Flooded Batteries vs. AGM Batteries: Understanding the Differences

When it comes to lead acid batteries, two popular options are flooded (sometimes called conventional) batteries and AGM (Absorbent Glass Mat) batteries. While both serve the purpose of energy storage, they have distinct differences in terms of construction, maintenance, performance, and applications.

In this blog post, we will explore the key differences between flooded batteries and AGM batteries to help you make an informed choice for your specific needs.

SLA Batteries in warehouse

What is the difference between flooded batteries and AGM batteries?

Flooded Batteries

Flooded batteries, also known as wet cell batteries, have a traditional design consisting of lead plates submerged in a liquid electrolyte solution. The plates are separated by separators, and the electrolyte level needs to be monitored and periodically topped with distilled water. They typically require regular maintenance to ensure optimal performance and longevity.

Maintenance activities include checking and maintaining proper fluid levels by adding distilled water as needed, monitoring electrolyte-specific gravity, and periodic equalization charging to address electrolyte stratification.

They require a specific charging regimen to maintain optimal performance. The charging rate for flooded batteries is typically expressed as a percentage of the battery’s capacity (C-rate). The recommended charging rate for flooded batteries is generally less than 10% of their capacity. For example, a 100Ah flooded battery should be charged at a rate of 10A or less.

Charging at a slower rate helps ensure a complete and thorough charge, allowing the electrolyte to mix properly and minimizing the risk of stratification. To learn more about C-rates visit battery universities’ article, “What is C-rate?”.

High Performance Flooded Lead Acid Battery

The bulk charge voltage of a 12V flooded lead-acid battery is typically 14.2 volts. This voltage is commonly used during the bulk charging stage to quickly replenish the battery’s charge. During bulk charging, a constant current is applied to the battery to rapidly restore its energy.

The higher voltage helps facilitate the electrochemical reactions within the battery, allowing for efficient charging. However, it’s important to note that the specific bulk voltage can vary depending on the battery manufacturer, design, and temperature conditions.

The float charge voltage is typically 13.4 volts. The float voltage is the voltage at which a fully charged battery is maintained during a standby or maintenance mode. During the float charging stage, a lower voltage is applied to the battery compared to the bulk charging stage.

This voltage level is intended to keep the battery at a fully charged state without overcharging or creating excessive gassing. It compensates for the self-discharge of the battery and any small loads connected to it, ensuring that the battery remains ready for use.

AGM Batteries

AGM batteries feature a sealed design with a fiberglass mat soaked in electrolyte. The mat is placed between the lead plates, effectively immobilizing the electrolyte. AGM batteries do not require the addition of water or electrolyte checks, as the design eliminates the need for maintenance associated with fluid levels.

High Performance AGM Battery

They have a different charging profile compared to flooded batteries. AGM batteries generally have a more controlled charging approach. The recommended charging rate for AGM batteries is typically around 10-25% of their capacity. Their unique glass mat separator design can be charged at higher rates than flooded batteries due to reduced internal resistance and better charge acceptance. The glass mat separator tightly holds the electrolyte in place, preventing spillage and excessive gassing, allowing for faster charging without damaging the battery. For more information on AGM technology visit our blog post, “The Complete Guide to AGM Batteries”.

The bulk charge voltage of a 12V AGM lead acid battery is typically 14.7 volts, and the float charge is typically 13.8 volts.

When should a flooded or AGM battery be used?

The choice between AGM or flooded batteries should be determined by the intended application, budget, and charging control.

Assess the specific needs of your application. Flooded batteries are the optimal choice for applications where cost is a primary concern, and regular maintenance can be performed. They can handle higher discharge rates and provide a higher surge current than AGM batteries. This makes them suitable for applications that require high-power output, such as automotive starting applications, off-grid renewable energy systems, and heavy-duty industrial equipment.

In comparison, AGM batteries are ideal for sealed and maintenance-free setups. AGM batteries have a lower self-discharge rate than flooded batteries. They can retain their charge for longer periods when not in use, which can be advantageous for applications with infrequent or seasonal use. Additionally, the glass mat separator makes them highly resistant to vibrations and shocks. They are commonly used in applications such as deep-cycle storage for RVs and boats, backup power systems, renewable energy storage, and UPS (Uninterruptible Power Supply) systems.

AGM Dual Purpose Marine Battery

Budgetary constraints should also be evaluated. Flooded batteries need periodic maintenance, including checking and topping up electrolyte levels, equalizing charges, and monitoring specific gravity (the ratio of the density of the electrolyte to the density of water). This maintenance requirement adds to the overall cost and effort of using flooded batteries. On the other hand, AGM batteries may have a higher upfront cost, but they offer convenience and durability, potentially reducing long-term costs.

Lastly, the charging control available in your system should be examined. AGM batteries may require more sophisticated charging control to prevent overcharging, while flooded batteries can tolerate higher charging voltages. We recommend our guide to charging sealed lead acid batteries.

Are AGM Batteries safer than flooded?

Flooded batteries can emit gases, including hydrogen and oxygen, during charging and discharging. Adequate ventilation is necessary to prevent the buildup of potentially hazardous gases. Additionally, the presence of liquid electrolyte in flooded batteries poses a risk of leakage and corrosion if not properly handled or maintained.

AGM batteries on the other hand, are designed with recombination capabilities, meaning they recombine the gases produced during charging. This recombination process significantly reduces the emission of hazardous gases, making AGM batteries safer to use in enclosed spaces. The sealed design of AGM batteries also eliminates the risk of electrolyte leakage and corrosion.

AGM Sealed Lead Acid Group


Flooded batteries and AGM batteries are two distinct types of lead acid batteries, each with its own set of advantages and considerations. Flooded batteries require regular maintenance and fluid level checks, while AGM batteries are maintenance-free. Flooded batteries can handle higher discharge rates. AGM batteries provide convenience, safety, and better resistance to vibration and shock. Understanding these differences will help you select the appropriate battery type based on your application requirements, maintenance preferences, and safety considerations.

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