Battery Equivalents and Replacements

AGM Battery Voltage Chart

Absorbent Glass Mat (AGM) batteries represent a significant advancement in lead-acid battery technology, offering enhanced performance and reliability for a wide range of applications.

From vehicles and marine applications to renewable energy systems and emergency backup, AGM batteries provide a durable, maintenance-free power solution. An AGM Battery Voltage Chart is a crucial tool for understanding the optimal operating voltages and state of charge (SoC) for these batteries.

By using this chart, users can ensure their AGM batteries operate efficiently and last longer, making the most out of their investment.

Published: April 9, 2024.

car battery multimeter 1

12V Lead-Acid Batteries Voltage Chart

The following chart lists the three most popular 12V lead-acid battery types and their voltage depending on the battery's State of Charge (SoC) (values are given for the room temperature):

SoC (State of Charge) Wet/Flooded Lead-Acid Battery Gel Lead-Acid Battery AGM Lead-Acid Battery
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

However, one must note that these are general values for batteries in resting state (at least 2 hours with no load and no charge), and they may differ slightly from battery to battery.

For example, Odyssey AGM batteries' SoC/Voltage relation is as follows:

  • 12.84+ V: 100% SoC,
  • 12.65 V: 85% SoC,
  • 12.50 V: 75% SoC,
  • 12.18 V: 50% SoC,
  • 12.00 V: 35% SoC,
  • 11.88 V: 25% SoC, etc.

These values are very similar to the values provided for AGM batteries in the Lead-Acid Batteries Voltage Chart, but they are not exactly the same.

Thus, whenever you can, check the Owner's Guide of the battery that you have and the exact voltages for the model that you have.

12V, 24V, 36V, 48V AGM Batteries Voltage Chart

The following chart lists 12V, 24V, 36V, and 48V AGM batteries' voltages depending on the battery's State of Charge (SoC) (values are given for the room temperature):

SoC 12V AGM Battery 24V AGM Battery 36V AGM Battery 48V AGM Battery
100% 12.80+ 25.60+ 38.40+ 51.20+
75% 12.60 25.20 37.80 50.40
50% 12.30 24.60 36.90 49.20
25% 12.00 24.00 36.00 48.00
0% 11.80 23.60 35.40 47.20

Again, these are general values for batteries in resting state (at least 2 hours with no load and no charge), and they may differ slightly from the battery pack to the battery pack.

When is the AGM Battery Discharged, at 11.80V or at 10.50V?

The state of discharge for AGM (Absorbent Glass Mat) batteries, like all batteries, is a critical factor in understanding their health and operational status.

Common thresholds like 11.80V and 10.50V often serve as reference points for determining whether an AGM battery is considered "discharged." However, the true point at which an AGM battery is deemed discharged is not solely dependent on voltage but also on discharge current and discharge time. This nuanced approach is crucial for maximizing the longevity and efficiency of these batteries.

AGM batteries, renowned for their efficiency and durability, operate differently under varying loads and discharge rates. At a high discharge current, the battery might reach a voltage of 11.80V relatively quickly, but this does not necessarily mean it is fully discharged.

High discharge rates can cause the voltage to drop more rapidly, but once the load is removed, the voltage can partially recover, indicating residual capacity.

Conversely, a lower discharge rate over a more extended period might bring the voltage down to 10.50V, suggesting a deeper level of discharge. Therefore, the voltage level at which an AGM battery is considered discharged is influenced significantly by the current it is supplying.

Understanding the relationship between discharge current, discharge time, and battery voltage is vital for the effective management of AGM batteries.

It allows users to set appropriate discharge limits to protect the battery's health and ensure a long service life.

For Example:

The following chart shows the Universal Power Group UB121000 AGM Deep Cycle Battery (Amazon link, the link opens in the new window) constant current discharge characteristics at 77°F (25°C), with the values given in Amps (A):

Cell End
Voltage (V)
Battery End
Voltage (V)
5 min 10 min 15 min 30 min 1 hr 2 hr 3 hr 4 hr 5 hr 8 hr 10 hr 20 hr
1.6 V 9.60 V 351.5 256.5 180.5 109.3 57.0 33.3 24.4 19.0 15.7 11.0 10.0 5.4
1.7 V 10.20 V 309.7 233.7 161.5 103.6 53.6 31.7 23.8 18.5 15.4 10.8 9.7 5.3
1.75 V 10.50 V 298.3 222.3 152.0 100.7 52.3 31.0 23.2 18.2 15.2 10.7 9.5 5.2
1.8 V 10.80 V 286.9 210.9 142.5 97.9 50.4 30.2 22.6 18.0 14.8 10.5 9.3 5.1
1.85 V 11.10 V 275.5 199.5 133.0 95.0 48.5 29.5 21.9 17.4 14.4 10.2 9.0 4.9

As one can see, the allowed end discharge voltage depends on the discharge current and time (bold numbers).

And when the lead-acid battery is discharged down to 100% DoD, be sure to recharge it as soon as possible to prevent the damage caused by the buildup of lead sulfate crystals.

Thus, determining when an AGM battery is discharged requires more than a simple voltage check. It necessitates a comprehensive understanding of the battery's application, including the discharge current and the expected duration of discharge.

Note: to prolong the lead-acid battery's lifetime, don't discharge it down to 100% DoD - 80% DoD is more than enough.

AGM Batteries: Deep Cycle vs. Dual Purpose vs. Starting

AGM (Absorbent Glass Mat) batteries are a type of sealed lead-acid battery that offer distinct advantages over traditional flooded lead-acid batteries, including maintenance-free operation, leak-proof construction, and improved safety.

AGM technology is versatile, supporting a range of battery designs tailored to specific applications: deep cycle, dual purpose, and starting. Understanding the differences and applications of each can help you choose the right battery for your needs.

Deep Cycle AGM Batteries

Deep cycle AGM batteries are designed to provide a steady amount of current over a long period. They are built to endure repetitive discharges down to 20% of their total capacity and are ideal for powering electrical loads over extended periods.

Characteristics

  • Thick Plates: Have thicker active plates and denser active material to withstand deep discharge cycles.
  • High Capacity and Durability: Designed for applications requiring a long, reliable service life and numerous charge-discharge cycles.
  • Low Self-Discharge Rate: Ideal for seasonal or infrequent use, as they hold their charge well over time.

Advantages

  • Suitable for off-grid power systems, RVs, marine applications, and anywhere consistent power is needed over an extended time.
  • They can be mounted in any orientation due to their leak-proof design.
  • More resistant to sulfation compared to flooded batteries, especially when left in a discharged state.

Disadvantages

  • Higher upfront cost compared to equivalent flooded or gel batteries.
  • Not designed for applications requiring high bursts of current, such as starting engines.

Applications

Solar and wind energy storage, RV house power, electric vehicles, marine applications, and any situation where batteries are regularly discharged and recharged.

Dual Purpose AGM Batteries

Dual purpose AGM batteries are designed to offer a compromise between starting and deep cycle capabilities. They can start engines and provide power for accessories and are an excellent choice for vehicles and boats that require both functionalities from a single battery.

Characteristics

  • Moderate Plate Thickness: Features a design that balances between thin plates for high current bursts and thick plates for deep discharge resilience.
  • Versatility: Capable of handling both starting and cycling duties, making them a flexible option for many users.
  • Durability and Maintenance-Free: Shares the same maintenance-free and leak-proof characteristics as other AGM batteries.

Advantages

  • Ideal for applications with limited space for multiple batteries.
  • Reduces the need for separate starting and deep cycle batteries.
  • Provides reliable starting power and moderate deep cycle capabilities.

Disadvantages:

  • May not offer the same lifespan in deep cycle applications as a dedicated deep cycle battery.
  • Not as efficient in starting applications as a dedicated starting battery.

Applications

Boats, RVs, and vehicles that require both starting power and energy for accessories, without the space or budget for multiple batteries.

Starting AGM Batteries

Starting AGM batteries are optimized for delivering quick, high-power bursts to start engines. They feature thin plates that maximize surface area, providing the instant energy needed to start cars, motorcycles, and boats.

Characteristics

  • Thin Plates: Optimized for high current output over short durations.
  • High Cranking Amps (CA) and Cold Cranking Amps (CCA): Designed to perform under various temperature conditions, especially crucial in cold weather.
  • Fast Recharge Rate: Can recover quickly from the initial starting discharge.

Advantages

  • Provides reliable starting power under a wide range of environmental conditions.
  • Leak-proof and maintenance-free, ideal for vehicles and equipment used in demanding conditions.
  • Resistant to vibration and shock, prolonging the battery's lifespan in mobile applications.

Disadvantages

  • Not suitable for deep discharge applications, as the thin plates are prone to damage if frequently deeply discharged.
  • Specific to starting applications and cannot be used as a stand-alone power source for accessories over a long period.

Applications

Cars, motorcycles, boats, and other engine-starting applications where reliability and performance are critical, particularly in harsh or variable climates.

jump a car battery

Choosing between deep cycle, dual purpose, and starting AGM batteries depends on your specific needs.

Deep cycle batteries are best for sustained power over long periods, dual-purpose batteries offer flexibility for vehicles and boats that need to start engines and power accessories, and starting batteries are optimized for providing quick, powerful bursts of energy to start engines.

Understanding these distinctions will help you select the right AGM battery for your application, ensuring optimal performance and longevity.

Lead-Acid Batteries: Wet/Flooded vs. Gel vs. AGM

Lead-acid batteries have been a cornerstone of energy storage systems for over a century, powering everything from vehicles to backup power systems.

Among the various types, wet/flooded, gel, and AGM (Absorbent Glass Mat) batteries represent the most common variations, each with its unique advantages and applications.

Wet/Flooded Lead-Acid Batteries

Wet or flooded lead-acid batteries are the traditional type, consisting of lead plates submerged in an electrolyte solution of sulfuric acid and water. These batteries require regular maintenance, including topping up the water lost during the charge-discharge cycle.

Advantages

  • Cost-Effective: They are generally less expensive than gel and AGM batteries, making them a cost-effective option for many applications.
  • High Performance: Offers high current delivery and good performance in high-temperature environments.
  • Maintainability: It's possible to check the electrolyte level and specific gravity, allowing for regular maintenance and long-term usage.

Disadvantages

  • Maintenance Required: Regular topping up of water and cleaning of terminals to prevent corrosion is necessary.
  • Risk of Spills and Gases: They can leak acid if tipped over and release hydrogen gas during charging, posing a risk of explosion if not properly ventilated.
  • Limited Placement: Must be kept upright to avoid leaks and cannot be used in sensitive environments.

Applications

Automotive starter batteries, large stationary power backups, and any application where the battery remains mostly stationary and can be regularly maintained.

Gel Lead-Acid Batteries

Gel batteries incorporate a silica additive that turns the electrolyte into a gel-like substance. This design prevents leakage and reduces the evaporation of the electrolyte, making these batteries virtually maintenance-free.

Advantages

  • Maintenance-Free: No need to top up with water or clean terminals, reducing upkeep efforts.
  • Leak-Proof and Spill-Proof: Can be mounted in any position without the risk of leaks, increasing flexibility in installation.
  • Vibration Resistance: The gel electrolyte minimizes movement inside the battery, offering better resistance to vibration and shocks.

Disadvantages

  • Lower Charge and Discharge Rates: Compared to wet/flooded batteries, gel batteries have lower charge and discharge rates.
  • Temperature Sensitivity: Performance can degrade in extreme temperatures.
  • Higher Cost: Generally more expensive than flooded lead-acid batteries due to their advanced design and materials.

Applications

Marine, RV, wheelchair, and off-grid solar applications where maintenance access is difficult or where the battery needs to be installed in unusual positions.

AGM (Absorbent Glass Mat) Batteries

AGM batteries feature glass mats that absorb the electrolyte between the battery plates. This design offers the benefits of a sealed battery with a lower internal resistance, allowing for higher discharge rates and shorter charging times.

Advantages

  • High Rate of Discharge and Charge: Ideal for applications requiring high power output in short bursts, such as starting engines.
  • Maintenance-Free: Like gel batteries, AGM batteries are sealed and do not require water to be added.
  • Durability and Vibration Resistance: The tight packing of the internal components reduces movement and damage from vibration.

Disadvantages

  • Cost: AGM batteries are typically more expensive than gel and wet/flooded batteries.
  • Sensitivity to Overcharging: Requires a well-regulated charging system to avoid damage.
  • Weight: Generally heavier than other types of lead-acid batteries, which may be a consideration for portable applications.

Applications

Ideal for vehicles, UPS systems, high-end motorcycles, military equipment, and any application that requires robust energy delivery or operates in a harsh environment.

Choosing the right type of lead-acid battery - wet/flooded, gel, or AGM - depends on your specific needs, including cost, maintenance capabilities, installation environment, and performance requirements.

Each type offers distinct advantages, making them suitable for a wide range of applications. Understanding these differences is key to selecting the best battery solution for your needs, ensuring reliability, efficiency, and longevity in your power storage system.