, cable, device, or charging conditions are responsible. The Charger & Cable Test can compare charging rates on a device whose browser reports its battery level.

Compare the charger and cable

Open the Charger & Cable Test on the device you are charging. If the browser reports its battery, use the test to observe the charging rate in percent per minute.
Start with a charger and cable that you know work with this device. Keep the device in the same place and note the rate after charging for several minutes.
Swap only the cable, then compare the rate under similar conditions. Next, put the original cable back and swap only the charger. Changing one part at a time helps show which part affects the result.
For a fairer comparison, keep the screen off, close demanding apps, use the same outlet, and compare at roughly the same battery level. Charging speed can vary even when you do not change the charger.

The test reports a change in battery percentage over time, not the charger's exact wattage. It appears only on devices whose browser reports battery information, so a missing reading does not prove the charger is incompatible.

What the result means

The battery charges, but more slowly with one charger

The device and charger may have fallen back to a slower charging mode. Fast charging requires a compatible power method, and the cable must also support the current the devices are trying to use. A charger with a high wattage rating may still charge slowly if it does not offer a mode your device accepts.

Changing the cable changes the rate

The cable may be limiting charging, or its connector may be worn or damaged. Some USB-C cables support less current than others. A cable that works for ordinary charging is not necessarily capable of every fast-charging rate.

The rate slows as the battery fills

This can be normal. Many devices reduce charging speed as the battery approaches full, and they may also slow charging when the battery is hot or cold. Compare chargers at a similar battery level rather than judging one by a brief reading near 100 percent.

The device does not charge with one combination

The charger, cable, or charging port may be faulty, or that combination may not work with the device. A browser test cannot identify the exact cause. Try a known-good compatible charger and cable before assuming the device itself is broken.

The browser shows no battery reading

Some browsers and devices do not provide battery information to web pages. The Charger & Cable Test cannot measure charging rate on those devices, and this result says nothing about whether the charger is safe or compatible.

How to fix slow or missing fast charging

Check the charger and cable for damage. Stop using a charger with a cracked case, bent connector, exposed wire, or signs of overheating. Do not test a damaged charger just to compare its speed.
Try a known-good combination. Use a reputable charger and cable that the device maker recommends or lists as compatible. For a laptop, check that the specific USB-C port accepts charging. Some ports transfer data but do not charge the computer.
Reconnect both ends. Unplug the cable from the device and charger, then connect it firmly again. If the device charges when the connector is held at an angle, stop bending it and have the cable or port checked.
Check the port for debris. Power the device off before inspecting it. Do not put metal tools or liquid into the port. If debris is packed inside or the connector is loose, ask a repair professional to clean or inspect it.
Let the device cool and reduce its workload. Charging may slow when the device is hot, in direct sun, or running a game or other demanding app. Move it to a cool, dry place and try again with the screen off. Do not put it in a refrigerator or freezer.
Check for a charge limit or battery protection setting. On an iPhone with iOS 18, check Settings › Battery › Charging; on earlier versions, check Settings › Battery › Battery Health & Charging. Android menu names vary, but look under Settings › Battery for charging optimization or battery protection. These options can limit charging near full, but they do not make an incompatible charger support fast charging.
Check the computer's charging information. In Windows 11, open Settings › System › Power & battery to review battery status. Some laptops have a separate manufacturer app for charge limits. On a Mac, open System Settings › Battery to review battery options. These screens can help explain a charge limit, but charger compatibility depends on the computer, port, charger, and cable.

When the charger or device may be faulty

If a known-good, compatible charger and cable also fail, test the original charger and cable with another compatible device if you can. If the problem follows the charger or cable, replace that part. If several known-good combinations fail only with one device, its charging port or battery may need service.

Stop charging if the charger, cable, or device becomes unusually hot, smells burnt, makes a buzzing sound, or shows physical damage. A charging-rate comparison is not an electrical safety test, and it cannot certify a charger as genuine or safe.

Why charger wattage is not the whole story

A charger does not force its maximum rated power into every device. With common USB-C Power Delivery chargers, the charger offers supported power options and the device requests one it can use. Other fast-charging systems may require a particular protocol or a charger from the device maker. If the charger and device cannot agree on a fast mode, they may use a slower fallback instead.

The cable is part of that agreement too. Some cables have current limits, and some higher-power USB-C combinations need a cable designed to carry more current. The device may also limit charging to protect its battery or reduce heat. So a slower rate can be normal, even with a working fast charger.