For How Long Can a Car Battery Charge a Cell Phone?

By David Robinson

Car batteries are available even in the most remote places. With a 12-volt adapter, these batteries can charge your phone. Knowing how long you can use the battery to keep charging your phone requires some basic math skills.

How Powerful Is The Battery

Car batteries deliver a steady 12 volts, but the current varies according to the design of the battery. Batteries are rated in amp-hours, describing how many amps the battery will deliver over a set period of time. High amp-hour batteries last longer than low amp-hour batteries.

How Much Power Does The Charger Use?

Phone chargers draw power from the battery and convert it to a voltage suitable for the phone. Putting power into the phone necessitates taking power out of the battery, and the faster the power is taken, the quicker the battery is discharged.

Typical Charger Power Usage

Research conducted at the Lawrence Berkeley National Laboratory shows a mobile phone charger, while charging a phone, uses an average of 3.68 watts. Current equals wattage divided by voltage, so the average phone charger is drawing 0.3 amps from a 12-volt battery.

How Long Will The Battery Last?

A fully charged 200 amp-hour battery will, in theory, deliver 20 amps for 10 hours or 2 amps for 100 hours. The charger draws 0.3 amps, so the battery will run the charger for a maximum of 666 hours ((20/0.3) amps * 10 hours). Due to inefficiencies in batteries, the actual time will be about 2/3 of this: 444 hours.