Battery Charger Voltage Mechanics: A Comprehensive Guide

Battery chargers are essential devices that restore the charge in rechargeable batteries. The voltage at which a battery charger operates is a critical factor in determining the charging characteristics and efficiency of the charger. This comprehensive guide delves into the technical specifications and mechanics of battery charger voltage, providing a detailed understanding for DIY enthusiasts and experts alike.

Understanding Battery Voltage and Charging

The voltage of a battery charger is typically specified in volts (V) and is determined by the number of cells in the battery being charged. For example, a 12-volt battery charger is designed to charge a 12-volt battery, which typically consists of six cells connected in series. Each cell in a lead-acid battery has a resting voltage of about 2.1 volts, so a 12-volt battery has a resting voltage of about 12.6 volts (6 cells x 2.1 volts).

To charge a lead-acid battery, a battery charger must apply a voltage that is higher than the resting voltage of the battery. This voltage is typically referred to as the charging voltage. The charging voltage is determined by the battery chemistry, the number of cells in the battery, and the charging method being used.

Constant Current Constant Voltage (CCCV) Charging

battery charger voltage mechanics

The lead-acid battery uses the constant current constant voltage (CCCV) charge method. In this method, a regulated current raises the terminal voltage until the upper voltage limit is reached. The correct setting of the charge voltage limit is critical for lead-acid batteries, as it is a compromise between fully charging the battery to get maximum capacity and avoiding sulfation on the negative plate, as well as not over-saturating the battery, which can cause grid corrosion on the positive plate, gassing, and water-loss.

The charge temperature coefficient of a lead-acid cell is –3mV/°C. Establishing 25°C (77°F) as the midpoint, the charge voltage should be reduced by 3mV per cell for every degree above 25°C and increased by 3mV per cell for every degree below 25°C. If this is not possible, it is better to choose a lower voltage for safety reasons.

Measuring Battery Charger Efficiency

The Energy Conservation Program: Test Procedure for Battery Chargers from the U.S. Department of Energy (DOE) applies to battery chargers that operate at either direct current (“DC”) or alternating current (“AC”) and provides a test procedure for measuring the efficiency of battery chargers. The test procedure measures the input power and output power of the charger and calculates the efficiency as the ratio of the output power to the input power.

Monitoring Charge State

Battery chargers generally use current, which eventually drops to near zero, as their monitored parameter to determine state of charge. More specifically, the change of the charging rate is monitored (the derivative (dx/dt) of current) and when it ‘flattens’ to a specific value, the battery has reached a certain charge state.

DIY Battery Charger Voltage Mechanics

When it comes to DIY battery charger voltage mechanics, it’s important to follow the manufacturer’s instructions for setting the charging voltage and current limit. Observe the battery temperature, voltage, and current during charge, and disconnect the charge once the battery is fully charged and the current has dropped to 3% of the rated Ah. If float charge is needed for operational readiness, lower the charge voltage to about 2.25V/cell.

When building a battery cart that automatically tells you which battery has the most charge, measure the voltage of the battery while charging, which will give you a rough estimation of the state of charge. However, voltage is not an indication of charge status on batteries, and a nearly dead battery with no load will measure 12 volts. Internal resistance is a good indication of battery charge.

Conclusion

The voltage of a battery charger is a critical factor in determining the charging characteristics and efficiency of the charger. Understanding the technical specifications and mechanics of battery charger voltage is essential for ensuring proper battery charging and maintaining the health and longevity of rechargeable batteries. This comprehensive guide has provided a detailed overview of the key concepts and best practices in battery charger voltage mechanics, equipping you with the knowledge to tackle your DIY projects and maintain your battery-powered devices effectively.

References

  1. Energy Conservation Program: Test Procedure for Battery Chargers, U.S. Department of Energy, 2022-08-24, https://energy.gov/sites/default/files/2022-08/battery-chargers-tp-fr2.pdf
  2. Charging Lead Acid, Battery University, https://batteryuniversity.com/article/bu-403-charging-lead-acid
  3. Measuring Voltage of a Charging Battery?, Chief Delphi, 2011-12-04, https://www.chiefdelphi.com/t/measuring-voltage-of-a-charging-battery/115312