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How to test an AGM battery

AGM (Absorbed Glass Mat) batteries are found in the most modern and powerful vehicles. These batteries last longer in nature and are considered to be more durable within certain parameters. AGM recombinant batteries have a very low internal resistance, which reduces heat at high charge/discharge rates, and a very low static charge loss of about 1% to 3% per month.

AGM batteries, electrically, have less internal resistance, and because their densely packed cell plates and battery mats are more reactive, they can be charged up to five times faster than flooded-cell batteries. Due to their fiberglass construction, AGM’s cold-weather performance is also less susceptible to negative temperatures than a typical flooded-cell battery.

When testing idle voltages, you are likely to see voltages in the 12.8V range rather than the 12.6V range seen in fully charged flooded cell batteries. AGMs also have a lower self-discharge rate, and AGM battery life is longer because an AGM can withstand a deep-discharge cycle of up to 80% of its capacity without affecting battery life. These combined features allow AGMs to operate more efficiently in vehicles equipped with multiple high-capacity accessories, such as heated seats and windshields.

AGM battery benefits

AGM batteries are used in vehicles with high potential load, therefore one AGM battery should only be replaced by another AGM battery of the same capacity. A common mistake is to replace an AGM battery with a normal wet lead-acid battery, which can destroy the normal wet battery within a few weeks due to the high requirements. When you need a new AGM battery, so read our review.

AGM battery voltage

Charging the AGM battery also requires special attention; the charging voltage must not exceed 14.8 volts, even for short periods of time, without damaging an AGM battery. AGM batteries can withstand very high charging currents, but not voltage; slow charging is always the best and most efficient method. The high heat from the engine compartment can also drastically reduce the life of the AGM battery; these batteries have never been designed for higher temperatures.

AGM battery life

To test a high-capacity AGM battery, you must start with a fully charged battery. As a rule, you can remove the surface charge by turning on the key for about three minutes. Conductivity battery testers can usually work well if they have the correct information, but appear to be more accurate with wet batteries than AGM models; AGM batteries sometimes give false-positive results. A stress test is the best way to avoid unnecessary comebacks.

Charging an AGM battery

According to the latest ADAC studies, around 40% of failures in 2017 were due to the battery. Reasons for this include the increasing electrification of vehicles. Nowadays, higher demands are placed on the battery – up to 150 electrical consumers and the automatic start-stop system in modern cars require sufficient power. A regular battery check by a specialist workshop is therefore recommended in order to detect an imminent battery failure before a final failure occurs.

Advantages

The electrolyte is bound in a glass fiber fleece, which is why they are also called dry batteries. Because of their lightweight, these automotive batteries are often installed in motorcycles. Other advantages include a long service life, very low self-discharge, and insensitivity to deep discharge – even at low temperatures. This makes them ideal for classic cars that are stationary for long periods, as well as for vehicles with automatic start-stop systems and recuperation brakes for energy recovery – here, energy is fed back through the braking process. AGM batteries are sensitive to heat, however, and are usually susceptible from 131F (55° Celsius).

This special feature makes it possible to pack plus and minus plates within the battery particularly tightly and to reduce the internal resistance. Due to the closed construction, gases produced are converted back to water in oxygen recombination, so that these batteries have a long shelf life and only a low self-discharge rate. They are leak-proof and generally more shock-resistant than wet batteries. Another advantage is that they are easier to transport and more flexible to install. Due to their high cycle density, AGM batteries are particularly suitable for automatic start-stop systems. AGM accumulators can even be charged without problems by short-term energy supply, e.g. during braking.

Best agm battery

Best choice

MOTOPOWER Digital Car Battery Tester 12V

It provides a quick reading to determine if a vehicle battery needs a jump or replacing and also the alternator output status. Various Colored LEDs indicate the battery or the alternator status, while the LCD digital screen give the accurate reading of the voltage simutaneously.

  • 4,9 Rating
  • Four-character lighted digital display, accuracy close to 0.01volt
  • Reverse-hookup and Over-Voltage protection
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Top rated agm battery

Top choice

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CARTMAN 12V Car Battery & Alternator Tester

  • 4,8 Rating
  • LED display indicates condition for 12V battery only
  • Indicate the percentage charging in the battery

Best choice

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MOTOPOWER Digital Car Battery Tester 12V

  • 4, 9 Rating
  • Four-character lighted digital display, accuracy close to 0.01volt
  • Reverse-hookup and Over-Voltage protection

Recommended

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Schumacher BT175 12V Handheld Digital Battery Tester

  • 4,7 Rating
  • For 12V SLI and VRLA/AGM/GEL batteries with up to 1200 CCA
  • Testing voltage of 7-15V

How To Test AGM Batteries

How to Test an AGM Battery

Best automotive battery testers

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1. Automotive Battery Tester 6V / 12V Digital Battery Analyzer Lead-acid Accumulators Automotive Battery Load Tester

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2. MOTOPOWER MP0514A 12V Digital Car Battery Tester Voltmeter

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3. SOLAR BA9 40-1200 CCA Digital Battery and System Tester

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4. CARTMAN 12V Car Battery & Alternator Tester

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5. Battery Tester FOXWELL BT705 Automotive 100-2000 CCA Battery Load Tester

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6. Schumacher BT175 12V Handheld Digital Battery Tester

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7. Performance Tool W2989 Battery/Alternator Tester Battery/Alternator Tester

Correct testing of the battery state by specialist workshops

Basic principles:

Absorbed glass mat batteries (AGM) are the latest improvement over lead batteries. AGM batteries are completely sealed units and maintenance-free. Unlike a conventional lead-acid battery with flooded cells containing sulfuric acid in the cells, an AGM battery contains no liquid. This means they are safer because even if the outer casing is damaged or torn, there is no leakage. AGM batteries are a popular choice for use in golf carts and motorized wheelchairs and are increasingly being used in seagoing vessels and motor vehicles.

agm battery load tester

AGM Multimeter Test

Step 1

The first thing you need to do is turn on the multimeter. Next you need to set it to measure voltage by turning the control dial or by pressing the appropriate button on the meter.

Step 2

Next, move the sensor to the end of the red wire that goes from the meter to the “+” terminal of the AGM battery. Place the probe on the “-” terminal of the AGM battery on the end of the black wire.

Read the multimeter reading. A fully charged AGM battery has an output voltage of approximately 13 volts. This is slightly higher than 12 volts on the battery side to provide resistance to the voltage in the battery cables.

However, if the measured value is below 10 volts, you will need to charge the battery for a while. After charging, check the voltage again, then leave it for a few hours and try again. If the voltage drops more than 1 or 2 volts, you may need to consider replacing it.

AGM load test

Step 1

To determine the load current, examine the label on the side of the AGM battery. CCA stands for Cold Start Current. It is followed by a number that indicates the ampere rating of the battery. For example, CCA 70.

The letters Ah – ampere hours – may also be indicated. In a golf cart, this value is followed by a number.

Step 2

Divide the CCA or Ah by 2 with a calculator. For example, if the label is CCA 70, divide 70 by 2 to get 35. This is the number you get from the load tester when your battery is charged and in good condition.

Step 3

Switch on your load tester. Attach the alligator clips to the battery clips; the clip at the end of the red wire goes to the clip “+” and the clip at the end of the black wire goes to the clip “-“.

Step 4

Press the start button for the load test. Run the test for 10 to 15 seconds and then stop the test. Most load testers stop automatically after 15 seconds.

Read the load test meter. Expect it to display the same result as the amperage you calculated earlier. If the reading is more than 10 percent below the value you calculated, charge your battery until it is full. Leave it for a few hours and test it again. If the number is still more than 10 percent below the number you calculated, consider getting a replacement.

AGM batteries should be tested for 15 seconds to determine their capacity:

  • 1/2 of the CCA rating (Cold Cranking Ampere);
  • 3 times the Ah rating (ampere-hour rating).

NOTE: The voltage must not drop below 9.7 volts for the duration of the test.

Battery Conditions

State of Charge Sealed or Flooded Lead Acid battery voltage Gel battery voltage AGM battery voltage
100% 12.70+ 12.85+ 12.80+
75% 12.40 12.65 12.60
50% 12.20 12.35 12.30
25% 12.00 12.00 12.00
0% 11.80 11.80 11.80

Three misunderstandings about AGM batteries

More batteries are placed in the trunks and under the seats of some vehicles. If the battery is not under the hood, it may be an absorbent glass mat (AGM) or gel cell battery. AGM batteries prevent the spilling of acid in accidents because they are sealed. They can also be mounted at odd angles.

Manufacturers can cut themselves off a few pounds from the vehicle as these batteries offer plenty of power for their size and weight. This is an important reason why they are increasingly found in the latest generation of vehicles.

Here are three misconceptions about AGM batteries

You can use a conventional battery charger for AGM-type batteries or with helium cells.

Incorrect. These batteries charge weakly and slowly. Many of these types of batteries have microprocessors that collect information from the battery and regulate current and voltage accordingly. Therefore, the settings for these batteries are different. Overcharging can damage these batteries.

Also, alternators are not chargers. If the battery is severely discharged and will not start the engine – use the correct charger.

AGM and gel cells can be tested in the same way as conventional batteries.

Incorrect. Batteries of this type have a lower internal resistance. Old-style capacitance testers will not be able to read these batteries. Basically all new battery analyzers have a mode for AGM/gel cell batteries.

Replacing an AGM or gel battery is the same as replacing a flooded battery.

Not quite the same. Additional installation steps are required to notify the vehicle of the battery replacement. Newer cars have a sensor to detect the battery or similar systems. Will need a scanner to recalibrate. Failure to recalibrate the system may result in overcharging of the new battery by the alternator, resulting in battery failure.

agm battery impedance tester

Battery test step by step

Please follow the manufacturer’s instructions!

  1. Connect the battery tester to the battery terminals to determine the state of charge and internal resistance. In principle: connect the red cable to the positive terminal and the black cable to the negative terminal. The sequence of coupling and uncoupling is irrelevant. To connect the tester to a battery in the trunk or passenger compartment, use the battery terminals in the trunk and not the jumper contacts in the engine compartment, as the resistance of the cable installed in the vehicle would affect the measurement.
  2. To connect the tester to a battery located in the trunk or passenger compartment. Connect to the battery terminals of the battery and not to the starting aid contacts in the engine compartment, as the resistance of the cable installed in the vehicle will affect the measurement.
  3. Set the battery tester to the correct battery type: starter battery, gel battery, EFB or AGM battery. The instrument uses a different test algorithm for each battery type, so a wrong setting would result in an incorrect reading. In addition, for some test devices it is important to know whether the test is performed with a battery installed in the vehicle or whether it is outside the vehicle.
  4. Enter the specified cold start current for the battery into the device, including the measurement method used. Common standards are DIN, EN, IEC, JIS and SAE. Details of the test standard can be found after the cold start current information on the battery label.
  5. The tester then automatically performs the test and delivers the result.

Conclusion

During the computerization of battery charging processes, the charging voltage is not only affected by operating conditions, but also by additional loads. For example, transmission control modules (in BMW) are programmed to adjust the charging power to compensate for normal battery plate wear. A modern charging system is controlled through various inputs to the engine control unit. And it can serve the AGM battery more efficiently than a simple voltage regulator.

Membrane battery test

  • To avoid overcharging, it is recommended that the alternator, voltage regulator, and battery share clean positive and negative battery connections.

AMG battery testing

  • Note that a voltage drop of even 0,5 Volts (depending on the current flowing through the circuit) will cause the charge voltage to increase above the 14,4 Volts limit for an AGM.

How to Test an AGM Battery Video

David Muench (Carnes Mechanical)

David Muench (Carnes Mechanical)

Hey, I’m David. I’ve worked in a cars store for 6 years. I write reviews and guides, helping people to choose the most suitable technicals and best product for them. I’m happy to finally share my knowledge of the industry here, on CarnesMechanical.

8 Comments

  • Thomas Peterson says:

    You made a good point that if the battery isn’t under the hood of a vehicle they could be industrial AGM batteries. I’ve been helping my uncle around the farm and he needed some help replacing the batteries. It’s good to know what the different batteries might go to and where they are located.

  • Petja H says:

    CCA stands for “Cold Cranking Amperes”, not Amplifier. The Battery Conditions table is wrong, those voltages are for a conventional lead acid battery, the corresponding voltages to each charge state are higher for AGM batteries.

  • Petja H says:

    You fixed the mistakes I pointed out and deleted my comment, lol 😀

  • D. B. says:

    Thanks for the good write up on AGM batteries. I have owned and worked an electronics business for 45 + years. So of course I know everything. (smiling) I was recently trying to figure out why a new, but discharged, AGM boat battery would test bad and not charge. I tried everything I could think of. I even used two different, modern, digital, charger/maintainers with AGM settings on them. I also tried the recondition mode on the chargers but it still tested bad. I gave up and returned it to the retailer. They checked it at 4 volts which did not surprise me. They then put it on a 20 amp charger to bring it up to full voltage. It then charged OK and load tested OK. It again load tested OK after setting for 24 hours. It has me frustrated because they really did nothing more than I did. Which brings me to my question: Can AGM batteries have an intermittent problem or are they more inclined to simply work or not work? Any help is appreciated.

    • Kyle Thomson says:

      Some ‘smarter’ chargers may not identify 4volts as a battery it is compatible with and not charge. Sometimes you might need a dumb charger to get enough voltage in for the ‘smart’ charger to see a flat battery and begin the charge. Had to do exactly this when our golf cart wouldn’t charge after going flat.

  • Kevin says:

    Thanks! I have been looking for a good article about choosing a battery for a long time.

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