Learn how to safely test and diagnose your car battery.
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Time 10 - 20 minutes
ComplexityBeginner
CostFree
Introduction
Car batteries are essential for vehicle operation, but like everything else, they don't last forever. They have a limited life span— usually no more than five years under the most favorable conditions. Unfortunately, one of the most common ways of finding out your battery is dead is when you try to fire it up and your car won't start or it turns over very weakly.
When this happens, it's important to determine if the battery is just drained and needs to be recharged or if it's just too old to keep a charge. Sometimes, if your battery is low or dead (maybe you left the lights on), you may just need to charge it. But when a battery loses its ability to hold a charge, it'll need replacement. So how do you tell?
Proper battery testing makes it easier to figure out what's wrong with your battery. You could use a multimeter, but you won't get as much information about what's going on inside the battery. To gain a more in-depth look into the health of your battery, do a detailed test using a dedicated battery tester.
We recommend this Topdon BT100 battery tester because it can not only check the charge on your battery, it can test its overall health, in addition to performing cranking, and charging tests.
Tools Required
- Car battery tester
- Multimeter or voltmeter (optional)
Project step-by-step (7)
Step 1
Prepare to Test the Battery
- Unless your vehicle won’t start or you already suspect battery issues, wait at least a few days without driving to test your car battery.
- Your car’s alternator charges the battery while the vehicle is running. Testing the battery after not driving for a few days will tell you how well your battery is holding a charge. If it runs down significantly, it probably requires replacement.
- Lift your vehicle’s hood and locate the battery.
- Note: If the battery isn’t under the hood, consult your vehicle’s manual to locate the battery compartment.
- Remove the red plastic protective cover from your battery’s positive terminal.
Step 2
Connect the Battery Tester
- Connect your battery tester’s red cable clip to the battery’s positive (red) terminal.
- Connect the tester’s black cable clip to the negative (black) terminal, then turn on the tester.
- Be sure to position the tester in a spot where it sits flat and won’t fall down into the engine later when you perform cranking tests with the vehicle running.
Step 3
Test Voltage
- Switch your battery tester to voltage testing mode to determine the battery’s state of charge.
- Most car batteries are called “12-volt,” but the actual voltage depends on the state of the charge. This test can be performed with a simple voltmeter or multimeter, but the battery tester will also determine this.
- Determine if your battery is within the healthy voltage range.
- Fully charged, it should read 12.6 volts or higher. This is just enough voltage to start your car but is only about 75 percent charged. If it reads 12.0 volts or lower, that’s a sign that the battery could be weak and might keep losing its charge.
Step 4
Test Cold Cranking Amps
- Switch your battery tester to CCA (cold cranking amps) mode.
- Cold cranking amps is a rating applied to car batteries expressing how much electricity flows from them at 0 F. Your battery’s CCA rating is probably written on its casing.
- Input your battery’s CCA rating number into your tester.
- Many testers require you to start the vehicle with the tester connected to determine actual CCA performance.
- Compare your battery’s CCA rating with reality using your battery tester.
- A healthy battery will deliver close to its rated CCA, while a failing battery won’t. The tester tells you the state of battery health based on actual CCA output.
Step 5
Perform Cranking Test
- Set your battery tester to cranking test mode, then start the vehicle. Your tester will record the lowest voltage that the battery is able to maintain while the starter motor is working.
- Make a note of your battery’s cranking reading.
- Healthy batteries should maintain between nine and 10 volts during cranking. If your test shows less than nine volts, your battery is too weak.
Step 6
Perform Charging Test
- Switch your battery tester to charge testing mode while the vehicle is running.
- If your battery and alternator are good, it should read between 14.2 and 14.5 volts. Lower than this might mean your alternator, or some other part of the charging system, needs work. This charging test can also be performed with a DC volt meter or multimeter.
Step 7
FAQ
How do temperature changes affect battery performance?
Temperature extremes can be tough for car batteries. In hot weather, the engine compartment can reach 140°F or higher! This causes the battery’s liquid electrolytes to evaporate, damaging the battery’s internal structure, corroding the lead plates, and causing the battery to discharge faster.
This situation can also lead to overcharging the battery, which shortens its lifespan. In prolonged deep cold (below 15 degrees), the battery’s resting capacity can be diminished, making the engine very hard to turn over. It doesn’t help that the oil is more sludge-like when it’s really cold, which puts even greater strain on the battery. Additionally, recharging tends to slow down in cold temperatures, so the battery may take longer to reach a full charge.
How do I test a car battery with a multimeter?
Set your multimeter to read volts. When the engine is off, touch the red lead of the multimeter to the positive battery terminal and touch the black lead to the negative terminal post. Your reading should be around 12.2 to 12.6 volts. If you run the very same test with the engine running (the alternator is then charging the battery), your reading should be in the range of 14.1 to 14.6 volts.
Originally Published: June 25, 2020
