Mobile Fundamentals · Inside a Phone · Lesson 4 of 4

Batteries & Charging

Lithium cells, charge cycles and what the port really does.

Why batteries wear out

Phone batteries are lithium-ion cells. They pack a lot of energy into a small, light space, but they slowly wear out. As a battery ages it holds less charge, so a full battery doesn't last as long as it did when the phone was new.

Wear is measured in charge cycles. One cycle means using 100% of the battery's capacity in total, whether that happens all at once or across several partial charges. Using 60% one day and 40% the next adds up to one cycle. Each cycle wears the cell a little.

Heat is the other big factor. Apple says its devices are most comfortable between 16 and 22 °C (62 to 72 °F), and that temperatures above 35 °C (95 °F) can permanently reduce battery capacity. Leaving a phone on a car dashboard in summer, or charging it somewhere hot, speeds up wear. Cold drains a battery faster in the moment, but that effect goes away once it warms up.

Apple rates iPhone 15 and later models to keep 80% of their original capacity after 1,000 cycles. Older iPhones are rated for 500.

More than power

The charging port on your phone (USB-C on most phones today) does more than charge it. It also carries data. That's how you copy photos to a laptop or back up a phone to a computer. It's also how forensic tools connect to a phone to pull out evidence.

Because a cable can carry data, phones are careful about what they connect to. An iPhone asks whether to Trust This Computer and then asks for your passcode. Android phones usually charge only, with no data shared, until you choose to allow file transfer.

A public charging port rigged to steal data or install malware is called juice jacking. The FBI and FCC have both warned about it, though confirmed real-world cases are rare. Avoiding it is simple.

• Plug your own charger into a wall outlet instead of using a public USB port.
• Carry a battery pack.
• Use a charge-only cable, which has no data wires.
• If your phone asks whether to trust a device or share data, choose no or charge only.