How to Test an AGM Battery? Simple 7 Steps for Two Methods
Absorbed glass mat or AGM batteries are built very differently from the traditional flooded lead acid battery. For example, they last longer, can meet a higher potential load and are typically more durable. This means they shouldn’t be tested the same way as your traditional battery. Yet these batteries can fully discharge and need to be maintained. Let’s learn how to test an AGM battery.
A. How to Use Multimeters to Test AGM Battery?
Step 1: Charge the battery.
The first step is to charge the battery until it is fully charged. You can charge it with a regular battery charger, though a smart battery charger that is designed to charge AGM batteries is the better choice.
Step 2: Apply a Light Load
Turn on an electrical item connected to the battery for a few minutes. Only turn on lights or another low level draw instead of something that drains the power like an air conditioner.
Step 3: Determine What Value Is Good for Your Battery
Check the voltage and ampere ratings on the battery label. The voltage will typically be rated as 12 volts, but this could be anywhere from 11.5 to 13 volts.
The ampere rating will be listed as amps or CCA. CCA is short for cold cranking amps.
Step 4: Measure the Voltage
Get out the multimeter. Set it to measure voltage. Put the tips of the multimeter on the battery terminals. The red tip goes on the positive terminal (the one with the plus sign) while the black goes on the negative terminal (the one with a dash on it). Read the voltage.
A 12 volt AGM battery should read somewhere between 12.4 and 13 volts. The fully charged AGM battery will have slightly higher voltage to offset the voltage resistance in the battery cables.
Step 5: Determine What to Do with the Battery Based on Test Results
If the voltage is below 12.4 volts, you may need to desulfate the battery or replace it altogether. Suppose the battery test comes back at 10 volts or lower.
The problem may be that the battery isn’t fully charged. You may not have fully charged it before running the test, or it may have experienced a chemical reset that prevents full charging. (That’s why we mentioned desulfating.)
How long does AGM battery take to charge?
Desulfate and recharge or simply resume charging for an hour or two. Then repeat the test. If the battery hasn’t fully recharged at this point, it should be discharged. If the battery has the right voltage level, you know it can recharge.
Since you’re concerned the battery isn’t holding its charge, wait two hours and then test the voltage again. If the voltage dropped by more than one or two volts, the battery should be replaced.
Why aren’t we recommending a conductive battery test?
That is because AGM batteries sometimes give false positive results. A stress test checked via a multimeter is going to be more accurate.
B. How to Do an AGM Battery Load Test?
Step 5: Know What the Right Reading for Your Battery Is
The first step is determining what a good load test reading would be with your AGM battery. You can calculate the load-test amperes by dividing the CCA number by two. For example, if the CCA is 60, the load-test amperes should be 30 if the battery is in good condition.
In other cases, the value is noted by the letters Ah. That stands for ampere-hours. This rating is commonly used on golf cart batteries. Multiply the Ah rating by 3 to determine the ideal result from a load test. If the Ah rating is 15, then you’ll expect a rating of 45 from the load-test.
Step 6: Charge It, Then Load It
We’re going to assume you have a fully charged battery and have briefly run a light load on it like connecting it to light bulbs or another low-power draw for a few minutes.
Step 7: Use the Load Tester
Get out your load tester. Put the prongs on the load tester on the battery terminals. Apply them to the two terminals at the same time. Leave them on the load tester for fifteen seconds. If you have an automatic load tester, it will stop after ten to fifteen seconds by itself. Then read the load-tester’s reading.
If you don’t think you did the test correctly, you can repeat the test. You have to repeat the test after recharging if the voltage drops below 9.7 volts.
You should get the same reading as you did just a few seconds ago. What if the reading is lower? If the load-test results drop by more than ten percent between tests that are only a few seconds apart, the battery probably needs to be replaced.
The multimeter test is easier to do and harder to mess up. However, both methods of testing an AGM battery are a good way to determine if the battery is still good.