How to test a 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.

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

Top rated agm battery

Top choice

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

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


Schumacher BT-100 Battery Load Tester 12V

  • 4,7 Rating
  • Tests 6V and 12V batteries up to 1000 CCA
  • Ideal for cars, light trucks, motorcycles, boats and lawn tractor batteries

How To Test AGM Batteries

How to Test an AGM Battery

Best automotive battery testers

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

2. MOTOPOWER MP0514A 12V Digital Car Battery Tester Voltmeter

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

4. CARTMAN 12V Car Battery & Alternator Tester

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

6. Schumacher BT-100 100 Amp Capacity Battery Load Tester

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

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. 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 a 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.

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

Turn on the multimeter. Set it to voltage measurement by turning the control dial or pressing the appropriate button on the meter.

Step 2

Place the sensor on the end of the red wire from the meter to the “+” terminal of the AGM battery. Place the sensor on the “-” terminal of the AGM battery at the end of the black wire.

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

However, if the measured value is below 10 volts, you will need to charge your battery for a while. After charging, check the voltage again, then leave for several hours and test again. If the voltage drops by more than about 1 or 2 volts, you may need to consider a replacement.

AGM load test

Step 1

Read the label on the side of the AGM battery to determine the amperage load. The label bears the letters CCA, means that it is a cold crank ampers, followed by a number referring to the ampere when the battery is intended for use in a vehicle. For example, CCA 70 may appear on the label. Alternatively, it can have the letters Ah, which means ampere-hours, followed by a number if the battery is used in a golf cart.

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 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 your normal charger with AGM or gel cell batteries.

Wrong. These batteries like to be charged slowly and weakly. Many AGM/gel cell battery chargers have microprocessors that collect information from the battery and adjust current and voltage accordingly. Some have different settings for charging flooded, gel and AGM batteries. Overcharging can destroy these batteries.

In addition, generators are not chargers. Do not rely on an alternator to do the work of a charger. If a battery is so discharged that it cannot start the vehicle, use a charger as soon as possible to ensure that the battery is fully charged.

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

Wrong. These types of batteries have a lower internal resistance than flooded batteries. Older capacitive battery testers/analyzers may not be able to read these batteries accurately. Most new battery analyzers have a special mode for AGM/gel cell batteries. Old school load testers may not provide meaningful results.

Replacing the AGM or gel cell battery is identical to replacing the flooded battery.

True and false. While the installation of the battery may be the same for both battery styles, some vehicles require an additional step to notify the vehicle that the battery has been replaced. Newer vehicles have a battery sensor module or similar systems. These systems require recalibration with a scan tool when the battery is replaced. If the system is not recalibrated, the alternator may overcharge the new battery and cause the battery to fail shortly after replacement.

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.


In the age of computer-controlled battery charging systems, the charging voltage can be influenced both by the operating conditions and by the additional load, the battery SOC and the battery SOH. For example, the powertrain control modules (PCMs) in some vehicles, such as BMWs, are programmed to adjust their charging power to compensate for normal wear on the battery plate. As a result, a modern charging system that is controlled via various inputs to the engine control unit (ECM) can maintain an AGM battery more efficiently than a conventional voltage regulator that only responds to battery voltage and ambient temperatures. With this note, I generally recommend replacing both the flooded cell and the AGM with an equivalent original device (OE).

Testing agm batteries

Since a conventional voltage regulated charging system is based on voltage feedback from the battery to the voltage regulator, I would also recommend that the alternator, voltage regulator and battery share clean positive and negative battery connections to avoid overcharging.

AMG battery testing

For off-road applications, I would recommend installing stainless steel washers at each screw connection and applying an aerosol anti-corrosion spray to all electrical and battery terminals. In any case, note that a voltage drop of more than 0.5 volts, depending on the current flowing through the circuit, can increase the charge voltage well beyond the 14.4 volt limit for AGMs.

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.


  • 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 😀

    • David Muench (Carnes Mechanical) says:

      Thank you! We made a small mistake. Thanks for your comment, we fixed everything. I will be more attentive next time.

  • 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.

  • Kevin says:

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

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