Here's how to test a CR2032 battery to see if it's good

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Your car key fob suddenly needs several presses to unlock the door. Your AirTag stops responding when you try to locate it. Before blaming either gadget, it's worth checking the battery inside. One common type is the CR2032, a 3V lithium coin cell also used in Samsung's SmartTag2 and some smart home sensors. Depending on the device and how often it's used, one can last way over a year.

A multimeter can help you check the battery's voltage, and it's a good addition to your toolbox if you don't already own one. There are cheaper ways to check, too, but don't mistake a voltage reading for a complete picture of battery health.

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Begin by checking the battery itself. If you spot crusty residue, rust-colored patches, swelling or leakage, skip the test and replace it. Have a look at the device's contacts too, since corrosion there can cause trouble even with a good battery.

Remove the CR2032 and set your digital multimeter to DC voltage (V with a solid line over a dashed line). Plug the black lead into COM and the red lead into the V or VΩ port. If your meter doesn't automatically select a range, set it above 3V, typically 20V.

Hold the battery by its edges over a nonconductive surface. Touch the red probe to the broad face marked with a plus sign (+) and the black probe to the center of the smaller negative face underneath. Keep the probe tips from touching each other or bridging the battery's terminals. A negative reading simply means you've reversed the probes.

A fresh CR2032 typically reads around 3V, while a reading closer to 2.5V is a good reason to bring in a replacement. But don't give that 3V reading too much credit though. A multimeter draws almost no current, so a battery might look fine on the display but struggle when your key fob or tracker actually needs power. There's no single pass-or-fail reading for every device. If it keeps acting up, try a fresh CR2032 unit before assuming the device is faulty.

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A dedicated coin-cell tester is another option. Some cost around $10, although certain models only measure voltage rather than how the battery performs under load. Basic multimeters can cost less than $20 and are good for checking other batteries or troubleshooting low-voltage electronics. Still, buying either just to diagnose one suspect $3 battery is a harder sell.

There's also the LED trick, assuming you have one lying around. A small red or yellow LED is a better choice than a blue or white one because it generally needs less voltage to light up. Touch its longer leg to the CR2032's positive side and the shorter leg to the negative side. If it lights up, the battery can supply some current. That's about all it proves. The LED won't tell you how much life is left, and one that stays dark doesn't necessarily mean the battery is dead.

AirTag owners can check Find My before opening anything. Select Items inside the app and tap your AirTag. If it says "Low Battery," or shows a depleted battery icon, skip the testing and replace the CR2032. Some CR2032 batteries have bitterant coatings that may prevent proper contact, even when they're brand new. Apple recommends packaging marked "Compatible with Apple AirTag" when replacing an AirTag battery, and says an AirTag's battery lasts more than a year, while Samsung rates the SmartTag2 for up to 500 days in normal mode.

After removing a CR2032, keep it out of children's reach and tape its terminals before taking them to a battery collection facility. If one is swallowed, seek emergency medical care immediately, as serious internal injuries can occur in as little as two hours.