The most accurate way to test your battery is with a multimeter. These digital devices are inexpensive and easy to use, and they’re available at most auto parts stores. A multimeter can tell you your battery’s state of charge (SOC) and whether it’s healthy or ready for replacement. The key is to understand the different settings on a multimeter and what they mean for testing a battery.
Here’s a quick guide to the different multimeter settings and how they work:
Looking for a multimeter setting for a car battery? Look no further! The voltage range for a car battery falls between 15 and 20 volts. You can test your battery by setting your multimeter to the 20V DC range.
Be Careful
You’re going to be dealing with some potentially dangerous current here, so be careful. First, turn off your car and make sure the keys are out of the ignition. Then disconnect the negative cable from the battery using a wrench or socket. This is where you’ll connect the black probe from your multimeter.
Connect the red probe to the positive terminal on your car battery using another wrench or socket. Your multimeter should now be connected across both terminals.
Set your Multimeter to the Right Scale
Make sure your multimeter is set to the correct scale for measuring voltage. Set it to 20V, a scale that can easily read both 12-volt and 6-volt batteries. If you have an analog multimeter, make sure the needle is set to zero before you take a reading — that way, any error in your meter will show as an offset instead of an offset plus a false reading.
Test the Battery with a Low Load
The next step is to turn off all accessories in your vehicle and test the battery’s voltage with a low load. You’ll need to find the positive and negative terminals on your battery to do this. Then, connect the red lead to the positive terminal and connect the black lead to the negative terminal.
If you see 12 volts or more on your display, it means that your car’s charging system works well, and there are no problems with your battery. If it shows anything lower than 12 volts, there’s an issue with either its charging system or the battery itself. For instance, a reading of 11 volts means that your car battery has a 50% charge left, while 10 volts means that there’s only 20% left.
Test the Battery with a High Load
When testing the battery under a heavy load, switch the meter to the 20-volt DC range. If you don’t have a high-load tester, use a 100-watt light bulb as a substitute. A 100-watt bulb draws about 8 amps from the battery when on and about 1 amp when off.
The best way to test the battery with a light bulb is to remove it from the socket of your car’s headlight or dome light circuit. With the ignition switch off, connect one end of the light bulb to the ground and touch one-meter probe to the other end of the bulb (Figure 2).
Have an assistant turn on the ignition switch while you watch the meter. If there is no voltage drop, both your battery and alternator are OK. If there is a voltage drop of more than 0.5 volts, you have a bad connection somewhere in either system.
DC vs AC
This is probably what you’re most familiar with. In this case, DC is the direct current, and AC is the alternating current. When testing car batteries, you’ll always use DC voltage, so make sure this is set correctly!
Resistance (ohms)
This setting tells you how much resistance there is in a circuit. The standard unit for measuring resistance is ohms, so this setting will usually be labeled “ohms.” This setting can help you measure resistance in wires and other components.
Voltage (V)
This setting allows you to measure the pressure of electricity between two points in the circuit. You can test this with your battery and alternator to see if it works properly. This will also be set to DC (direct current) when testing car batteries since that’s the principle behind their working. (1)
Amperage (A)
Grab your multimeter and find the amperage (A) setting. You should see a little symbol that looks like a snake biting its own tail—that’s the symbol for amperage. (2)
Next, find your battery’s positive (+) and negative (-) terminals. They’re usually marked in red and black, respectively. If not, look for the little “+” and “-” symbols next to them.
Put one of your multimeter leads on the positive terminal and the other on the negative terminal. The lead with a red tip will go on the positive terminal, while the black-tipped lead will go on the negative terminal.
Now look at your multimeter’s display: if it shows a number between 10 and 13 amps, your battery is in good shape! The numbers will be lower if you haven’t driven it recently, but they should be back up after running it for a while. Remember that all batteries lose charge over time—even if they’re working fine now.
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References
(1) electricity – https://www.eia.gov/energyexplained/electricity/
(2) snake biting – https://www.cdc.gov/niosh/topics/snakes/symptoms.html
Video References
ChrisFix
Junky DIY guy