What is a “Tail Current” in a battery?

“Tail Current” How Do I Calculate This and Is It Even An Option In My Battery Monitor?

• What Is The Tail Current In A Battery?
• Does My Battery Monitor Have The Tail Current Option?
• How Do I Calculate The Tail Current In My Battery Bank?
• Tail Current Setting Example

What Is The Tail Current In A Battery?

The term “tail current” in the context of batteries is often associated with the charging or discharging process of a battery. It refers to the small, residual current that flows in or out of a battery after the majority of the charging or discharging has taken place.

When a battery is charged or discharged, there is an initial higher current that gradually decreases as the battery approaches its fully charged or fully discharged state. The tail current is the small amount of current that continues to flow even after the primary charging or discharging process is complete.

In a charging scenario, the tail current could represent the ongoing trickle charge that helps to maintain the battery at its fully charged state. In a discharging scenario, the tail current might be the small current drawn by devices connected to the battery even after the main discharge is complete.

Monitoring and understanding the tail current can be important in battery management, especially in applications where precise control of the state of charge is critical. Excessive or prolonged tail currents can sometimes indicate issues with the battery, such as internal leakage or self-discharge, which might impact the overall performance and lifespan of the battery.

The tail current is typically measured in amperes (A) and is considered during the design and management of battery systems to ensure proper charging and discharging characteristics. It’s worth noting that the term might be used differently in specific contexts or industries, so it’s always a good idea to refer to the technical documentation or specifications provided by the battery manufacturer for precise information.

Does My Battery Monitor Have The Tail Current Option?

It’s important to note that not all battery monitors provide users with direct control over tail current settings. Some systems may have preset algorithms for charging phases, including the tail current, without user-adjustable parameters.

In the Magnum Energy battery monitor for instance or the BMK, the charging process the display signals a fully charged state (SOC = 100%) when the following three conditions are satisfied:

1. Stabilization of Charging Voltage: The voltage during the charging process achieves a consistent and stabilized level over time.
2. Low Charging Current (Preset Tail Current): The charging current decreases to a minimal percentage of the Ampere-Hour (AH) capacity, typically below 2% and this value is predefined as a “Tail Current.”
3. Ampere-Hours (AH) Balance: The AH removed from the battery align within 1% of being fully replenished.

Absorb Done Amps: Also for the Magnum inverter chargers, in the Absorb Charge mode, as the battery undergoes charging, there is a reduction in the current needed by the battery. The term “Done amps” denotes the point at which the done amps consistently falls below the Absorb Done Amps setting for a continuous duration of two minutes. At this juncture, the charger transitions from the Absorption charge cycle to the final stage of the charging process. Although this is great if you don’t have any loads on while charging, the best way to do this without overcharging your batteries while having loads on would be to install a BMK.

Refer to your charger system’s user manual to ensure the proper and adjustable settings are configured. If you have any uncertainties regarding these settings, feel free to reach out to us at IOTG Solar. We are readily available to provide assistance and answer any questions you may have. Whether through a call or by  contacting us here , we are committed to offering guidance to ensure your charger system is configured optimally for your specific needs.

How Do I Calculate The Tail Current In My Battery Bank?

Flooded Lead Acid, AGM, Lead Carbon:

The concept of a “tail current” or “maintenance current” can vary between different battery types and charge profiles. Calculating the tail current in your battery bank involves determining a small, residual charging current that persists during the final stages of the charging cycle. For lead-acid batteries, the tail current is often expressed as a percentage of the battery’s capacity. Typically, it is set to a low value, such as 1.5% or 3% of the total capacity, and serves as a threshold for transitioning from the absorb to float stage. For example, in a 200 Ah lead-acid battery bank, a 1.5% tail current would be 3 amps. In lithium batteries, tail current considerations may differ due to their distinct charging characteristics. Precision in tail current settings is crucial for optimizing the charging process, ensuring efficient energy absorption, and prolonging the overall lifespan of both lead-acid and lithium battery systems.

Lithium Batteries:

Diverging from the traditional model observed in lead-acid batteries, lithium batteries do not strictly adhere to a continuous full capacity regimen. Instead, post-bulk charging, a tail current of 5 to 6% is considered acceptable for optimal lithium battery performance. Unlike their lead-acid counterparts, lithium batteries do not necessarily derive benefits from prolonged periods at maximum charge; on the contrary, such practices could have detrimental effects over time. After the bulk stage, a brief ten-minute absorb phase takes place, during which the Battery Management System (BMS) undertakes essential cell balancing operations. This ensures peak cell health without necessitating prolonged periods of full charge, aligning seamlessly with the distinctive characteristics and requirements of lithium battery technology. Embracing the 80/20 rule—charging to 80% and discharging to 20%—proves to be an effective strategy for cycling lithium batteries, with occasional returns to 100% charge. This approach, prioritizing partial charge cycles, stands as a proven method to enhance the overall longevity and efficiency of lithium batteries.

Tail Current Setting Example

In the depicted screenshot, which showcases a system employing a Victron battery monitor, attention is drawn to a Lead Acid battery bank boasting a total capacity of 320 Ah. The system adopts a tailored strategy by configuring the tail current to 1.5%, translating to 4.8 Ah (1.5% of 320 Ah). This strategic customization becomes evident during the later absorb stage of the charging process, where the incoming current gradually diminishes as the battery nears full capacity. The predetermined tail current of 4.8 amps establishes a pivotal threshold; when the charging current dips just below this parameter, the system smoothly transitions into the float stage. This precision in tail current settings underscores the sophisticated control mechanisms inherent in advanced battery monitoring systems such as Victron, Schneider and others, optimizing the charging process for heightened efficiency and prolonged battery life.

Now, envision an off-grid scenario where the charging source is a gas-powered generator. In a strategic move to conserve fuel as the charging process approaches the designated tail current of 4.8 amps, a decision is made to adjust the tail current setting to 3%. Consequently, this adjustment translates to 9.6 amps (3% of 320 Ah). In this scenario, the absorb stage would seamlessly transition to float mode with just under 9.6 amps remaining to full capacity, as opposed to the earlier setting of 4.8 amps. This thoughtful customization in tail current settings not only showcases a keen consideration for fuel efficiency but also demonstrates the flexibility and optimization capabilities embedded in the Victron battery monitoring system.

Conclusion

In summary, the concept of “tail current” within battery monitoring denotes the residual, low-level current persisting during the concluding phase of a charging or discharging cycle, typically when the battery approaches full or empty status. This critical element in battery management systems serves a pivotal role in balancing operations for configurations with multiple cells. By optimizing the charging process, it ensures the harmonious distribution of charge among individual cells. Tail current settings, present in certain systems and customizable in others, contribute to efficient battery performance. A comprehensive grasp of and vigilance over the tail current are indispensable for sustaining battery health and enhancing the overall efficacy of the system.

Should you require assistance, feel free to reach out to us at IOTG Solar—we are readily available to provide support and guidance either through a call or our online  contact option here .

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