How to Test a Car Battery


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The car battery is probably one of the most understated pieces of critical equipment in a car, regardless of its vintage, classic, or modern. The battery typically sits there, doing its job admirably and silently, deteriorating over time, until it finally fails. As Murphy’s Law dictates, if anything can wrong, it will go wrong, and in terms of the car battery failing, this ‘going wrong’ can occur at the most inopportune of moments, such as when you are leaving home for work and are already late, about to embark on a long trip, or need to rush for an emergency. How can you mitigate this unpleasant surprise? Let us take you through how to test a car battery.

Why should you test your car battery? The car battery deteriorates over time, much like the other components on your car. However, it gives precious little warning before completely failing, and can cause a serious issue if you are pressed for time or faced with a transportation urgency. Thankfully, there are ways you can periodically test your car battery, and utilize diagnostic solutions such as Carly to monitor and maintain it in good condition throughout its lifespan.


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What Does the Battery Do in My Car?

How to test a car battery

The simplest activity that the battery undertakes is to provide electricity to the starter motor so that you can crank the engine to life with the turn of a key or push of a button, rather than using a hand crank, which can be dangerous if improperly used. However, a car battery bears much greater responsibility than this, especially in modern cars with cutting-edge technology and advanced computer systems.

The car battery keeps the car’s electronics alive when the car is parked. Do not assume that the electrical system has been disconnected completely when you turn off the car and remove the key, especially if you’re considering how to test a car battery.

Do you remember how your car’s clock resets, and you lose your radio presets when the battery goes flat and has to be replaced? This is only the tip of the iceberg; the computer systems themselves will reset under a dead battery. In fact, on older and simpler EFI cars, disconnecting the battery is one way of resetting the ECU or Engine Control Unit. However, on newer cars with complex computer systems, you do not want to do this — no matter which way you decide how to test a car battery.

How to Test a Car Battery: Why Do Batteries Go Bad?

The car battery is continually charging and discharging throughout its life. It is charged while the engine is running, slowly discharges when the car is parked and faces an abrupt discharge when the starter motor is activated. That is why it’s recommended to connect a trickle charger to the battery if a car is going to be parked for the winter, for example. Temperature extremes can also accelerate the deterioration of the battery. The average lifespan of a car battery is around three to five years, but if you’re using cheaper batteries in temperature extremes, two years of life is not uncommon.

Here’s How to Test a Car Battery

How to test a car battery

If you’re wondering how to test a car battery, the simplest method is to borrow a multimeter. Since this device is a wonderful piece of equipment to have in your toolkit and can be used for a myriad of purposes around your home, why not buy one? You can find them cheaply, from $20 onwards. Set the multimeter to its voltage measuring mode, typically 20 or 50 volts, touch the red probe to the positive terminal on your car battery, and the black probe to the negative terminal.

If your multimeter is showing a reading of around 12.2 volts or more when the car is switched off, the battery is good. If this value is closer to 12.0 volts or below, the battery needs to be replaced, although this may also be the case if your car has been parked for a few weeks without being started.

Now switch on the engine and with it running, take another voltage reading. Be careful not to mix up the positive and negative probes, remember red is positive and black is negative. With the engine running, the voltage should read between 14 and 14.5 volts. If the reading is lower, there are two possible outcomes. One is that your battery is weak, and two is that your car’s charging system has a fault.

However, this method gives you a very rough and rudimentary indicator of the health of your car battery, and to obtain a more in-depth verdict, consider investing in a consumer-grade diagnostic solution that can tell you not just the health of your battery, but of your entire car and its onboard computer systems. There are hundreds of options out there, and we recommend Carly.

Why Should I Pick Carly?

Carly is a two-part solution for diagnostic scanning, that’s been developed to work synergistically. The hardware component is the Carly Universal OBD Scanner which is an unobtrusive dongle that plugs straight into your vehicle’s OBD2 port. If you’re wondering where the port is, your vehicle owner’s manual will reveal the location, as will a quick internet search with your vehicle make and model. If you’re thinking about whether your vehicle has an OBD2 port, if it was manufactured after 2005 it’s basically a certainty.

The software component of Carly is the Carly app which lives right on your smartphone, making it a great solution in how to test a car battery! Using wireless communication, it liaises with the Carly Universal OBD Scanner and unlocks a world of possibilities for you. The Carly app is available for Apple and Android devices from the official app stores, and there’s even a Carly lite app that allows you to check what’s supported on your car before spending any money. Talking about money, Carly costs under $100 for the app and adaptor, which is tremendous value, especially as regular software updates are provided too.

How to Test a Car Battery with Carly?

How to test a car battery

How to test a car battery is simple with Carly. Simply plug in the Carly Universal OBD Scanner to your car’s OBD2 port, fire up the Carly app on your smartphone, wait for the two to establish a connection and you’re up and running. If you’ve got a BMW, there’s an additional benefit as you can register your battery with Carly and obtain a deep analysis of its health condition. If your vehicle is of another make, you can still view standard battery errors under the Health Diagnostic section of the Carly app.

What Else Can Carly Do?

Carly can perform in-depth diagnostics on your vehicle’s computer control units, detecting faults and presenting them to you with an easy-to-understand ‘traffic light’ system, where faults are separated into critical ones, important ones, and those you don’t need to worry about. Carly also allows you to perform basic maintenance on your car such as oil, filter, and battery changes, by ensuring that the requisite maintenance minders are reset and initializations performed, which can be a lifesaver for enthusiast owners who know how to perform such maintenance but lack the OBD-specific equipment to complete the task.

If you enjoy customizing certain aspects of your car, Carly’s coding function is on hand, offering a simple interface and a way to reset any changes should they not be to your liking. Finally, the Carly Used Car Check is a powerful tool that can be used to verify if a used car has faced mileage tampering.

Keep in mind, that every car has different software and modules built-in. Therefore specific features will vary with every model.

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