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Why Does My Battery Voltage Drop When I Stop Charging?

Why Does My Battery Voltage Drop When I Stop Charging?

27 volts drops to 25 when I shut my charger off. Why?

  • Why Does My Voltage Drop When I Stop Charging?
  • What Is Nominal Voltage?
  • Conclusion

Why Does My Voltage Drop When I Stop Charging?

While actively charging or during the float phase, the battery voltage registers at 27.2 volts in my solar system. However, upon turning off the solar controller and disconnecting any loads, the voltage unexpectedly drops to 25.2 volts. I anticipated the voltage to remain closer to the float voltage, as it was just before shutting off the controllers. Could you assist me in understanding why this voltage drop is occurring?

My customer, who has a substantial battery bank in their off-grid solar system, has raised an important question. Given that batteries are a pivotal component and often represent a significant investment in such systems, it’s crucial to understand their behavior during the charging process and when disconnected from the charging source. Additionally, we need to explore how batteries behave when they are simply left idle with no connections. This understanding is paramount as it directly influences the efficiency, performance, and longevity of the entire off-grid solar system.

person holding low battery smartphone

What Is Nominal Voltage?

Nominal voltage refers to the standard or average voltage of a battery. It’s a commonly used term in the context of batteries to represent a typical or designated voltage for a particular type of battery. The nominal voltage is not an exact value but rather a rough average that is used for general categorization.

For example:

Lead-acid batteries often have a nominal voltage of 2.2 or 2.3 volts per cell, so a 12-volt battery would have six cells with a nominal voltage of 2.3 or so volts each.

Common nominal voltages include 1.2V for nickel-cadmium (NiCd) or nickel-metal hydride (NiMH) rechargeable batteries, and 3.2V for lithium-ion (Li-ion) batteries.

Resting Voltage After Charging:

After charging, the resting voltage of a battery should ideally be close to its nominal voltage. However, the actual voltage can vary depending on factors such as the type of battery, the charging conditions, and the state of charge.

As a general guideline:

A fully charged lead-acid battery might have a resting voltage around 6.2 to 6.3 for (6 volt)12.6 to 12.8 volts for (12 volt)

A 24 volt battery bank should sit around 25.2, and a 48 volt battery bank should sit around 50.4

A fully charged lithium-ion battery might have a resting voltage around 3.7 volts per cell with a nominal or resting voltage of around 3.2 to 3.4 volts.

A standard 12-volt lithium battery is expected to rest within the range of 13.4 to 13.6 volts.

It’s essential to acknowledge that these readings can vary based on temperature, the specific manufacturer, and model of the battery. Different battery brands and models may have slightly different resting voltage values.

It’s important to note that battery voltage naturally decreases over time, and the rate of discharge depends on factors like self-discharge, connected loads, and battery health.

After Removing the Charge: When you remove the charge (stop charging), the voltage will typically drop slightly due to factors like surface charge dissipation. A healthy battery should stabilize at a voltage close to its nominal voltage after the surface charge dissipates.


Understanding nominal voltage, resting voltage after charging, and the behavior of batteries after removing the charge is crucial for maintaining the health and performance of a battery system. Nominal voltage provides a standardized reference, allowing for general categorization of different battery types. Monitoring the resting voltage after charging offers insights into the battery’s state and helps ensure it aligns with the expected nominal values.

As batteries naturally lose voltage over time, factors such as self-discharge, connected loads, and overall battery health play significant roles. After removing the charge, observing a slight drop in voltage is normal, attributed to surface charge dissipation. However, a rapid or substantial decline may indicate underlying issues, such as a high self-discharge rate or a faulty cell.

In every instance, it is crucial to strictly follow the manufacturer’s specifications tailored to the specific battery type. Consistent monitoring, adopting appropriate charging practices, and promptly investigating any irregularities in voltage behavior are key factors that enhance the durability and reliability of battery systems, especially in off-grid solar setups and other applications.

If you ever encounter questions or concerns regarding your batteries, their charging, or discharging processes, please feel free to reach out to us here at . We are dedicated to providing assistance and support to ensure the optimal performance of your solar and energy storage systems. Your satisfaction and peace of mind are our top priorities.

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