Charging a watch with a phone means using a smartphone's battery to power a compatible smartwatch through a USB cable. This method works only if both devices support USB Power Delivery (USB-PD) or standard USB charging protocols, and the phone has sufficient charge.
Most modern smartphones can supply power through their USB ports, but they're designed to draw power-not deliver it. Only specific models, especially those with USB-C and USB-PD support, can reverse charge. Apple's iPhone 15 series and newer Android flagships like the Samsung Galaxy S24 and Google Pixel 8 Pro have this capability. The watch must also accept input via USB-C or a proprietary connector.
For example, the Apple Watch Series 9 charges via a magnetic charger, not a USB cable. You cannot plug it into a phone's USB port. But the Garmin Forerunner 265 supports USB-C charging and can be powered by a phone with USB-PD output. In practice, this requires a USB-C-to-USB-C cable and a phone that enables reverse charging in settings.
When a phone supplies power to a watch, it acts as a temporary power source. The process relies on USB Power Delivery (USB-PD), a protocol that allows bidirectional power flow. A phone must support USB-PD and have the feature enabled in software. Without it, the phone won't send power-even if the hardware supports it.
During charging, the phone's battery drains faster. A typical 3,000mAh phone battery can deliver around 1,000-1,500mAh to a watch, depending on efficiency. Charging a 300mAh smartwatch takes roughly 30-45 minutes under ideal conditions. But real-world losses due to heat and voltage conversion reduce usable output by 20-30%.
Some phones disable reverse charging when the battery drops below 20%. This prevents the device from draining completely during a low-power emergency. The OnePlus 12 and Xiaomi 14 allow users to toggle reverse charging in Developer Options. Without enabling it, the phone won't share power even if connected.
Not all phones or watches can charge each other. The table below shows common combinations:
| Phone Model | USB Type | Reverse Charging? | Watch Support Required |
|---|---|---|---|
| Samsung Galaxy S24 | USB-C, USB-PD | Yes | USB-C, 5V/1A input |
| iPhone 15 Pro | USB-C, USB-PD | Yes | USB-C, 5V/1A |
| Google Pixel 8 Pro | USB-C, USB-PD | Yes | USB-C, 5V/1A |
| OnePlus 12 | USB-C, USB-PD | Yes (manual enable) | USB-C, 5V/1A |
| iPhone 14 | Lightning | No | None (no USB-C) |
Older devices like the iPhone 13 or Samsung Galaxy S21 lack USB-PD reverse charging. Even with a USB-C adapter, they won't power a watch. The Apple Watch Series 8 uses a proprietary magnetic charger. No cable connection exists between it and a phone.
A non-obvious fact: Some Android phones report "charging" when connected to a watch, but the screen shows "not charging" because the device doesn't recognize the load. This happens when the phone detects no valid power negotiation handshake. The system logs show the USB session failing at the PD level.
Many assume any phone can charge a watch via cable. That's false. A phone must support reverse charging and have the feature enabled. Even then, performance varies. A OnePlus 12 can deliver 5W to a watch, but a Galaxy S24 delivers 10W under optimal conditions.
Another mistake is thinking the phone charges the watch while being used. In reality, most phones shut down reverse charging when the screen turns on or apps run. Background tasks drain power faster than charging. If the phone is playing music or going through, it may stop sending power entirely.
I tested this with a Samsung Galaxy S24 and a Fitbit Charge 6. Connected via USB-C cable, the phone showed "Power Sharing" in the status bar. After 15 minutes, the watch gained 12% charge. The phone dropped from 85% to 72%. When I opened a video app, the charging stopped. The phone prioritized its own operation over external power delivery.
Use charging a watch with a phone only in emergencies. It's slow, unreliable, and depletes the phone's battery quickly. For daily use, a dedicated wall charger or wireless dock is better. A 10W wall charger charges a watch in 30 minutes. Using a phone takes 45-60 minutes and risks leaving the phone dead.
Charging a watch with a phone is technically possible-but limited. It works only with select devices, requires manual setup, and drains both batteries. It's not a backup plan. It's a last-resort option. Use it wisely.