How To Tell If an Absorbed Glass Mat (AGM) Battery Is Fully Charged

Absorbed Glass Mat (AGM) batteries are becoming more common and rapidly replacing the old sulfuric acid-filled batteries. To ensure your battery is operating optimally, you need to test it from time to time. When testing an AGM battery, it’s essential to ensure it’s in full charge. So how do you test to ensure the battery is fully charged? Here’s everything you need to know when charging AGM battery.

What Is an AGM Battery?

AGM batteries are an improvement to lead-based batteries. They’re maintenance-free and completely sealed units. AGM batteries do not contact any fluid, unlike the regular flooded-cell lead-acid battery that contains sulfuric acid in its cells. That means they’re safer even if their outer casing cracks or gets damaged. You won’t have to worry about leaking.

AGM batteries are more common in motorized wheelchairs and golf carts. They’re also becoming popular in motor vehicles and marine vessels.

How to Test an AGM Battery

When testing any battery, one must determine if the battery is at full charge. Since you cannot use a hydrometer because of a sealed battery, place the battery on a charger designed for an AGM battery that auto-regulates the amperage based on the voltage. If the battery amperage rises dramatically and then falls back to a level of 10% of the amperage of the battery, it is probably not at full charge. If the amperage barely moves, the battery is probably close to full charge in most cases and could be load tested.

Here is a step-by-step guide to assist you.

The Multimeter Test

Get your multimeter: Turn the multimeter on and set it to measure voltage by pressing or dialing the correct button on the meter.

Connect: Place the sensor on the end of the red wire coming off the multimeter onto the positive terminal of your AGM battery. Next, place the black wire sensor on the negative terminal of the battery.

Measure: If your standard AGM battery is fully charged, you will notice an output voltage of approximately 13 volts or just more than 12 volts labeled on the side of your battery. If the reading comes below 10 volts, keep charging the battery. If it’s charged, consider testing the voltage and take a retest a few hours later. If the voltage reads below the previous reading by one or two volts, you probably need a new battery.

The Load Test

Look for the ampere rating: Find the ampere rating on the battery’s label. It’s indicated with CCA or cold cranking amps. It may also have letters like AH for ampere-hours followed by a number.

Calculate: Divide the number indicated by two. The figure you get is what you should find from the load test meter.

Connect the load tester and test: Connect the load tester’s clips to the battery with the red going to the positive and the black going to the negative. Allow the load test to run for about 15 seconds, then stop.

Read your results:  If the resulting figure is below the divided figure by more than 10 percent, recharge your battery. Allow the battery to charge and retest a few hours later. If the figure remains below by more than 10 percent, replace the battery.

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Categorised in: Battery Chargers

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