While it takes a bit of time, this process of charging and then resting the battery gives you the most accurate measurement with the voltmeter.

Buy a digital voltmeter at any auto parts or home improvement store, or online.

If you’re checking a vehicle battery, you don’t have to disconnect the cables attached to the terminals. Just be sure you’re touching the actual battery terminal, not part of the cable attached to it. In some vehicles, you may have to lift off a red plastic cap to access the positive terminal. [2] X Research source

Always touch the positive probe to the positive terminal first, then touch the negative probe to the negative terminal. If you connect the negative probe first and the positive probe touches any conductive material, you may short out the battery—which may damage it or, in rare cases, cause a dangerous explosion.

If your vehicle battery has a voltage reading below 12. 4, it’s not holding a charge properly. In this case, either the battery itself is failing or it’s being drained by a “power parasite” in your vehicle—for instance, a map light you left on or a tablet you left connected to a charger in the vehicle. Pull the negative and then positive probes away from the battery terminals once you’ve completed the test. This greatly reduces the possibility of accidentally creating a short circuit.

If you’re not sure how to charge the battery, check the product manual. Checking an open-cell lead acid battery—that is, a lead acid battery with caps that can be opened to access the liquid inside—with a battery hydrometer is most accurate when the battery is fully charged. Closed-cell lead acid batteries without the access caps cannot be tested this way. Use a digital voltmeter for a more basic checkup, or have an auto mechanic or other trained professional conduct more detailed testing.

Don’t wear clothing made of natural fabrics like cotton, since sulfuric acid quickly dissolves these.

Car batteries typically require the use of a ratchet set or a crescent wrench to disconnect the battery cables. In other cases, the cables may be attached by snaps or pinch clamps that are easy to remove by hand. Refer to your product guide if you need specific instructions. Allowing the battery to rest while disconnected for 8 hours makes for a more accurate test. If you don’t have that amount of time, try to wait at least 2 hours.

Set the caps aside so you don’t lose them. Each cap corresponds to a separate chamber of liquid (or “cell”) inside the battery. Each cell is connected in series to make up the battery’s total voltage—for instance, 3 cells, each at 2 volts, for a total of 6 volts.

Look for battery hydrometers at auto parts retailers or online.

You’re essentially “warming up” the hydrometer at this point—that is, acclimating it to the temperature of the liquid inside the battery.

Hold the hydrometer completely upright to get the most accurate reading with the float.

The float might, for instance, line up with the marker labeled 1. 270 on the tube. This is the specific gravity of the liquid. Water has a specific gravity of 1. 000. Some hydrometers, however, omit the decimal point and will read 1270 for the battery liquid and 1000 for water. Specific gravity is not measured in units (like grams or milliliters) because it is a density ratio between a chosen liquid and water. Release the liquid from the hydrometer back into the battery cell once you’ve recorded your measurements.

Never place a metal-tipped thermometer (or any other type of metal) into the cell, or you may cause an unpredictable—and potentially dangerous—chemical reaction.

For instance, if your specific gravity reading was 1. 270 and the temperature reading was 90 °F (32 °C), add 0. 040 to get 1. 310 as the adjusted specific gravity. Your battery manufacturer may provide more specific temperature adjustments, possibly including basic to somewhat complex mathematical formulas. Consult the battery’s product guide or check the manufacturer’s website.

If the readings you get aren’t very close, your battery may need to be repaired or replaced. In a healthy battery, all the cell specific gravity readings should be within 0. 050 (and ideally much closer) of each other.

For the most accurate information, check the manual for your battery, vehicle, or other battery-powered product for a specific DoD table. Otherwise, find a general DoD table online, such as at https://rimstar. org/renewnrg/measure_battery_electrolyte_specific_gravity_with_hydrometer. htm. For example, the DoD table might show that a 12-volt battery should have a specific gravity of 1. 265 at 0% DoD. At 30% DoD, that figure might instead be 1. 218, and at 50%, it might be 1. 190.

Keep your safety gear on while re-capping the battery and cleaning the hydrometer. Rinse the used distilled water down the drain, rinse out and dispose of the cup, and rinse out the sink basin with clean water.