How to check battery voltage with a multimeter

Turn the meter to DC volts, put the red probe on the positive (+) terminal and the black probe on the negative (-) terminal, and read the number. On a 12 V lead-acid car battery that has rested, about 12.6 V to 12.9 V means full, and anything near 12.0 V means it is mostly empty. The exact full-charge figure depends on who you ask, which is why the table below shows three sources side by side.

Set up the meter

Plug the black lead into the COM jack and the red lead into the jack marked V. Turn the dial to DC volts, the setting with a straight line and a dotted line, sometimes labeled VDC. Skip the AC setting, the wavy line.

If your meter picks its own range, you are done. On a manual-range meter, choose a range above the battery’s voltage. Interstate Batteries tells car owners to use a DC range above 15 volts, and on most meters that means the 20 V setting. A 12 V battery on a 2 V range will just show an overload mark.

Touch the red probe to the positive terminal, marked + and usually under a red cover. Touch the black probe to the negative terminal, marked -. If you swap them, a digital meter shows the right number with a minus sign in front. Hold the probes still until the reading stops moving.

Let the battery rest first

A battery that was just charged, by a charger or by the alternator, reads higher than it really is. That is called surface charge. Two different tricks fix it for a car:

• Fluke suggests turning the headlights on for about 1 minute after driving, then off, before you measure.
• Interstate suggests about 2 minutes with the headlights on.

Turn off every accessory before you measure, and take the reading with the engine off. For a deep-cycle battery in an RV, boat or off-grid bank, Trojan says to let it stand disconnected for at least one hour so the voltage settles. Disconnect the loads and the charger, then wait.

What the reading means: 12 V lead-acid

The table uses each source’s own labels. Percentages are what each source called the row, so a few rows pair slightly different charge levels.

The three sources do not agree to the hundredth of a volt. Interstate’s own glossary says a fully charged 12 V battery produces at least 12.66 V, while Fluke rounds the full figure to 12.6 V. A practical way to read the table: 12.6 V and up is a healthy full battery, about 12.4 V is a battery that could use a charge, about 12.2 V is half full, and 12.0 V or below is discharged.

A car that struggles to start at 12.3 V is worth a charge and a proper test: on Interstate’s chart that is under half full.

Renogy calls 12.2 V or lower with the engine off a bad battery, and Fluke reads 12.2 V as only half charged, so take it as a warning level. A resting voltage only tells you how full the battery is, and Interstate says a multimeter reading is not a measure of how long the battery will last.

Engine running: the charging check

Start the engine and measure again at the terminals. Fluke says a good alternator should produce 13.7 to 14.7 V, and Interstate says a healthy battery on a running car shows about 14 V. If the reading stays at the resting figure, the alternator is not charging. If it climbs well above the top of that range, the charging system also needs a look. In both cases a shop or a parts store can run a proper charging-system test.

Starting and load checks

For a quick look at what happens when you crank, set the meter to its MIN/MAX function, as Fluke recommends, and have a helper start the car. The meter holds the lowest value it saw. Fluke also says to check voltage before you do any load test, because a battery that is already low will fail the load test for the wrong reason.

A load test pulls real current out of the battery, and Interstate warns that repeated load tests can drain a battery flat. Shops do this with a dedicated tester. The multimeter’s job is the first step: tell you whether the battery is full enough to be tested at all.

LiFePO4 batteries read differently

A 12 V lithium iron phosphate (LiFePO4) battery is four 3.2 V cells in series, which makes 12.8 V nominal. Epoch lists a 12 V 100 Ah LiFePO4 battery as 12.8 V and 1,280 Wh, and Victron sells a 12.8 V line of the same chemistry. Nominal voltage is the label value, not the resting reading of a full battery.

The curve is flat. Most of the capacity sits in a narrow band, and the voltage only falls fast near empty. Anker SOLIX says a full resting voltage is often 13.4 to 13.6 V and that the curve is flat enough that voltage alone should be treated as an estimate, not a fuel gauge. Renogy and Anker agree on the ends and on the long flat middle:

The other rows between 90% and 20% differ by up to a few tenths of a volt between the two charts, so they are left out. The usable takeaway is that a LiFePO4 battery reading 13.0 V is not necessarily full, and the meter cannot tell 70% from 40% with any confidence. Anker also notes that a battery that is being charged shows a higher number than its true resting voltage. If you want an accurate percentage on a lithium bank, a battery monitor that counts amps in and out does the job; a multimeter only flags the extremes.

24 V and 48 V banks

A 24 V bank is two 12 V batteries in series, and a 48 V bank is four. Multiply the per-battery readings by 2 or 4. Renogy’s LiFePO4 chart shows exactly that: 13.6 V per 12 V block is 27.2 V on 24 V and 54.4 V on 48 V. For lead-acid, measure the whole bank and then each battery separately.

Turn the reading into watt-hours

Once you know the battery is mostly full, the next question is usually how long it will run something. Watt-hours equal volts times amp-hours, so a 12.8 V, 100 Ah LiFePO4 battery holds about 1,280 Wh. How much of that you can use depends on the chemistry. Discover Battery gives 50% as the recommended maximum depth of discharge for flooded lead-acid, and Renogy and Samlex plan lithium at 80%, with some rating it to 100%.

Here is a lead-acid example. A 12 V, 100 Ah battery holds 12 x 100 = 1,200 Wh. At a 50% depth of discharge, that is 600 Wh you can use. The amp hours to watt hours calculator does this for any voltage and chemistry and adds inverter losses. If the battery will feed 120 V appliances, the inverter size calculator tells you what size inverter the loads need. If you plan to recharge it from the sun, the solar panel calculator sizes panels from your monthly energy use and your state’s sun hours.

Safety around batteries

A lead-acid battery can throw acid and give off hydrogen. IADC says to wear safety goggles and a face shield around a battery, because one droplet can damage an eye, and that a lead-acid battery gives off explosive hydrogen gas while charging, which a spark or flame can ignite. That gas is released while the car is running as well as on a charger. Trojan and Interstate both say to wear gloves and eye protection and to charge only in a well-ventilated area, and Trojan adds to keep sparks and flames away from a charging battery. Take off jewelry and keep metal away from the terminals, as Interstate advises.