The simplest way to measure a battery is to check its voltage with a digital multimeter. A fresh AA battery reads about 1.54 volts, a healthy car battery sits around 12.6 volts, and a fully charged lithium-ion cell reads 3.6 to 3.7 volts. Comparing your reading to these benchmarks tells you whether a battery is full, partially drained, or dead.
What You Need
A basic digital multimeter is the only tool required. You can find one for under $20 at any hardware store, and most models work the same way. You’ll use two test leads: a red one and a black one. The black lead plugs into the port labeled COM (common ground), and the red lead plugs into the port labeled V/Ω (voltage/resistance). That’s the standard setup for measuring any battery.
Setting Up Your Multimeter
Turn the dial to DC voltage, usually labeled VDC or marked with a solid line above a dashed line. Batteries produce direct current, so you need the DC setting rather than the AC setting used for household outlets.
If your multimeter has manual range selection, choose a range higher than the battery’s expected voltage. For a 1.5V AA or AAA battery, select the 2V or 6V range. For a 9V battery, select the 20V range. For a 12V car battery, select the 20V or 60V range. If your multimeter is auto-ranging, it handles this step for you.
Taking the Reading
Touch the red probe to the battery’s positive terminal (marked with a +) and the black probe to the negative terminal (marked with a −). The voltage reading appears on the screen almost instantly. If you accidentally reverse the probes, the multimeter will simply show a negative number. That won’t damage anything, but flip the probes to get a clean positive reading.
For loose batteries like AAs and 9Vs, hold the probes firmly against the metal contacts. For a car battery, press the probes directly onto the lead terminals under the hood. A steady, stable number means your connection is good. A flickering reading usually means the probe isn’t making solid contact.
Reading Results for Common Batteries
AA and AAA Alkaline Batteries
A brand-new alkaline cell starts at about 1.54 volts. The voltage drops steadily with use, hitting roughly 1.25 volts at the halfway point. Once the reading drops to about 1.0 volt, the battery is effectively dead and most devices will stop working with it. If you’re sorting through a junk drawer of loose batteries, anything reading 1.3 volts or above still has useful life. Below 1.1 volts, toss it.
9V Batteries
A 9V battery is six 1.5V cells stacked in series, so the same proportional drop applies. A fresh one reads around 9.5 volts. Below about 7 volts, it’s spent.
Car Batteries
A fully charged 12V car battery reads 12.5 to 12.66 volts with the engine off. This is called the resting voltage, and you should measure it after the car has been sitting for at least a couple of hours so the surface charge dissipates. A reading of 12.4 volts is the minimum threshold for a battery with enough charge to start reliably.
You can also test how a car battery performs under load by having someone crank the engine while you watch the multimeter. A healthy battery will drop 1 to 1.5 volts during cranking, then bounce back quickly once the starter disengages. If the voltage plunges to 8 or 9 volts (or lower) during cranking, the battery is likely failing and needs replacement.
Lithium-Ion Cells
Standard lithium-ion cells, like the 18650s found in laptop packs and power tools, have a nominal voltage of 3.6 volts. Fully charged, they read about 4.2 volts. A reading below 3.0 volts means the cell is deeply discharged, and repeatedly draining them that low causes permanent damage. Lithium-ion battery packs in consumer devices often have multiple cells wired together, so a “12V” lithium pack typically contains cells in series adding up to that total.
Voltage vs. Actual Capacity
A voltage reading tells you the state of charge: how full or empty the battery is right now. It does not tell you about the battery’s overall health, which is how much total energy it can still hold compared to when it was new. Think of it like a gas gauge versus the size of the tank. An old car battery might read 12.6 volts (full gauge) but fail under load because its total capacity has degraded.
Measuring actual capacity in milliamp-hours (mAh) requires a discharge test. You drain the battery through a known load at a controlled rate while tracking how long it lasts. Dedicated battery testers and hobby chargers can do this automatically, but it takes hours and isn’t necessary for everyday checks. For most people, the voltage reading combined with a load test is enough to judge whether a battery is worth keeping.
Testing Internal Resistance
As batteries age, their internal resistance rises, which limits how much current they can deliver. You can estimate internal resistance at home with a multimeter and a known resistor. First, measure the battery’s voltage with nothing connected (open-circuit voltage). Then connect a resistor across the terminals, measure the voltage again under that load, and use the difference to calculate the resistance.
For example, if a AA battery reads 1.5 volts open-circuit but drops to 1.4 volts with a load, that 0.1 volt difference is being lost across the battery’s internal resistance. Using Ohm’s Law with the load current gives you the resistance value. A healthy AA cell has an internal resistance well under 1 ohm (around 0.2 to 0.3 ohms). Higher values mean the battery will struggle to power demanding devices even if the voltage looks fine.
Why the Bounce Test Doesn’t Work
You may have seen videos suggesting you can test an alkaline battery by dropping it on a hard surface: fresh batteries thud and stay put, while dead ones bounce. Researchers at Princeton University found this test is unreliable. The bouncing happens because the zinc inside the battery gradually converts to zinc oxide as the cell discharges, forming tiny bridges between particles that act like springs. But that springy network reaches its maximum bounce well before the battery is actually dead. So a battery that bounces might be half-drained or fully drained. The bounce tells you the battery isn’t brand new, but it can’t distinguish between “partially used” and “genuinely dead.” A multimeter takes ten seconds and gives you an actual number.
Quick Reference Voltages
- AA/AAA alkaline: 1.5V fresh, 1.25V at 50%, 1.0V dead
- 9V alkaline: 9.5V fresh, under 7V dead
- 12V car battery: 12.5–12.66V full, 12.4V minimum for starting
- Lithium-ion cell: 4.2V full, 3.6V nominal, below 3.0V over-discharged