Multimeter Guide

What is a multimeter?

A multimeter is a versatile device that can be used to accurately measure different electrical values in batteries, components and circuits. When it comes to batteries and DC voltage, a multimeter can be commonly used to measure current, voltage, and resistance of circuits or components. They can also be used to perform a continuity test, which allows the detection of broken circuits.

Main Measurement Functions:

• Voltage DC (V): Measures the voltage between any two points in a DC circuit.
• Current DC (A): Indicates the rate of flow of electrical charge through a circuit or component, usually 10A maximum. For higher values, use a clamp meter.
• Resistance (Ω): Indicates how strongly a component resists the flow of current.
• Continuity: Makes an audible noise when a circuit is complete (closed). If the circuit is broken (open) the multimeter will not make a noise .

Common uses:

Troubleshooting: Locating electrical faults in vehicles, 12V setups and electronic devices.
Component Testing: Checking whether batteries, fuses, and other parts are working properly.
System Maintenance: Verifying that voltage and current are within expected ranges in electrical systems.

How do I use a multimeter to test a battery’s voltage? (step by step)

Safety First!

Before you begin, it's crucial to follow a few safety precautions. Car batteries contain corrosive acid and can produce flammable gases.

• Turn everything off: Make sure your car's engine, headlights, radio, and all other electronics are off.
• Wear your gear: Put on your safety glasses and gloves.
• Ensure good ventilation: Work in a well-ventilated area, like an open garage or outdoors.
• Keep metal away: Be careful not to let any metal tools touch both battery terminals at the same time, as this can cause a short circuit. This includes spanners, screwdrivers, watches and jewellery.

Step 1: Locate the battery

Ensure that you can access the two terminals on top of the battery. Most batteries are in the engine bay, but can also be located in the boot or under a seat. Check the manual or call your local R&J branch for more help locating the battery. You may be required to remove a plastic cover or some trim pieces to access the battery.

Step 2: Ensure the vehicle is switched off:

For an accurate reading of the battery voltage, ensure the vehicle is switched off. You may need to isolate the battery from the system by disconnecting the positive or negative terminal.

Step 3: Connect the Multimeter:

Turn the multimeter dial or settings to Voltage DC position. Connect the positive probe to the positive terminal and the negative probe to the negative terminal; you’ll see the terminals marked with '+' and '-'.

Step 4: Read the voltage:

Observe the reading on the multimeters display.

If you see a reading with a minus sign in front (-12.6V instead of 12.6V), that means the probes are possibly reversed. Ideally, the resting voltage of a lead acid battery should be no lower than 12.6V.

A fully charged lead acid car battery should have a healthy voltage. Here’s what the numbers mean:

• 12.6V or above: Your battery is fully charged and in good shape.
• 12.1V to 12.5V: Your battery is partially discharged and should be charged ASAP.
• 12.0V or below: Your battery is severely discharged and should be charged ASAP. It may have a problem holding a charge. You should get the battery checked by a professional as soon as possible.

Voltage Drop Under Load (cranking)

A weak battery can struggle to maintain a stable voltage, especially under load or during engine cranking. Over time the battery’s internal components can degrade which can reduce the battery’s ability to hold charge or deliver sufficient power.

Using a multimeter regularly can help you stay on top of your battery’s health. This can increase the life of the battery and avoid early replacements. We recommend the REDMAX multimeter for easy testing. It is a Digital Pen Type Multimeter which is easy to use and carry. Learn more about the product here: https://www.rjbatt.com.au/redmax#multimeter

What’s the difference between a multimeter and a battery tester?

Both a multimeter and a battery tester can help you assess the condition of a car battery. Here's a breakdown of the key differences:

1. Functionality

• Multimeter: A multimeter measures various electrical properties, such as voltage, current, and resistance. When testing a car battery, it primarily measures the battery's voltage to determine its state of charge. However, it doesn't provide a comprehensive analysis of the battery's overall health.
• Electronic Battery Tester: This tool is specifically designed for testing starting batteries. It not only measures the voltage but, depending on the model, also evaluates the battery's starting power, internal resistance, and its ability to perform under load. It provides a more detailed assessment of the battery's health and whether it needs to be replaced.

2. Ease of Use

• Multimeter: The multimeter is super quick and easy to use. It’s handy for a quick battery health check, especially if you’re using a 2 handed Digital Pen type multimeter like the REDMAX Multimeter.
• Battery Tester: These don’t take much longer than the multimeter to use but provide more information. They require running a load through the battery and taking diagnostics. Many models also have a digital display that provides clear, straightforward results, often with recommendations like "Good," "Recharge," or "Replace."

3. Purpose

• Multimeter: A general-purpose tool for diagnosing electrical issues in various systems, not just car batteries. It’s useful for testing circuits, fuses, and other electrical components.
• Electronic Battery Tester: A specialised tool for diagnosing the health and performance of car batteries and, in some cases, the vehicle's charging system (alternator).

4. Cost

• Multimeter: Generally more affordable and widely available. It’s a great tool for DIY enthusiasts who need a multipurpose device.
• Electronic Battery Tester: Often more expensive due to its specialised features, but it’s a worthwhile investment for those who frequently test batteries or work in automotive repair.

5. Output

• Multimeter: Displays raw data (e.g., voltage or resistance), leaving the interpretation up to the user.
• Electronic Battery Tester: Provides detailed diagnostics and actionable insights , such as the battery's condition , and whether it needs replacement. It can also perform a starting and charging system test in some vehicles.