[Instructions] How to charge an AGM battery?

Table of contents

• What is an AGM battery?
• What type of charger do I need to charge AGM batteries?
• Battery chargers for AGM batteries
• How to charge AGM batteries?
• FAQs about charging AGM batteries

What is an AGM battery?

An AGM battery is a type of sealed lead-acid battery in which the electrolyte is absorbed into fiberglass mats, making the battery spill-proof and low maintenance. AGM technology offers several advantages over traditional flooded lead-acid batteries, such as faster charging times, higher vibration resistance, and better performance in extreme temperatures.

AGM batteries are sensitive to improper charging, which can lead to overcharging, sulfation or reduced capacity. Following the correct charging procedure maintains the health of the battery and extends its life, making it a worthwhile investment in performance and durability.

What type of charger do I need to charge AGM batteries?

Charging AGM (Absorbent Glass Mat) batteries requires a compatible charger that can deliver the correct voltage and current without damaging the battery. AGM batteries are sensitive to overcharging and undercharging, so using the right charger is critical to maintaining the health and performance of the battery. Below you'll learn what to look for when choosing a charger for your AGM battery.

AGM battery charging logic

AGM (Absorbent Glass Mat) batteries are sealed lead-acid batteries that require special charging protocols to maximize their lifespan and performance. The charging logic for AGM batteries occurs in several stages to prevent overcharging, reduce heat generation, and ensure a full charge without damaging the battery. Below is a breakdown of the typical charging stages used to effectively charge AGM batteries.

1st main charging phase

The bulk charge phase is the first phase in which the charger applies a high current (measured in amps) to the battery to quickly bring it to about 80% of its full capacity.

• Tension: The charger gradually increases the voltage to about 14.4-14.7V for a 12V AGM battery.
• current: The current remains high so that the battery can be charged quickly.
• Objective: To quickly restore the battery to most of its capacity without overloading or overheating it.

This phase is important to get the battery back into a usable condition as quickly as possible, especially in situations where short charging times are required.

2nd absorption charging stage

Once the battery reaches about 80% of its capacity, it enters the absorption phase.Here the charger keeps the voltage constant while the current is gradually reduced.

• Tension: The voltage remains constant, usually between 14.4-14.7V for a 12V AGM battery.
• Electricity: The current decreases as the battery approaches full capacity.
• Objective: Charge the battery to near 100%, avoiding overcharging which can lead to overheating or gassing (although AGM batteries are sealed, excessive gassing can cause internal pressure build-up).

The absorption phase is crucial to charge the battery and ensure that it is fully charged without stressing or damaging the internal plates.

3. Maintenance charge phase (maintenance mode)
After the battery is almost fully charged, the charger enters the maintenance phase, also known as trickle charging or float charging. During this phase, the battery remains fully charged but does not overcharge.

• Tension: The voltage is reduced to a safe level, about 13.2-13.6V for a 12V battery.
• current: A minimal current flows to keep the battery full without damaging it.
• Objective: Keeping the battery in a ready-to-use condition, which is ideal for batteries that are kept on standby or stored.

The maintenance stage is particularly useful for AGM batteries used in standby applications, such as emergency generators or seasonal equipment, as it maintains the charge without causing sulfation or degradation.

4. Equalization charge (occasional equalization)

Although this is generally not required as often with AGM batteries as with flooded lead-acid batteries, some chargers perform an equalization stage to distribute the charge across all battery cells. This is only required in certain cases or with certain chargers.

• Tension: For a limited time, a slightly higher voltage is applied to balance the cell voltage.
• Frequency: Rarely performed on AGM batteries and usually avoided unless recommended by the manufacturer.
• Objective: Prevent cell imbalance and increase battery life.

For AGM batteries, equalization should only be carried out under the manufacturer's instructions, as excessive or improper equalization can damage the battery.

Battery chargers for AGM batteries

Not all battery chargers are suitable for AGM batteries. Look for chargers that are specifically designed for AGM batteries or sealed lead acid batteries. These chargers typically have adjustable settings to suit the unique charging profile of AGM batteries, including regulated voltage and current.

1. Intelligent chargers (automatic chargers)

An intelligent charger is ideal for AGM batteries because it automatically adjusts the charging process to the condition of the battery. These chargers usually work in several stages:

• Bulk Level: The charger applies maximum current until the battery reaches about 80% of its capacity.
• absorption phase: The charger gradually reduces the current to bring the battery close to a full charge.
• trickle charge: The charger maintains a low current to fully charge the battery without overcharging it.

These stages ensure that the battery receives the optimal amount of current without damaging the internal structure. Smart chargers with an AGM mode can be a good investment as they help extend the life of the battery.

2. Charger with voltage regulation

AGM batteries require specific voltage settings to be charged correctly:

• For a 12V AGM battery, the charging voltage range is usually between 14.4V and 14.7V during the main charging and absorption phase.
• When trickle charging, the voltage should be lower, usually between 13.2V and 13.6V.

Using a charger that allows you to manually control or adjust the voltage can be beneficial for maintaining the health of the AGM battery.

3. Temperature compensation

Some high-quality chargers are equipped with temperature compensation. Since temperature affects the charging performance of the battery, a charger with temperature sensors can adjust the charging voltage to the temperature of the environment, thus preventing overcharging in warm conditions or undercharging in cold conditions.

4. Avoid standard car chargers (unless AGM compatible)

Standard car chargers are not always suitable for AGM batteries as they can overcharge or overheat the battery and damage its internal structure. If you already have a car charger, check if it has an AGM setting; if not, you should invest in a special AGM compatible charger.

5. Consider a low amp charger for trickle charging

If you need to maintain the charge of a battery that is not used frequently (such as in a backup power system), a low-amperage (1-3 amps) trickle charger with float mode can be an ideal choice. These chargers provide a small amount of current to charge the battery without overcharging it, which is especially helpful for AGM batteries in storage.

How to charge AGM batteries?

Safely and effectively charging an AGM (Absorbent Glass Mat) battery requires careful preparation and the right equipment. Follow these steps to properly charge your AGM battery:

1. Safety first

Before handling the battery, remove any jewelry and wear safety glasses. Go to a well-ventilated area or outdoors to avoid contact with battery fumes.

2. Prepare your charger

Plug in your AGM compatible battery charger and make sure it is ready to use. Follow all specific instructions for your charger model to ensure safe operation.

3. Connection of an emergency power supply (optional for car batteries)

When charging an AGM car battery, connect a backup power supply to the vehicle's OBD-II port. This will prevent settings and codes from being reset while the battery is disconnected.

4. Disconnect the negative terminal

Carefully disconnect the negative terminal of the vehicle. Wrap the cable with a glove, rag or other non-conductive material to prevent it from accidentally touching metal. The negative cable is usually black or marked with a minus sign (-).

5. Set your charger to AGM mode.

Important: Select “AGM” or “Absorbed” on your charger to match your battery type.

Choose the correct voltage (usually 12V for most AGM batteries) to ensure proper charging. If you're using a smart charger, it may automatically detect the battery type, but it's best to check these settings manually.

6. Connecting the charger to the battery

Attach the charger clamps to the battery terminals (positive to positive, negative to negative). Make sure the indicator lights on your charger show the correct settings (AGM or Absorber).

7. Start of the charging process

Start charging and allow the charger to work for approximately two to eight hours, depending on the battery's capacity and discharge level. Check your charger's progress indicators to monitor the charging status.

8. Safely disconnecting the charger

When the battery is fully charged, disconnect the charger from the battery. Avoid allowing the charger clamps to touch the loose negative battery terminal to prevent sparking.

9. Reconnect the negative pole and remove the emergency power supply

Reconnect the vehicle's negative terminal to the battery. If you are using a backup power supply, disconnect it once the negative terminal is secured.

Important tips for charging

• Slow charging preserves the battery life: Slowly charging your AGM battery will help prevent overheating. Overheating causes the water inside the battery to evaporate, making the electrolyte more acidic, which can corrode the battery's internal components and shorten its lifespan.
• Temperature monitoring: Pay attention to the temperature of the battery while charging. If it feels excessively warm, stop charging and allow it to cool down before continuing.

Suggest reading: Can I charge the LiFePO4 with a normal charger? Explained!

FAQs about charging AGM batteries

1. Can I use a normal car charger for an AGM battery?

It is best to use a charger with an AGM setting or a smart charger specifically designed for AGM batteries. A regular car charger may deliver too much current and risk overcharging.

2. How long does it take to charge an AGM battery?

The charging time depends on the battery capacity and the current of the charger. In general, it takes 4-6 hours to charge a 12V AGM battery with a smart charger.

3.What is the ideal charging voltage for an AGM battery?

The ideal charging voltage for a 12V AGM battery is between 14.4V and 14.7V. Always check the manufacturer's guidelines to ensure proper charging.

Conclusion

Properly charging an AGM battery can have a significant impact on its performance and longevity. By following the right steps and avoiding common mistakes, you can keep your AGM battery in top condition, ensuring reliable power for years to come.

If you are looking for a more efficient and longer lasting battery option, consider switching to LiFePO4 lithium batteries LiFePO4 batteries, such as those from Timeusb offer higher energy efficiency, faster charging and a longer lifespan than conventional AGM batteries. They also have built-in security featuresthat prevent problems like overcharging and overheating, making them an excellent choice for reliable, maintenance-free power.