Batteries

How batteries work: types (AA, AAA, 9 V, button cells, LiPo), what V and mAh mean, why a battery gets weaker, and rules for using batteries safely.

5 Nov 2026 Electronics Info

Assorted batteries: AA and AAA batteries, a 9 V battery and its clip, a holder for four AA batteries, button cells and a small LiPo battery with wires

What is a battery?

A battery stores chemical energy. When we connect it into a closed circuit, the chemicals inside start to react and the battery drives a current around the circuit: from plus (+), through the wires and components, back to minus (−). When the chemicals are used up, the battery is flat.

Every battery has two terminals, plus and minus. They are marked + and − on the battery, and in a schematic a battery has its own symbol: the longer line is plus.

What is inside a battery?

An alkaline AA battery cut lengthwise and connected to a lamp. The numbers mark the steel can with the plus terminal at the top (1), the black manganese dioxide along the wall (2), the thin separator (3), the grey zinc gel in the middle (4), the brass pin (5), the plastic seal (6) and the minus terminal at the bottom (7). The arrows show the current from plus through the lamp to minus

The picture shows an alkaline AA battery cut lengthwise and connected to a lamp:

The steel can. It is also the plus terminal, with the small bump at the top.
Manganese dioxide, the black substance along the wall of the can. It is mixed with graphite so that it conducts better.
The separator, a thin paper that keeps the plus and minus sides apart.
Zinc powder in a thick gel, in the middle of the battery.
A brass pin connects the zinc to the minus terminal.
A plastic seal closes the battery and keeps the minus terminal apart from the can, which is plus.
The minus terminal, a metal disc at the bottom.

The zinc and the manganese dioxide can only react when the current has a path outside: from plus through the wires and the lamp back to minus. Inside the battery the circuit is closed by the electrolyte, an alkaline liquid that soaks the separator and the zinc gel. The separator lets the electrolyte through, but doesn’t let the zinc and the manganese dioxide touch. While the circuit is open, the reaction almost stops, so a new battery in a drawer keeps its energy for years. When the zinc is used up, the battery is flat. The electrolyte burns the skin, and it is what comes out of a leaking battery.

Strictly speaking, what we call an AA battery is a single cell. A real battery is several cells joined together, like the 9 V battery. Inside it there are six small 1.5 V cells, joined one after another by metal strips, the plus of one to the minus of the next, so their voltages add up: 6 × 1.5 V = 9 V.

On the left a 9 V battery, on the right the six small 1.5 V cells from inside it, laid out in a row. They stand alternately the other way round and are joined by metal strips, the plus of one to the minus of the next, so together they give 6 × 1.5 V = 9 V between the minus and plus snaps

In some 9 V batteries the cells are small cylinders, as in the picture, and in others they are flat plates stacked on top of each other. In everyday speech we call them all batteries, and so will we.

Which batteries do we use?

Battery	Voltage	Where we use it	Can it be recharged?
AA (code LR6)	1.5 V	the Maqueen robot (three of them)	no
AAA (code LR03)	1.5 V	the micro:bit battery holder (two of them)	no
9 V (code 6LR61)	9 V	first circuits, measurements	no
button cell CR2032	3 V	small lights, watches, remote controls	no
rechargeable AA (NiMH)	1.2 V	instead of an ordinary AA	yes, with its own charger
LiPo and Li-ion	3.7 V	mobile phones, drones	yes, only with a special charger

The code LR6, LR03 or 6LR61 is printed on the battery in small letters. The letter L means it is an alkaline battery.

What is written on a battery?

Voltage (V): how hard the battery drives a current. A new AA battery has a little over 1.5 V, and a flat one about 1 V.
Capacity (mAh): how long the battery can keep supplying a current. More mAh doesn’t mean a stronger battery, but one that lasts longer. This is explained in misconception 7.
A crossed-out bin: don’t throw the battery in the household rubbish, but in a box for old batteries.

When we put batteries one after another, their voltages add up: the three AA batteries in the Maqueen give 3 × 1.5 V = 4.5 V. Other ways of connecting batteries are explained in misconception 6.

Why does a battery get weaker?

As a battery runs down, its voltage slowly drops. That is why a robot with worn-out batteries drives more slowly and an LED shines more dimly.

The voltage also drops when the battery has to supply a large current. A battery that shows a good voltage on the multimeter can give out when it drives a motor. While we only measure, a tiny current flows out of it. When the motor switches on, the current rises and the battery’s voltage falls. If a robot jerks oddly, or the micro:bit restarts by itself when the motors start, change the batteries first.

Rules for batteries

Mind plus and minus. The holder shows which way round each battery goes.
Don't mix old and new batteries, or batteries of different kinds.
Never charge a battery that doesn't say it can be recharged. It can leak or burst.
Don't connect a battery's plus and minus with a wire or a piece of metal: that is a short circuit. Don't carry a 9 V battery in a pocket with keys and coins.
If a battery gets hot, swells up or leaks, switch the circuit off at once and call the leader. Don't touch the white or crystal-like substance around a leaking battery: it burns the skin.
Take the batteries out of a device you won't use for a long time, because over time they can leak.
Take old batteries to a box for old batteries. Many shops and schools have one.

Button cells are not for little ones

A button cell looks like a sweet or a coin, and if it is swallowed it is very dangerous: within a few hours it can burn the inside of the throat. Keep button cells, new and used, away from younger brothers and sisters. If someone swallows a battery, they must go to hospital at once, without waiting for signs to appear.

Rechargeable batteries and LiPo

This part is for the Arduino workshop and for projects such as the Drone.

NiMH

A rechargeable AA battery (NiMH) has 1.2 V instead of 1.5 V. Most devices work just as well with it, because the voltage of an ordinary AA battery soon drops below 1.5 V anyway. Three rechargeable batteries give 3.6 V instead of 4.5 V, so the robot may drive a little more slowly. They are charged only with a NiMH charger.

Li-ion and LiPo

Lithium batteries store a lot of energy in a small, light package, so they are in mobile phones, laptops and drones. One cell has 3.7 V. Full, it has 4.2 V, and while it is working its voltage mustn’t fall below about 3.3 V. A cell that is run down further is permanently damaged.

A LiPo battery can supply a very large current. How much it may supply is given by the C rating: a 500 mAh, 20C battery may supply up to 20 × 0.5 A = 10 A. That same ability makes it dangerous: in a short circuit the current rises to tens of amps, and the battery can catch fire.

It is also dangerous when a short circuit happens inside the battery. Inside, a LiPo cell is a stack of thin layers:

the pouch, made of aluminium foil covered with plastic,
plus electrodes: aluminium foil coated with a lithium compound,
separators: a thin plastic film soaked in electrolyte, only about 20 micrometres thick, thinner than a hair,
minus electrodes: copper foil coated with graphite.

The layers inside a LiPo cell. Left, a cell in good condition: inside the pouch (1), plus electrodes (2), separators (3) and minus electrodes (4) take turns; all the plus electrodes are joined to the plus tab on the left, and all the minus ones to the minus tab on the right. Right, a nail has gone through all the layers and joined the plus and minus electrodes, and the spot around it glows and heats up

All the plus electrodes are joined to the plus tab, and all the minus electrodes to the minus tab. Between every plus and minus electrode there is an insulating separator. The picture shows only three plus and two minus electrodes; a real battery has many more. If the battery is knocked, bent or punctured, like the nail in the picture on the right, the separator is damaged and a plus and a minus electrode touch. A large current flows through that spot and it heats up, the heat damages the separator around it, and the battery heats itself up more and more. Such a fire is hard to put out. A real-world example is the Galaxy Note 7 phones on the Short circuit page.

Mobile phones and ready-made battery packs have a protection circuit that disconnects the battery when the current is too high or the voltage too low. LiPo batteries for models and drones often have none, so we have to look after the battery ourselves.

Rules for LiPo batteries

A LiPo battery is charged only by an adult, with a LiPo charger, in a fireproof bag and never unattended.
Don't bend, puncture, squash or short-circuit the battery. Don't carry it loose in a bag.
Keep the cell voltage between 3.3 V and 4.2 V. If you won't use the battery for a long time, leave it half charged (about 3.8 V per cell).
Hand a swollen, knocked or damaged battery to the leader straight away. Don't use it or charge it.
For measurements on the test bench, use a bench power supply with current limiting instead of a LiPo battery.

How long does a 9 V battery last?

A 9 V battery has a small capacity, about 500 mAh. An Arduino Uno with nothing connected uses about 50 mA, so such a battery would power it for about ten hours:

\[ t \approx \frac{500\ \text{mAh}}{50\ \text{mA}} = 10\ \text{h} \]

With LEDs, a buzzer or a servo it lasts much less, and a motor runs it down quickly. That is why a 9 V battery is not good for powering an Arduino for a long time. USB or an adapter is better, and how a steady DC voltage is made from AC is explained on the Rectifiers and voltage regulators page.

What did we learn?

A battery stores chemical energy and drives a current from plus, around the circuit, back to minus.
Inside a battery the plus and minus sides are kept apart by a separator. The reaction only runs while the circuit is closed.
Voltage (V) tells us how hard the battery drives a current, and capacity (mAh) for how long it can.
A battery gets weaker as it runs down and when it has to supply a large current, for example to a motor.
We don’t mix batteries, short-circuit them or charge them unless they say they can be recharged. We keep button cells away from small children.
LiPo batteries store a lot of energy and can catch fire, so only an adult charges them, with a special charger and under supervision.

Short quiz

1. What voltage do three AA batteries give when put one after another?

The voltages add up: 3 × 1.5 V = 4.5 V. That is how the batteries in the Maqueen robot are connected.

2. Yesterday the robot drove fast, and today it drives slowly and jerks. What do you check first?

The batteries. When they are worn out, their voltage drops, especially when the motors need a large current. I replace all the batteries with new ones at once.

3. May you have one new and two old batteries in the same holder?

No. The old ones run down first, and the new one keeps driving a current through them, so they can leak. We always change all the batteries at once.

4. A younger brother is playing with a button cell. Why is that dangerous?

If he swallows it, within a few hours the battery can burn the inside of his throat. The battery should be taken from him and put away where he can’t reach it. If he has swallowed it, he must go to hospital at once.

5. A 1000 mAh LiPo battery is rated 25C. What is the largest current it may supply?

25 × 1 A = 25 A.

6. You notice that a LiPo battery has swollen up. What do you do?

I don’t use it or charge it, but hand it to the leader straight away. A swollen battery is damaged inside and can catch fire.

7. What is the separator in a battery for?

It keeps the plus and minus sides apart, but lets the electrolyte through. That is why the reaction only runs when the outside circuit is closed. If the separator is damaged, plus and minus touch and there is a short circuit inside the battery.