Testing Your Battery with a Voltmeter
Charge the battery fully, then let it rest for 4 hours. If you’re testing an automobile battery, take the vehicle for a 20+ minute drive, then shut off the engine for 4 hours. For other types of lead acid batteries, charge them all the way before letting them rest for 4 hours. 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.
Put on safety gear and turn on your digital voltmeter. While you won’t be opening the battery cells to do this test, it’s best to play it safe and put on thick gloves, long sleeves, and protective eyewear. Remove any dangling jewelry as well. Turn on the digital voltmeter by pressing its power button and watching for the display screen to show “0.0”. Buy a digital voltmeter at any auto parts or home improvement store, or online.
Touch the voltmeter’s positive probe to the battery’s positive terminal. The digital voltmeter has 2 probes, one red and one black, connected to the voltmeter device. Place the metal tip of the red, positive (+) probe to the red, positive terminal of the lead acid battery. 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.
Touch the voltmeter’s negative probe to the battery’s negative terminal. Follow the same process as before, but this time with the black, negative (-) probe touching the black, negative battery terminal. Do this while continuing to hold the positive probe to the positive terminal. 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.
Check the display reading on the digital voltmeter. Under normal circumstances, a 12-volt lead acid automobile battery should give a reading between 12.4 and 12.7 volts. Other types of lead acid batteries have varying ideal voltage readings, so check your battery’s product manual or look on the manufacturer’s website. 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.
Using a Hydrometer in an Open-Cell Battery
Charge the battery fully at least 8 hours before testing it. Lead acid batteries recharge in various manners based on their function and manner of installation. For a lead acid vehicle battery, drive the vehicle around for at least 20 minutes. For a lead acid battery connected to solar panels, let the battery charge fully on a sunny day. 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.
Put on safety gear such as—at minimum—gloves and goggles. Lead acid batteries contain dangerous sulfuric acid, so protective gear is a must. At the very least, put on thick rubber or PVC gloves and protective goggles. Even better, also put on a rubber or PVC work apron and heavy work boots. Don’t wear clothing made of natural fabrics like cotton, since sulfuric acid quickly dissolves these.
Disconnect the battery and leave it that way for 8 hours. With your safety gear on, disconnect the cable connected to the negative (-) terminal and set the cable aside where it can’t touch the battery. After that, disconnect the cable connected to the positive (+) terminal and set it aside. When setting aside the positive cable, make sure it does not touch any metal surface of the vehicle or other item powered by the battery—it still carries a small charge that may damage the vehicle/product’s electronics system. 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.
Remove the caps on top of the disconnected battery. If you took off your safety gear while waiting, be sure to put it back on first! Then, identify the series of access caps on top of the battery—there may be 1, 2, 3, or more, depending on the battery type, so check the product manual for confirmation. In most cases, the caps can be removed by unscrewing them counterclockwise by hand. 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.
Squeeze the bulb of a battery hydrometer and put the open end into the liquid. Choose any of the cell openings for this task. The hydrometer is a bit like a turkey baster—squeezing the bulb end before placing the open end in the liquid allows for the liquid to be drawn up into the tube section whenever you release your squeeze. Look for battery hydrometers at auto parts retailers or online.
Draw liquid into the hydrometer, squeeze it out, and repeat twice. Release your squeeze on the bulb to draw liquid from the cell up into the tube of the hydrometer. Then, without lifting the hydrometer from the cell, squeeze the bulb again to release the liquid back into the cell. Repeat the entire process 2 more times without removing the hydrometer's open end from the cell. You’re essentially “warming up” the hydrometer at this point—that is, acclimating it to the temperature of the liquid inside the battery.
Draw in liquid again and identify the location of the hydrometer’s float. Suck up liquid from the same cell yet again, but this time keep it in the tube instead of squeezing it right back out. Locate the “float”—which, as the name indicates, is a piece within the tube that floats when liquid is inside it. The location of the float determines the specific gravity reading you’ll use. Hold the hydrometer completely upright to get the most accurate reading with the float.
Write down the specific gravity and temperature measurements. Using the specific gravity scale markings on the hydrometer tube, write down the measurement that corresponds with the location of the float. Find the separate temperature gauge in the tube—which looks and works a bit like an old-style mercury thermometer—and write down the temperature measurement as well. 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.
Test the liquid temperature separately if you can’t use the hydrometer. If your hydrometer does not have a temperature gauge, aim an infrared no-touch thermometer (the type available at kitchen supply stores) at the liquid inside the cell. Write down the temperature reading alongside the specific gravity you recorded. 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.
Adjust your specific gravity reading based on the liquid’s temperature. The specific gravity chart for lead acid batteries assumes a liquid temperature of 80 °F (27 °C). That said, the liquid in your battery probably isn’t at this ideal temperature. For a general adjustment, add 0.040 to the specific gravity reading for every 10 °F (6 °C) above the ideal temperature, and subtract the same amount for every 10 °F (6 °C) below the ideal. 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.
Take readings from the battery’s other cells as well. This isn’t absolutely necessary, but you’ll get a better grasp of your battery’s overall health if you check each cell individually. Follow the same process as before and remember to adjust the specific gravity reading based on temperature. 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.
Compare your readings to a depth of discharge (DoD) table. This type of table shows what the specific gravity reading for your particular type of battery should be at various stages (or “depths”) of discharge. Since you fully charged your battery before testing it, your specific gravity readings should match those listed for the 0% DoD level. If your readings match the numbers at the 20%, 30%, 60%, etc. DoD levels, your battery is not functioning at full health. 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.
Re-cap the battery cells and rinse out the hydrometer. Once you’re finished testing the battery, screw the caps back onto each cell compartment. Then, either reconnect the cables to the terminals (positive first, then negative) if you’re keeping the battery, or replace it with a new one. Clean the hydrometer by dipping it into a cup of distilled water and filling and emptying the tube several times. 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.
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