Reverse Wireless Charging: What It Is & How to Use It

Reverse wireless charging lets a smartphone or tablet act as a wireless charging pad, topping up another device—like earbuds, a smartwatch, or another phone—by simply placing it on the back. You enter the battery level of the device doing the charging and the device being charged, and the calculator below estimates how much power the receiver gets and how long it takes. The result shows the estimated charging time and the power transfer efficiency based on typical real-world losses.

Formula

The calculator uses two standard formulas. First, the energy delivered in watt-hours (Wh):

Energy (Wh) = Power (W) × Time (h) × Efficiency (%) / 100

Second, the estimated charge added in milliampere-hours (mAh), assuming a typical 3.7V lithium-ion battery voltage:

Charge (mAh) = Energy (Wh) / 3.7V × 1000

Variables: Power is the output of the transmitting device in watts (W). Time is how long you charge in minutes. Efficiency is the percentage of power that actually reaches the receiving battery—wireless charging loses energy as heat. The 3.7V figure is the nominal voltage of most phone batteries.

Worked Example

Using the default values: 30 minutes, 5W output, 70% efficiency.

1. Convert time to hours: 30 min ÷ 60 = 0.5 h
2. Energy delivered: 5W × 0.5h × 0.70 = 1.75 Wh
3. Charge added: 1.75 Wh ÷ 3.7V × 1000 = 473 mAh

The calculator shows 1.75 Wh and 473 mAh. This matches the formula exactly. In practice, a 4000 mAh phone battery would gain about 12% from this session.

What Is Reverse Wireless Charging?

Reverse wireless charging is a feature built into many modern smartphones that lets the phone broadcast power outward through its back panel, turning it into a portable charging pad. It uses the same Qi standard that powers normal wireless charging, but the direction of energy flow is reversed. Instead of receiving power from a pad, the phone transmits it to a device placed on its back.

The term “reverse” simply means the power flows opposite to the usual direction. It is also called “wireless power share” or “battery share” by some manufacturers, but the underlying technology is identical. The feature first appeared in flagship phones around 2018 and has since become common on mid-range models.

How It Works

Inside a phone with reverse wireless charging, there is a copper coil near the back cover. Normally, this coil receives alternating magnetic fields from a charging pad and converts them into electrical current. In reverse mode, the phone’s battery supplies DC power to an inverter circuit, which turns it into AC. That AC current flows through the coil, generating a magnetic field. A second device placed on top has its own coil that picks up this field and converts it back to DC to charge its battery.

Key technical points:

• Qi standard: Most phones use the Qi protocol, so they can charge any Qi-certified device, including earbuds, smartwatches, and other phones.
• Power output: Reverse charging typically delivers 2.5W to 10W, much lower than a wall charger (15W–65W).
• Activation: It is usually turned on in Settings under Battery or Connections, and often shuts off automatically when the phone’s battery drops below 20–30%.
• Alignment: The receiving device must be centered on the coil area, usually marked on the back of the phone.

When to Use It (and When Not To)

Reverse wireless charging is best for small, low-power devices or emergency top-ups. Good use cases:

• Charging wireless earbuds or a smartwatch while traveling, without carrying extra cables.
• Giving a friend’s phone a few percent to make an urgent call.
• Charging a stylus, smart ring, or other small Qi accessory.

Avoid using it for:

• Charging a large tablet or laptop—the power output is too low and the process would take hours.
• Charging your own phone from another phone—it is slow and drains the donor phone quickly.
• Leaving devices stacked overnight—heat buildup can degrade batteries.

Real-World Performance and Limitations

In practice, reverse wireless charging is inefficient. Typical efficiency is 60–75%, meaning 25–40% of the energy is lost as heat. A phone with a 4000 mAh battery can transfer only about 1500–2000 mAh to another device before it drops to 20% and stops. That is roughly enough to charge a pair of earbuds twice or boost another phone by 30–40%.

Charging speed also depends on the receiving device. Most earbuds accept only 1–2W, so they charge slowly but safely. Smartwatches may require specific alignment. Phones with larger batteries can accept more power, but the donor phone heats up noticeably.

Common mistake: assuming reverse charging is as fast as a wall charger. It is not. A 10-minute reverse charge might add 3–5% to a phone, while a wall charger could add 30% or more in the same time.

Safety, Heat, and Battery Health

Heat is the main concern with reverse wireless charging. The transmitting coil generates heat, and the receiving device adds its own. Prolonged exposure to high temperatures can degrade lithium-ion batteries. Manufacturers build in safeguards:

• Automatic shutoff if the donor phone gets too hot.
• Power reduction when the battery is below a certain level.
• Foreign object detection—if a metal object (like a coin) is placed on the back, the phone stops transmitting.

To protect your devices:

• Remove thick cases, especially those with metal or magnetic attachments.
• Do not use reverse charging while the phone is itself wirelessly charging.
• Stop if either device feels hot to the touch.
• Use it sparingly—it is a convenience, not a primary charging method.

Alternatives and Comparisons

If you need to charge multiple devices, consider these alternatives:

• Multi-device wireless pads: Charge a phone, earbuds, and watch simultaneously from a wall outlet. Faster and more efficient than reverse charging.
• Power banks: A 10,000 mAh power bank costs little and can charge a phone fully several times. Reverse charging is essentially a built-in, low-capacity power bank.
• Cables: A simple USB-C cable is faster and more efficient than any wireless method. Use it when you have a wall outlet.
• Battery cases: For specific phones, these add bulk but provide reliable backup power.

The main advantage of reverse wireless charging is convenience—no cables, no extra device. The disadvantage is speed and efficiency. It is a “better than nothing” feature for emergencies, not a replacement for a proper charger.

How to Choose and Use It Correctly

If your phone has reverse wireless charging, here is how to get the most out of it:

1. Enable it in Settings (look for “Wireless power share,” “Battery share,” or “Reverse charging”).
2. Place the receiving device face-down on the back of the phone, centered on the marked coil area.
3. Wait for the confirmation vibration or on-screen notification.
4. Keep both devices still—movement breaks the connection and wastes power.
5. Stop when the receiving device reaches 100% or when the donor phone drops below 20%.

Check your phone’s manual for the exact location of the coil, as it varies by model. Some phones require the receiving device to be placed with its own coil aligned—usually the center of the back. If charging does not start, reposition the device or remove its case.

Frequently Asked Questions

Does reverse wireless charging work with any phone? No. The donor phone must have the feature, and the receiving device must support Qi wireless charging. Most modern phones from major brands support Qi, but some budget models do not.

Can I charge my phone from another phone? Yes, if both support reverse wireless charging. However, it is slow and drains the donor phone. You will typically get 10–20% in 15–20 minutes.

Is reverse wireless charging safe for my battery? Yes, when used occasionally. The phone’s software limits heat and power. Frequent use, especially in hot environments, can accelerate battery wear.

Why is reverse charging so slow? The phone’s coil is small and designed for low power. Also, wireless transfer is inherently less efficient than wired. Most phones cap reverse output at 5W or less to manage heat.

Can I charge my smartwatch with reverse charging? Yes, if the watch supports Qi. Many modern smartwatches do, but some use proprietary chargers. Check the watch’s specs before relying on reverse charging.

Does reverse wireless charging work with a case? It works with thin plastic or silicone cases, but thick cases, metal, or magnetic attachments can block the signal. Remove the case if charging fails.

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in t | Charging time | min | 30 | 0
in p | Power output | W | 5 | 0
in e | Efficiency | % | 70 | 0
out energy | Energy delivered | Wh | p * t / 60 * e / 100 | 2
out charge | Charge added | mAh | p * t / 60 * e / 100 / 3.7 * 1000 | 0