Why a charger is fast with one phone but slow with another

You plug two phones into the same charger, but one gains charge much faster. That does not automatically mean the charger or the slower phone is broken. Phones can accept different amounts of power, use different charging standards, and slow charging for several reasons.

Compare both phones under the same conditions with the Charger & Cable Test. It measures charging speed as battery percentage gained per minute, which can help show whether the difference is repeatable.

Compare the phones fairly

1. Open the Charger & Cable Test on the first phone. If the browser asks for permission to access battery information, allow it.
2. Use the same wall outlet, charger, and cable for each phone. Connect one phone at a time, and leave the other unplugged.
3. Start each comparison at a similar battery level, such as 20 to 40 percent. Let the phone charge for at least 10 minutes while the screen is off and you are not using demanding apps.
4. Note the reported percentage per minute, then repeat the test on the other phone using the same setup. If the browser does not provide battery data, the test may not be available on that phone.

Do not compare one phone charging from 10 percent with another charging from 85 percent. Phones commonly reduce charging speed as the battery fills, so a high battery level can make a healthy charger look slow.

What the result means

One phone is consistently faster

The phones may have different maximum charging rates or may support different charging standards. A charger's label shows what it can provide, not what every phone will accept. The phone and charger negotiate a compatible power level, and a phone that supports only a lower level will charge more slowly even when connected to a high-wattage adapter.

Charging speed in percentage per minute is not a direct measurement of watts. Phones have different battery capacities, so the same amount of electrical power can produce different percentage increases. Use the test to compare a single phone across chargers or cables, rather than treating two phones' percentages as a precise power comparison.

Both phones are slow with this charger

The adapter, cable, outlet, or charging conditions may be limiting both. A cable can support charging but still lack the capabilities needed for a phone's faster charging mode. A multiport charger may also reduce the power available to one port when another device is connected.

The speed changes during the test

That can be normal. Charging slows as the battery approaches full, and a phone may reduce power when it gets warm, is in heavy use, or is protecting its battery. A brief reading can also vary, so compare measurements taken over similar periods instead of relying on a single moment.

The phone does not show charging data

Some mobile browsers do not provide the battery information the test needs. This does not mean the phone is failing to charge. Check the charging icon and the phone's own battery screen, or compare how much charge it gains over the same timed period.

What to check

1. Match the charging conditions. Compare at similar battery levels, with both phones idle and cool. Remove a thick case if the phone feels warm, and keep the screen off during the timed comparison.
2. Check the adapter's output. Read the small print on the charger for its supported output and charging standards. A charger marked with a high maximum wattage may offer that rate only with compatible devices, ports, and cables.
3. Try a known-compatible cable. Use the cable supplied with the phone or one rated for the phone's charging standard and power level. Test the same phone with another cable, changing only the cable so you can tell whether it makes a difference.
4. Try another port and outlet. If the charger has multiple ports, test one port at a time. Plug it directly into a wall outlet rather than a computer, hub, or low-power USB port.
5. Check for heat or heavy use. Navigation, games, video calls, and hotspot use can consume power while the phone charges. On Android, check Settings › Battery for battery use and any charging or temperature notice. On iPhone, check Settings › Battery and look for a charging message on the Lock Screen.
6. Inspect the charging port. Look for lint, corrosion, or a loose fit. Do not push metal objects into the port. If the plug does not sit securely, stop using that cable and have the port checked.
7. Restart and retest. A restart can clear a temporary software issue. Afterward, repeat the comparison with the same outlet, charger, cable, starting battery level, and screen-off conditions.

When it may be a hardware problem

If one phone remains much slower with several known-compatible chargers and cables, while the other charges normally, the phone may have a worn battery, damaged port, or internal charging fault. Check for a system warning about battery health or moisture before testing again. If the connector is loose, the phone gets unusually hot, or charging repeatedly stops and starts, unplug it and arrange service rather than continuing to test.

If the same cable or charger is slow with multiple compatible phones, stop using it if it is frayed, unusually hot, or damaged. Otherwise, compare its output with the device makers' charging requirements and replace it with a compatible, reputable adapter or cable if needed.

Why charger wattage is not the whole story

Fast charging depends on an agreement between the charger and the phone. USB-C Power Delivery, PPS, and manufacturers' own charging systems can use different voltage and current combinations. If a phone and charger do not share a fast-charging mode, they fall back to a slower level they both support. The cable also has to carry the negotiated power.

Battery percentage adds another wrinkle. A large battery may gain fewer percentage points per minute than a small battery at the same charging power. For a useful comparison, test each phone with the same setup and similar starting charge, then judge each phone against its own normal behavior.