How do batteries power things?

1. Remote-control cars, clocks, flashlights and phones all run on batteries. A battery is a little package of energy you can carry anywhere.
2. Inside, a battery has two different chemicals kept apart by a thin wall. Together they store energy, a bit like the energy stored in food.
3. Connect a circuit and the chemicals react. The negative end gives out electrons, which travel through the wire and the bulb to the positive end. Inside, charged particles move to keep things balanced.
4. Some devices are hungrier than others. A tiny LED sips energy slowly, while a toy car's motor gulps it down, so the same battery runs out much sooner.
5. When the chemicals are used up, the reaction stops and the battery is flat. The energy didn't vanish: it became motion, light, sound and heat.
6. Rechargeable batteries, like the one in a phone, can be refilled. A charger pushes electrons backward and runs the chemical reaction in reverse.

This is a simplified cutaway of an AA battery connected to a bulb. Tap each part to see its job.

• Positive end (+). The bump on top is the positive terminal. Electrons that have traveled around the circuit come back into the battery here.
• Negative end (−). The flat end is the negative terminal. Electrons released by the chemicals flow out of here into the wire.
• Zinc powder (negative electrode). The middle is packed with zinc powder in a gel. When the battery works, the zinc slowly changes and gives away electrons. That's where the energy comes from.
• Manganese dioxide (positive electrode). A ring of dark manganese dioxide lines the inside of the can. It takes in the electrons that come back around the circuit.
• Separator. A thin paper-like wall keeps the two chemicals from touching. Charged particles can slip through it, but the chemicals can't mix.
• Electrolyte. A damp chemical paste called the electrolyte soaks everything. It lets charged particles called ions move inside the battery to keep the loop going.
• Current collector. A brass pin runs down the middle of the zinc. It collects the electrons and carries them to the negative end.
• Steel can. The steel can holds everything together and connects the positive chemicals to the positive end. A label wraps around the outside.

Connect each device to a fresh battery and watch the energy level fall. Time is sped up. Which device empties the battery fastest?

Experiment: Which device will empty the battery fastest?

1. Press New battery, then choose a device.
2. Watch how fast the energy level falls and how many hours pass.
3. Try the LED and the toy car. Which one makes the battery go flat first?

What we found: The toy car empties the battery fastest because its motor needs lots of power. The LED and the clock sip energy so slowly that the same battery lasts days or even years. The battery holds the same energy each time: what changes is how fast the device uses it.

Turn on JavaScript to try it yourself.

Your tent needs a light and a fan from 8 pm to 6 am. Choose devices so the battery lasts all night.

A battery stores energy in chemicals. Inside an AA battery are two different materials: zinc powder in the middle and manganese dioxide around the outside, with a damp paste called the electrolyte and a thin wall called the separator between them.

When you connect the battery in a circuit, the chemicals start to react. The zinc gives away tiny charged particles called electrons. They can't get through the separator, so they travel out of the negative end, through the wires and your device, and back in at the positive end. That flow of electrons is the electric current.

The battery doesn't make energy from nothing. It changes chemical energy into electrical energy, and the device changes that into light, sound, motion or heat. When the zinc is used up, the reaction stops and the battery is flat.

Different devices use energy at different rates. A clock or an LED sips it slowly, so a battery can last months. A toy car's motor gulps it quickly. Rechargeable batteries can be refilled by a charger, which runs the reaction backward. Old batteries should always be recycled, never thrown in the trash or opened.

Key words

Electrode

One of the two chemical ends inside a battery, positive or negative.

Electrolyte

A damp paste inside a battery that lets charged particles move.

Electron

A tiny particle with negative charge. A flow of electrons in a wire is an electric current.

Power

How fast a device uses energy. Measured in watts.

Watt-hour

The energy used by a 1-watt device running for 1 hour. Battery capacity is often given in watt-hours.

Rechargeable

A battery whose chemical reaction can be reversed by a charger so it can be used again.

Check your thinking

1. What kind of energy does a battery store?

   Show the answer

   Chemical energy. The chemicals inside store energy until a circuit lets them react.

2. Which device would make a battery go flat fastest?

   Show the answer

   A toy car with a motor. Motors need lots of power, so they use the stored energy quickly.

3. Where do electrons leave a battery?

   Show the answer

   The negative (−) end. The zinc releases electrons, which flow out of the negative end, around the circuit and back in at the positive end.

4. What should you do with a used-up battery?

   Show the answer

   Recycle it with a grown-up's help. Batteries hold chemicals that can sting or leak, so they should be recycled, never opened or burned.

5. Why do batteries run down faster in very cold weather?

   Show the answer

   The chemical reactions slow down. Chemical reactions are slower when cold, so the cell can't deliver as much of its stored energy.