about charging AGM batteries: AGM batteries “love” ideal conditions. Voltage. Current. Temperature. The degree of charge. Only with an ideal combination of these parameters throughout the entire service life of the battery will be 7-10 years. The stronger the deviation from ideal conditions, the faster the battery will order a long life. For many motorists, vaunted centenarians die in a year or two. And this despite the fact that the cost of AGM batteries is 2-4 times higher than conventional ones.

Features of AGM batteries

Absolutely all the features of charging an AGM battery are directly or indirectly related to their device. On the one hand, it is precisely due to their design that they are able to deliver relatively large currents and recover quickly. On the other hand, due to the peculiarities of the AGM device, batteries “survive” high voltage, low or high temperature, as well as defective charges extremely poorly.

AGM battery device

There is nothing more innovative in the design. The same lead plates (with additives). The same liquid electrolyte of water and acid. The principle of operation is exactly the same as that of other lead-acid batteries. The only fundamental difference is that the electrolyte here is not poured into "cans", as is done in the classics. It is impregnated with fiberglass mats, which are in close contact with lead plates.

AGM battery device
AGM battery device

Hence the name of the technology. AGM is an abbreviation for Absorbent Glass Mat, which literally means absorbent fiberglass mat. They are impregnated with electrolyte. Liquid. This is important to understand so as not to confuse AGM technology with GEL . In the batteries under consideration, the electrolyte is not gel, but liquid. Actually, charging gel batteries has its own characteristics.

The volume of electrolyte and lead

Compared to classic bulk lead-acid batteries, AGM contains much less electrolyte. This is possible just thanks to a special device and the use of impregnated mats. For charging AGM batteries, this feature plays a crucial role. Since there is less electrolyte, then its losses in the case of "boiling" faster can be critical.

Common AGM battery configuration
Common AGM battery configuration

On the other hand, due to the smaller volume of electrolyte, more lead can be crammed into the battery without increasing the size of the case. This is easy to see if you compare the weight of an AGM battery with a classic of the same size. And the more lead, the greater the capacity of the battery. But that's not the point. The main thing is that an increase in the surface area of the lead plates allows you to get high currents from the battery. It also works in reverse. That is, it is also possible to charge AGM batteries with high currents.

Internal resistance

The larger the area of lead in contact with the electrolyte, the lower the internal resistance of the battery. And for AGM, this is one of the design features. The area of the plates is increased due to the fact that space is “saving” due to the reduced volume of electrolyte. Their low internal resistance affects the charging of AGM batteries by the fact that they are able to accept large currents almost “painlessly”.

Detailed material about the internal resistance of the battery

Starting and charging currents

One of the advantages of the technology is the ability to send and receive large currents. Thanks to this feature, you can charge an AGM battery much faster than a regular one. Also, it is possible to remove large starting currents from it. This is fully used on cars with a start-stop system. Even increased starting current makes it easier to operate machines with diesel engines. Although in winter it is also problematic to start a gasoline engine with a weak battery.

Tendency to freeze and overheat


These tendencies are already among the disadvantages of technology. They are explained by the fact that the electrolyte is relatively small in volume. And the smaller the volume of liquid, the more intensely it reacts to critical temperatures. For this reason, during AGM charging , batteries often overheat, especially if the voltage mode is not observed. Batteries also freeze faster than usual. In addition, in this state they absolutely do not accept a charge.

Deep discharge resistance

This feature is an advantage of AGM technology. Especially when compared with calcium, for which a deep discharge is an irretrievable loss of capacity. Accordingly, the need to charge AGM batteries from a stationary charger occurs much less frequently than in the case of the mentioned calcium ones. By the way, the starting current of an almost discharged AGM battery is maintained at a decent level, which allows you to successfully start the engine even from an almost dead battery.

Memory effect

Another drawback of the technology is the memory effect. It consists in the following. If the AGM battery is regularly charged not up to 100%, then it will quickly lose the capacity declared on the case. Some sources even state that the capacity drops significantly due to systematic undercharging. It is not true. The resource, of course, will decrease from such exploitation. But not as quickly and critically as it is often said. However, it will not be superfluous to keep in mind this feature of AGM.

7 rules for charging an AGM battery

Charging an AGM battery with an automatic charger
Charging an AGM battery with an automatic charger

Charge AGM batteries in the following cases:

  • You just bought a battery, and there is no guarantee that the seller looked after it while it was gathering dust in the window or in the warehouse. Read more about this in the article “Do I need to charge a new battery”.
  • The car is rarely used or for short trips. In such cases, the battery does not have time to recover from the generator.
  • Low on-board voltage, due to which the battery on the car is never fully charged.
  • Long term parking. Especially if the car was in the cold.

Also, the AGM battery has to be recharged regularly when its resource is running out. If the on-board voltage is normal, you drive regularly and a lot, at night in winter the car is not on the street - the AGM battery can easily serve its entire operational life without a single recharge from a stationary charger.

Rule number 1. Frozen battery AGM - do not charge

Here it must be understood that not every AGM battery should be considered frozen if it has stood overnight in the cold. If he was well charged before, then he is not afraid of negative temperatures. If the battery is run down, then its electrolyte density is low. And this means that there is almost only water in the electrolyte, without acid. Such a battery has a very high chance of freezing. Moreover, this is AGM, where the electrolyte is small in volume.

Therefore, if a run-down AGM battery has been in the cold, you should not charge it in this state. First, the battery needs to be warmed up. To do this, bring it into a heated room for a couple of hours.

Advice! It is strongly not recommended to accelerate the heating of the battery with a building hair dryer, near a household heater or on a heating battery. This can lead to irreversible deformation of the lead plates and battery failure.

Rule number 2. Memory without voltage control - do not use

To charge the AGM, you must use a voltage-limiting charger
To charge the AGM, you must use a voltage-limiting charger

For starters, it is recommended to read - how to choose a battery charger. Next, you should learn that charging an AGM battery with an increased voltage leads to boiling of the electrolyte in it, as well as to excessive overheating. What is the first, what is the second - is extremely detrimental to battery life.

It follows that the memory for charging the AGM battery should be limited either automatically or at least manually. Uncontrolled charging is not acceptable for this type of battery.

Rule number 3. The charge voltage cannot be increased


Ideal Voltage
Ideal Voltage

The normal charging voltage for an AGM battery is 14.4V. If it is higher, the battery at the last stage will begin to boil, warm up, lose electrolyte, which cannot be topped up due to the design features. Voltage below the specified norm is also bad. If the charger is not capable of delivering 14.4 V, then the battery will never be able to charge to 100%, which is undesirable for AGM technology.

Advice! Even if the charger is automatic or manually adjustable, always check the charging voltage of the AGM battery with a multimeter. It must be connected directly to the battery terminals. This precaution will avoid overcharging.

Rule number 4. Charging current should not be overestimated without the need

AGM battery charge current
AGM battery charge current

AGM charging current can be higher than the classic 10% of capacity. That is, a battery for 70 ampere-hours can be charged with currents not 7 amperes, but 10-15. But there are two nuances here. Firstly, the capacity of a used battery is always less than indicated on the case. Natural wear and tear has not been canceled. Secondly, absolutely any battery is charged the better, the lower currents it is done. This also has a positive effect on the battery life.

Advice! Any batteries that you have on the farm (in a car, in smartphones, tablets), if possible, try to charge as little current as possible. This will increase the charging time, but will significantly improve its quality and extend the service life.

Rule number 5. AGM on charge "does not like" heat and frost

The ideal ambient temperature for charging an AGM battery is 20-25°C. If the air temperature is lower, the speed and quality of the charge will decrease. It is generally not recommended to charge such batteries in the cold. Therefore, in winter it is better to do this in a heated room.

The heat is much worse than negative temperatures for AGM batteries. This is one of the reasons why on many modern cars the place for the battery is provided further away from the engine - in the passenger compartment or in the trunk. Accordingly, it is not worth charging the battery from the charger at an ambient temperature above 30°C. Temperatures above 40°C are considered critical for AGM technology.

Rule number 6. Monitor battery temperature

Consequence of overheating
Consequence of overheating

If the ambient temperature is normal, and the AGM battery heats up during charging, then you are doing something wrong. The allowable charge voltage may have been exceeded. Or the charge current is much higher than the battery can accept in its current state (remember that the capacity decreases over time, and the current must be selected taking this fact into account).

Advice! To control the heating temperature of the AGM battery during the charging process, buy an inexpensive digital thermometer with an external sensor. Most of all, when charging, the batteries heat up, as a rule, in the area of positive terminal. There the thermometer sensor should be attached for the duration of charging.

Rule number 7. Charge your AGM battery up to 100%

If you have an automatic charger, then it will charge the battery itself until it charges up to 100%. For other cases, a sign of a fully charged battery can be a charging current that does not decrease for two hours. If your charger does not have an informative digital ammeter, the charge current at the last stage can be controlled using a conventional multimeter, included in the appropriate mode and in the open circuit between charging and battery.

Brief summary


There is nothing complicated and unusual in charging AGM batteries. The main thing to remember is how these batteries "love" ideal conditions. Voltage - 14.4 V. Current - the lower the better. Ambient temperature - 20-25°C. Heating of the battery case - no more than 40°C. The degree of charge is closer to 100%.

VIDEO: charging AGM battery