When you connect a laptop to a phone or iPad, the laptop typically acts as the power source for charging. This behavior is primarily dictated by the USB specifications and the power delivery capabilities of the devices involved.

How Charging Works

1. Power Source Roles: In a standard USB connection, one device is designated as the “host” (usually the laptop), while the other device (the phone or iPad) is the “peripheral.” The host provides power to the peripheral. This is due to the USB protocol, which specifies that host devices must supply power at a voltage of 5V and a certain current level.

2. Device Power Requirements: Different devices have varying power requirements. For instance, an iPad typically requires more power (around 10W) than an iPhone (around 5W). Many laptops’ USB ports may not provide sufficient power to charge an iPad effectively, which can lead to situations where it shows “not charging” even when connected.

3. USB-C and Power Delivery: With the advent of USB-C, there is more flexibility in power delivery. Both devices can negotiate who supplies power based on their capabilities. However, in most cases, laptops are still configured as hosts and will provide power to charge connected devices.

Why Phones Don’t Charge Laptops

• Design Limitations: Laptops are designed to receive power through their charging ports rather than through USB ports. The circuitry in most laptops does not allow them to draw power through USB connections; they are configured to output power instead.

• Charging Protocols: While technically possible for a phone to charge a laptop using USB-C under specific conditions (like using an OTG adapter), this is not the default behavior and typically requires special configurations or protocols that are not present in standard setups.

Summary

In essence, when you connect your phone or iPad to a laptop, the laptop acts as a charger due to its role as a host device in the USB architecture. The design of laptops generally prevents them from being charged via their USB ports, while phones and tablets are equipped to draw power from them.

References

1. Apple Discussions - How Does a Laptop Automatically Charge a Phone?
2. Reddit - ELI5: How Does a Laptop Automatically Charge a Phone?
3. RepairMate - Is Charging Phone from Laptop Bad?
4. Apple Support - Charging Your iPhone from a Computer
5. Apple Support - Charging Your iPad from a Computer
6. PCWorld - iPad USB Charging