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

Mobile phones often require mobile power supplies.

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.

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