How to Test an AGM Battery With a Multimeter

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Batteries in general, including AGM batteries, could fail in time. In most cases, you can consider these batteries to be dead if they are deeply discharged. Here’s where the problem lies: most battery charging devices come with built-in features of safety, which stop recharging faulty batteries. That’s especially true, even if they may be deeply discharged. 

Introducing AGM Battery: What Are They? 

An AGM (Absorbed Glass Mat) battery is a recent battery technology increasingly typical in today’s vehicles. It’s an option to the highly popular wet cell or flooded battery that has been utilized for many years.

An AGM battery has lead plates and a solution of water and sulfuric acid. The difference is an additional spongy glass mat dividing every plate. The glass mats press up against the plates, holding them strongly in place and letting more of them be packed into one case. 

You see, the liquid solution is held within the sponges. That’s why there’s never the need to fill those batteries. It’s worth mentioning as well that its design lowers the risk of spillage in the event when the case is tipped or broken on its side. 

How Does it Function? 

The construction in AGM-type batteries follows the same basic as standard SLA and the addition of a fiberglass mat that is placed between every positive and negative plate to absorb the electrolyte. 

The mat functions like a sponge with the electrolyte, and thus the battery becomes non-spillable. 

This battery holds the electrolyte in place and functions by letting the electrolyte be passed through the fiberglass mat. 

That makes maximum surface area for the electrolyte to touch the plates without it flooding the battery with excessive fluid. 

You see, AGM batteries have only sufficient electrolytes to keep the mat wet. No free liquid will leak out even if the battery is broken. How awesome is that? 

That enables less electrolyte in the battery while still giving the same energy as standard SLA batteries. 

What Are the Factors that Impact Battery Life? 

Make sure you remember the factors in mind, especially if you are considering your AGM battery life:

  • Regular car maintenance
  • Condition of the charging system such as alternator
  • Driving style
  • Temperature
  • Time 

Common Reasons AGM Batteries Die 

AGM batteries are created for one purpose—offer a powerful, fast, high-amperage current to the starter to start the engine, and then continuously have that charge of 12.4 volts sustained by the alternator once the vehicle is running.

However, the following things could cause an AGM battery to either not supply the right current or not retain that charge. 

  • Overcharging, rapid discharging, or alternator failure

An overcharge or rapid discharge often occurs when an alternator—the battery’s source for keeping charge—fails and causes problems. The electrolyte could boil over, leak, and fail if the alternator starts to overcharge the battery.

On the contrary, if the alternator stops charging, the vehicle’s entire ignition and electrical charging are running off the battery. That causes a rapid and instant drain. Often, the battery fails simultaneously as the alternator because of that fast discharge. It will take many days until it is found. 

  • Structural failure 

AGM batteries are composed of lead grids series submerged in an electrolyte. In this case, it’s sulfuric acid. They also live a tough life, bumped around with a vehicle’s suspension and movements and subject to major, rapid temperature changes. 

Because of this, batteries could have an internal structural failure that most people have called a dead cell. The failure is mainly because of electrolyte loss that causes the cells or grids to be exposed to air. 

  • Slow discharging or recharging 

If left unattended, all batteries can slowly discharge from 12.4 volts. However, a deep-cycle battery like AGM is not intended to be discharged beyond its starting cycle. Sometimes, that rears its ugly head when a battery is left in a car that has not started for a very long time. 

When that happens, the battery should be recharged, or worse, the car is jumped, and the alternator is left to perform the work of charging a dead battery—that it is not intended to do.

How to Use a Multimeter to Test an AGM Battery? 

So now that you finally understood the common reasons for your AGM battery dying, what’s next? It is time that you learn the steps to do when testing it using a multimeter. 

Remember that a multimeter is a practical and simple way to interpret the value of current, voltage, and resistance in your battery. You can rest assured that you’ll get exact values.

Testing your AGM battery voltage could be done using a power probe or a multimeter. But in this case, we will show the steps of using a multimeter.

  1. Take out the AGM battery’s terminal cover and clean the terminals thoroughly.
  2. Set your multimeter to the lowest voltage setting that is more than 15 volts.
  3. Connect the multimeter’s red (positive) lead to the battery’s terminal.
  4. Touch the multimeters black (negative) lead to the battery’s negative terminal.
  5. Once done, check the multimeter’s reading. 

A voltage reading of 12.4 to 12.7 volts indicates your AGM battery is in top condition. Meanwhile, a reading under 12.4 volt means the battery should be charged. 

Final Thoughts

An AGM battery is safer to deal with because it will not leak caustic fluids, unlike other batteries out there, not to mention it could have a longer lifespan. But in the event of a dying battery, you must equip yourself with proper information. 

Testing your battery’s voltage allows you to locate issues with the battery, like high levels of sulfation that will lead to a major decline in voltage when the battery is loaded. This test could also help you find badly selected or under-sized batteries that drop below their safe minimum voltage when loaded. 

This test is very easy to do, and a digital multimeter is the best tool you can use.

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