Genset Starting Education Module #6

Understanding Battery Charging Fundamentals

Battery charging is essential to maintaining the reliability and longevity of your energy storage systems. Batteries, like reservoirs, can be discharged, refilled, and kept full to offset natural leakage. This analogy helps explain fundamental charging concepts, including what is float charging and boost charging, and how they align with battery characteristics like self-discharge and internal resistance.

Charging Modes Analogy

Imagine a water tower representing a battery's storage capacity:

• The force of gravity equates to the battery's electrical potential (volts).
• A leaky valve symbolizes the natural self-discharge of a battery.
• Water flowing into the tower mirrors the charging current entering a battery.

Float Charging of Batteries: This mode offsets the natural self-discharge of batteries by applying a low, steady voltage to keep them at full capacity, much like a trickle of water compensating for the leak. Learn more about how float current monitoring systems can optimize this process.

Boost Charging of Batteries: Boost mode delivers a higher voltage for a limited time, accelerating charge restoration after discharge. This is like filling a water tower quickly after significant use.

How Charging Works

Battery chargers operate by providing voltage higher than the battery’s current voltage, allowing the battery to accept a controlled flow of current.¹

• A larger voltage difference allows for more current flow, accelerating the charge—ideal for boost charging.
• A smaller difference slows the current flow, perfect for precision float charging.

As depicted in the illustration below, the storage reservoir represents the battery, and the water main simulates the charger. This visual analogy highlights how charging currents fill the battery (reservoir) to maintain its capacity while compensating for leakage and ensuring reliable performance. For additional guidance, explore answers to common battery charging questions.

Illustration: Battery & water tower analogy; Storage reservoir = battery Water main = charger

How Batteries and Chargers Work Together: The Basics of Float and Boost Charging

Relationship of battery self-discharge to float voltage and current.

Battery charging relies on the interaction between a battery's voltage and the voltage supplied by the charger. Understanding this relationship is critical for maintaining battery health and efficiency, especially in systems that require both float charging and boost charging modes.

A battery charger does not force current into a battery. Instead, the battery accepts current when the charger’s voltage is higher than the battery’s current voltage. The greater the voltage difference, the more current the battery can accept—up to the charger’s current limit. This concept ensures controlled and efficient battery charging that avoids damage or overcharging.

Key points to consider:

• Float Charging of Batteries: In float mode, the charger provides just enough voltage to compensate for self-discharge, keeping the battery at full capacity without overcharging.²
  
  In AGM/VRLA batteries, this process involves the oxygen recombination reaction, where hydrogen and oxygen gases are combined back into water inside each battery cell under slight pressure. This mechanism ensures efficient maintenance of battery electrolyte levels without requiring external intervention.³
  
  
• Boost Charging of Batteries: When a battery is deeply discharged, boost mode delivers a higher voltage for a limited time to restore capacity more quickly.

These modes ensure the battery remains in optimal condition, whether it’s operating in float mode or undergoing recovery with boost charging.

By understanding the relationship between charging voltage and current flow, operators can ensure safe and efficient use of battery float charge systems for critical applications. Let me know if further refinements are needed!

Accelerating Recharge: The Role of Boost Charging in Battery Systems

“Boost charging” shortens recharge time by enabling the charger to spend more time delivering its maximum current

Recharge time can be significantly reduced by employing boost charging—a method that temporarily increases the charger’s voltage above the normal float level during recharge. This approach allows the battery to accept the charger’s maximum current for a longer period, compensating for the impact of internal resistance.

Key Concepts:

• Battery Internal Resistance: All batteries have inherent internal resistance, which restricts the efficiency of charging. As charge current increases, more energy is consumed by this resistance rather than being stored in the battery.
• Boost Voltage Advantage: Temporarily applying a higher voltage (boost mode) counteracts losses caused by internal resistance, maximizing the effective charging current and shortening recharge time.
• Importance of Transition to Float Mode: Once the battery reaches full charge, the charger must reduce its voltage back to float levels to prevent overcharging and potential battery damage.

This balance between boost charging and float charging ensures batteries are charged efficiently without compromising their longevity or performance.

Alternatives for controlling when the charger operates in boost or float mode

The charging performance gain enabled by boost charging is accompanied by risk of overcharging.  Several alternatives are available to control the charger’s operating mode so that the charger reverts to float mode once the battery is charged. The strengths and weaknesses of these are compared below in the table below. Fully automatic boost control systems are strongly recommended for genset battery charging. 

Table: Comparison of boost mode control methods

Summary of Key Points

• The charger does not force current into the battery; rather the battery accepts current from the charger when the charger’s voltage is higher than battery voltage.
  
  
• Float charging counters battery self-discharge. Float is the voltage at which a fully charged battery is maintained at a state of high charge. During float charging, current into the battery very slightly exceeds the battery’s self-discharge rate.
  
  
• “Boost” charge is an elevated voltage mode that shortens recharge time by enabling the charger to spend more time delivering its maximum current.
  
  
• The most effective way to reduce charging time is to temporarily increase charging voltage above the normal float setting during battery recharge.
  
  
• Fully automatic boost charging mode control systems are strongly recommended for genset battery charging. 

References

1. Battery chargers are current-limited to prevent excess current flow from damaging the charger or operating its protective fuses or circuit breakers.
2. Float current into the battery minus self-discharge current. 
3. Also known as “recombinant lead-acid”.
4. “Equalize” charging is the application of the boost charge voltage to an already charged battery. This deliberately overcharges the battery for the purpose of increasing capacity of the weakest cells in the battery string.
5. “Equalizing" is an overcharge performed on flooded lead acid batteries after they have been fully charged. It reverses the buildup of negative chemical effects like stratification, a condition where acid concentration is greater at the bottom of the battery than at the top. Equalizing also helps to remove sulfate crystals that might have built up on the plates. If left unchecked, this condition, called sulfation, will reduce the overall capacity of the battery. Many experts recommend that batteries be equalized periodically, ranging anywhere from once a month to once or twice per year. However, Trojan only recommends equalizing when low or wide-ranging specific gravity (+/- 0.015) are detected after fully charging a battery.” Source Trojan Battery Company.