Why charging slows when you use the phone
When a phone is plugged in, not all of the power from the charger necessarily goes straight into the battery. The phone is also using power at the same time.
The screen, processor, mobile data, Wi-Fi, GPS, camera, video playback, gaming, and other active tasks can consume part of the incoming power. That leaves less power available for actually charging the battery.
The effect is usually most noticeable during demanding activities such as gaming, video calls, navigation, recording video, or using the phone at high screen brightness. You can also browse our Charging Problems section for more smartphone charging issues.
Heat can reduce charging speed even further
Using a phone while it charges can also make it warmer.
Modern smartphones monitor battery and device temperature. If the phone becomes too warm, charging speed may be reduced automatically to protect the battery and other components. In some cases, fast charging may temporarily stop until the device cools down.
This is why a phone may charge normally when the screen is off but charge much more slowly when it is being actively used.
Check whether the charger is supplying enough power
A low-power charger can make the difference more obvious.
For example, if the charger supplies only slightly more power than the phone is consuming, the battery percentage may rise very slowly. During heavy use, it may barely increase at all.
Use a charger and cable that support the charging standard and power level recommended for the phone. A damaged cable, unsupported adapter, loose connection, or dirty charging port can also reduce the available charging power.
A simple way to test the cause
Plug the phone into the same charger and leave the screen off for about 15 to 20 minutes.
Then compare that with another 15 to 20 minutes while actively using the phone.
If charging is clearly faster while the phone is idle, active power use or heat is probably contributing to the slower charging.
If charging remains unusually slow even when the phone is cool and unused, the charger, cable, charging port, battery, or charging settings may need further checking. For additional guidance, see Apple's official information about charging and temperature.
When heat is a warning sign
A phone becoming slightly warm while charging can be normal.
However, stop using and charging the phone if it becomes extremely hot, shows a temperature warning, has a swollen battery, produces an unusual smell, or shows signs of physical battery damage.
Allow an overheated phone to cool in a safe place before charging it again. Do not try to cool it rapidly with ice, a freezer, or water.