SOC/Capacity and Voltage of AGM Battery

AUG.01,2023

AGM battery’s end voltage is crucial since it determines if the battery is over discharged. So, how do you amend the end voltage?

An AGM battery’s end voltage is crucial since it determines if the battery is over discharged.

The End Voltage: How Is It Set?

The AGM battery’s voltage is variable and depends on the battery’s SOC (State of Charge). Even when the SOC is the same under various circumstances, the voltage varies. The voltage of an AGM battery in the open-circuit, discharge condition, and charge condition is explained in this page.

I’ll explain how to establish the end voltage and how it becomes appropriate. A 12V battery is used as an example in the following. Divide or multiply by 2 proportionately depending on whether your battery is 6V or 24V.

A pplication of Deep Cycle and AGM Batteries

Please take this definition from Wikipedia: An AGM battery is a lead-acid battery with an AGM separator. Cycle life and DOD (depth of discharge) settings are crucial for deep-cycle AGM batteries. They directly impact life of battery and stable performance.

Long-term, 50% DOD is appropriate given the cost and battery life. Although DOD of 70% to 80% is also very common for specific niche applications, the final voltage is a crucial factor in achieving a suitable DOD arrangement.

AGM Battery's Open-circuit Voltage

OCV, or open circuit voltage, is the voltage reading following two hours of idleness without a load or charge. It may be regarded as either the highest voltage during discharge or the lowest voltage while charging. The voltage may change depending on the battery’s age or newness, temperature, or the manufacturer’s material.

In contrast to DOD, State of Charge relates to the remaining capacity of battery.

The final voltage, for instance, can be set at 12.25V if you select 50% DOD. However, the real capacity will be greater than 50% if the voltage falls to 12.25V while under load. For the reason that under the discharge condition the OCV voltage is higher than the related voltage. The voltage decreases as the current increases.

The following table demonstrates that at 12.25V and 0.1C discharge, there is still 80% of the available capacity.

AGM Battery Voltage at 0.1C of Discharge

For a 12V, 100Ah battery, 0.1C is equal to 10A. 0.1C stands for 0.1*(Ampere Hour) current.

According to the preceding table, 11.95V should be used as the final voltage for 50% DOD. The remaining capacity (SOC), even at low current or no discharge, is still at least 25%, therefore the battery won’t suffer any harm. But it is NOT advised to set the final voltage to 11.35V for 80% DOD. The residual energy is already zero percent in the case of very low discharge current or an open circuit. It will seriously harm the battery.

The battery needs to be protected more in this situation. Therefore, advising to set the end voltage to 11.90V or 11.80V according to chart above. 10% to 20% of the original capacity will still be available.

It is more crucial to adjust the battery system depending on the amount of energy utilized per day or cycle rather than just voltage. The end voltage will appear to prevent damage to the AGM battery when the energy usage sporadically surpasses the safety level.

A Quick Overview of UPS Battery High Current Discharge

The aforementioned suggestions are predicated on a low current battery system, like a 0.1C discharge system. A high current system, like a backup uninterruptable power supply, is a various matter.

The voltage can decrease to 9.6V quickly and the current can reach 3–4C in a 5-minute backup system. The end voltage in this instance is typically set at 9.60V. When the strong current stops, the battery voltage will quickly revert to normal.

Various Factors' Effects on Battery Voltage

The battery voltage is impacted by various charging or discharging currents under charging or discharging conditions. In a charging scenario, voltage increases as current increases. In a discharging scenario, when the current goes high, he voltage decreases.

The battery voltage may change across manufacturers and production materials. At changing temperatures, the battery’s voltage will change; the voltage will decrease as the temperature drops. The battery voltage will drop after prolonged use.

Conclusion

In order to protect the batter, the typical ESS’s end voltage, for example, a solar system for home, with a capacity discharge of approximately 0.1C, can be adjusted to 11.80–11.90V.

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