How to check charge rate before school

Your phone or laptop is low on battery before a school day, and you are not sure whether the charger will give it enough charge in time. A quick comparison can show whether the charger and cable are working together as expected.

Use the Charger & Cable Test to measure how many battery percentage points the device gains per minute. The result is most useful when you compare chargers under similar conditions, rather than treating one reading as a guaranteed charge time.

Check the charge rate

Open the Charger & Cable Test on the phone, tablet, or laptop you want to check. The tool appears only on devices whose browser reports battery information.
Note the current battery percentage. For a useful comparison, begin each test at roughly the same level, preferably below 60 percent. Charging often slows as the battery gets closer to full.
Connect the charger and cable you want to check. Keep the test page open and let the device charge for about 10 minutes. Avoid using demanding apps during the measurement.
Read the charge rate in percent per minute. If you have another charger or cable, repeat the test while changing just one item at a time.

Use the same device, outlet, approximate starting charge, and screen activity for each run. If the browser does not show the test on your device, it may not report battery data to websites. That does not mean the charger is faulty.

What the result means

The percentage rises at a steady rate

The device is receiving charge during the test. Compare this result with another cable or charger to see which setup charges faster in the same conditions. For example, a reading near 0.5 percent per minute would suggest about 10 percentage points in 20 minutes if the rate stayed constant. In practice, charging speed can change, so treat that as a rough estimate, not a promise.

The rate is lower than you expected

The cable, charger, outlet, device temperature, or current battery level may be limiting the rate. The number is based on battery percentage, not a direct measurement of electrical power. A small change in percentage can also be hard to measure accurately over a very short test.

The percentage does not rise

First check that the device recognizes the charger and that the cable is fully connected. If the battery level continues to fall while plugged in, the device may be using power faster than the charger supplies it, or charging may have paused because of heat or a device setting.

How to fix a slow charge

Check the connection and outlet. Seat both ends of the cable firmly and try a different wall outlet. If you are using a power strip, test directly from the outlet to rule out a loose connection.
Try another cable. Use a cable that supports charging for your device, not just one that fits the port. Check for fraying, bent connectors, or a loose fit. Stop using a cable that is damaged or becomes unusually hot.
Compare a compatible charger. A phone or laptop may charge slowly from a low-power USB port or an adapter that does not support its charging standard. Try the device's recommended charger or another compatible one. Change only the charger or cable between tests so you know what made a difference.
Reduce activity and heat. Close games, video calls, navigation, and other demanding apps while charging. Move the device out of direct sun and remove a thick case if it is trapping heat. A hot device may slow or pause charging to protect its battery.
Check the device's battery settings. On Windows 11, open Settings › System › Power & battery. On a Mac, open System Settings › Battery. On an iPhone, check Settings › Battery; on Android, check Settings › Battery, though names vary by model. Battery protection or optimized charging may pause or limit charging in some situations. Do not turn off a battery protection feature just to force a brief test.
Restart the device. If the charge indicator seems stuck after checking the cable and settings, restart the phone or computer, reconnect the charger, and compare again.

When the charger or device may be faulty

If the rate stays very low with a known-good compatible charger and cable, test the device at a different outlet and check whether its charging port feels loose. A port that only charges when the plug is held at an angle may need repair. Stop charging if the connector, cable, or adapter becomes very hot, smells unusual, or shows damage.

If the same charger and cable charge another compatible device normally but yours barely gains charge, the issue may be the device's port, battery, or charging circuitry. Contact the manufacturer or a repair shop, especially if the device is still under warranty. Do not insert metal objects into the charging port.

Why the number changes

Percent per minute is a convenient comparison, but it is not the charger's wattage. The device controls how much power it accepts, and that can change with battery level, temperature, charging limits, and whether the screen or apps are active. The battery percentage is also rounded, so a short test can make the calculated rate jump around.

For a school-day estimate, use a longer test and compare setups under matching conditions. A steady result can help you choose between chargers, but leave extra time before you head out instead of relying on a precise minute-by-minute prediction.