Set the multimeter to voltage mode before touching anything

A multimeter has a dial with different settings. To measure voltage, you turn the dial to the voltage symbol — it looks like a V with a wavy or straight line next to it. Most multimeters have two voltage settings: DC (straight line) and AC (wavy line). DC voltage is what batteries produce. AC voltage is what comes from wall outlets and power lines.

If you are not sure which one you need, start with DC. Most household electronics and batteries run on DC. Wall outlets are AC, but you should never test wall outlets yourself — that is dangerous and not something a beginner should do.

Key Takeaways

  • Turn the dial to the V symbol (DC or AC) before you touch the probes to anything.
  • The red probe measures the positive side, the black probe is ground or negative — always plug them into the correct ports on the multimeter.
  • Touch the red probe to the positive terminal and the black probe to the negative terminal or ground, then read the number on the screen.
  • If the number is 0 or very close to 0, the battery or power source is dead or not connected properly.
  • Never test wall outlets, power cords, or anything plugged into the wall without training — the voltage can hurt or kill you.

Plug the probes into the correct ports

A multimeter has two or three ports where the probes plug in. The black probe always goes into the port labeled COM (common). The red probe goes into the port labeled V or VΩmA (depending on what you are measuring). Some multimeters have a separate port for measuring current, but for voltage, the red probe goes into the V port.

Check your multimeter's manual or the label on the back if you are not sure. Using the wrong port will not break the multimeter, but you will not get a reading.

Touch the probes to the power source in the right order

Hold the red probe against the positive terminal or wire. Hold the black probe against the negative terminal, ground, or the other side of what you are testing. The order matters: red first, then black. If you reverse them, the multimeter will show a negative number instead of a positive one — the reading is still correct, just with a minus sign.

For a battery, the positive terminal is usually marked with a plus sign and is often the bump on top. The negative terminal is marked with a minus sign and is usually flat. For a wire or connector, you may need to look at the label or diagram to know which side is positive.

Read the number on the screen

The multimeter will show a number in volts. A fresh AA battery reads around 1.5 volts. A fresh 9-volt battery reads around 9 volts. A car battery reads around 12 volts when the engine is off. If the number is 0 or very close to 0, the battery is dead or you are not touching the terminals properly.

Some multimeters have an analog dial with a needle instead of a digital screen. The needle will move to point at the voltage. Read where the needle stops on the scale labeled for voltage.

What the reading tells you

If the voltage is close to what you expect, the power source is working. If it is much lower than expected, the battery is weak or dying. If it is 0, the battery is dead or the connection is broken. If the number bounces around or changes quickly, there may be a loose connection or a problem with the power source.

A battery does not have to read exactly the labeled voltage to work. A 9-volt battery that reads 8 volts is still usable. A 1.5-volt battery that reads 1.2 volts is getting weak but may still power a device. Once a battery drops below about 0.9 volts per cell, most devices will not use it.

Common mistakes that give wrong readings

The most common mistake is forgetting to set the dial to voltage mode before you start. If you leave it on resistance or current mode, you will get a wrong number or no reading at all. The second mistake is plugging the probes into the wrong ports — check the labels on the back of the multimeter.

A third mistake is not making good contact with the terminals. If you are barely touching the probe to the battery, the reading may be 0 or unstable. Press the probe firmly against the terminal. If you are testing a wire, strip a small bit of insulation so the probe touches bare metal, or use the probe to pierce the insulation slightly.

When not to use a multimeter

Do not test wall outlets, power cords, or anything plugged into the wall. Wall voltage is 120 volts in the United States and 230 volts in many other countries. That much electricity can cause a serious shock or burn. If you need to know whether a wall outlet is working, use a plug-in outlet tester instead — they are made for this and cost less than ten dollars.

Do not test the voltage while the circuit is powering a device unless you know what you are doing. Testing a battery while it is running a motor or light will give you a lower reading because the device is drawing power. If you want to know the true voltage of a battery, test it when nothing is connected to it.

Frequently Asked Questions

What does it mean if the multimeter shows a negative number?

It means you reversed the probes — the red probe is on the negative side and the black probe is on the positive side. The voltage is still correct, just shown as negative. Swap the probes and you will see a positive number. Both readings are accurate; the sign just tells you which way you connected them.

Can I test a battery while it is still in a device?

Yes, but the reading will be lower than the battery's true voltage because the device is using power. If you want to know if a battery is good, remove it from the device first, wait a few seconds, then test it. This gives you the battery's actual voltage without the device pulling power.

Why does my multimeter show 0 volts on a battery I know is not dead?

The probes are probably not touching the terminals properly. Make sure you are pressing the probe firmly against the positive and negative terminals. If the battery is old or corroded, the terminals may be dirty — wipe them with a dry cloth and try again. Also check that the dial is set to the V symbol, not another setting.

Is it safe to test a phone charger or laptop power adapter?

Only test the output side — the part where the cable plugs into the device, not the part that plugs into the wall. The wall-plug side carries dangerous voltage. If you test the output side while nothing is plugged in, you should see 0 volts. If something is plugged in and drawing power, you may see the rated voltage (usually 5, 12, or 20 volts depending on the device).

What if the voltage keeps changing when I hold the probes still?

A bouncing or changing reading usually means the connection is loose or the terminals are dirty. Try pressing harder with the probes, or clean the terminals with a dry cloth and try again. If the reading is still unstable, the battery may be damaged or the multimeter may have a problem. Try testing a different battery to see if the multimeter works correctly.