Charging Information For Lead Acid Batteries Battery University. Learn how to optimize charging conditions to extend service life. The lead acid battery uses the constant current constant voltage CCCV charge method. A regulated current raises the terminal voltage until the upper charge voltage limit is reached, at which point the current drops due to saturation. The charge time is 1. With higher charge currents and multi stage charge methods, the charge time can be reduced to 81. Lead acid is sluggish and cannot be charged as quickly as other battery systems. See BU 2. 02 New Lead Acid Systems. Lead acid batteries should be charged in three stages, which are 1 constant current charge, 2 topping charge and 3 float charge. The constant current charge applies the bulk of the charge and takes up roughly half of the required charge time the topping charge continues at a lower charge current and provides saturation, and the float charge compensates for the loss caused by self discharge. During the constant current charge, the battery charges to about 7. The topping charge is essential for the well being of the battery and can be compared to a little rest after a good meal. If continually deprived, the battery will eventually lose the ability to accept a full charge and the performance will decrease due to sulfation. The float charge in the third stage maintains the battery at full charge. Figure 1 illustrates these three stages. The switch from Stage 1 to 2 occurs seamlessly and happens when the battery reaches the set voltage limit. The current begins to drop as the battery starts to saturate full charge is reached when the current decreases to 35 percent of the Ah rating. A battery with high leakage may never attain this low saturation current, and a plateau timer takes over to end the charge. The correct setting of the charge voltage limit is critical and ranges from 2. V to 2. 4. 5V per cell. Setting the voltage threshold is a compromise and battery experts refer to this as dancing on the head of a needle. On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate on the other hand, over saturation by not switching to float charge causes grid corrosion on the positive plate. This also leads to gassing and water loss. Energy storage battery solutions for solar, utility, commercial and much more. Experts for over 20 years. Authorized wholesale supplier. Temperature changes the voltage and this makes dancing on the head of a needle more difficult. A warmer ambient requires a slightly lower voltage threshold and a colder temperature prefers a higher setting. Chargers exposed to temperature fluctuations include temperature sensors to adjust the charge voltage for optimum charge efficiency. The charge temperature coefficient of a lead acid cell is 3m. VC. Establishing 2. C 7. 7F as the midpoint, the charge voltage should be reduced by 3m. V per cell for every degree above 2. C and increased by 3m. V per cell for every degree below 2. C. If this is not possible, it is better to choose a lower voltage for safety reasons. Table 2 compares the advantages and limitations of various peak voltage settings. V to 2. 3. 5Vcell. V to 2. 4. 5Vcell. Advantages. Maximum service life battery stays cool charge temperature can exceed 3. C 8. 6F. Higher and more consistent capacity readings less sulfation. Limitations. Slow charge time capacity readings may be inconsistent and declining with each cycle. Sulfation may occur without equalizing charge. Subject to corrosion and gassing. Needs water refill. Not suitable for charging at high room temperatures, causing severe overcharge. Table 2 Effects of charge voltage on a small lead acid battery. Cylindrical lead acid cells have higher voltage settings than VRLA and starter batteries. Once fully charged through saturation, the battery should not dwell at the topping voltage for more than 4. This is especially critical for sealed systems because they are less tolerant to overcharge than the flooded type. Charging beyond the specified limits turns redundant energy into heat and the battery begins to gas. The recommended float voltage of most flooded lead acid batteries is 2. V to 2. 2. 7Vcell. Large stationary batteries at 2. C 7. 7F typically float at 2. Vcell. Manufacturers recommend lowering the float charge when the ambient temperature rises above 2. C 8. 5F. Figure 3 illustrate the life of a lead acid battery that is kept at a float voltage of 2. V to 2. 3. 0Vcell and at a temperature of 2. C to 2. 5C 6. 0F to 7. F. After 4 years of operation permanent capacity losses become visible, crossing the 8. This loss is larger if the battery requires periodic deep discharges. Elevated heat also reduces battery life. See also BU 8. 06a How Heat and Loading affect Battery Life. Figure 3 Capacity loss on standby. Permanent capacity loss can be minimized with operating at a moderate room temperature and a float voltage of 2. Vcell. Source Power Sonic. Not all chargers feature float charge and very few road vehicles have this provision. If your charger stays on topping charge and does not drop below 2. Vcell, remove the charge after 4. Recharge every 6 months while in storage AGM every 61. These described voltage settings apply to flooded cells and batteries with a pressure relief valve of about 3. Pa 5psi. Cylindrical sealed lead acid, such as the Hawker Cyclon cell, requires higher voltage settings and the limits should be set to manufacturers specifications. Failing to apply the recommended voltage will cause a gradual decrease in capacity due to sulfation. The Hawker Cyclon cell has a pressure relief setting of 3. Pa 5. 0psi. This allows some recombination of the gases generated during charge. Aging batteries pose a challenge when setting the float charge voltage because each cell has its own unique condition. Connected in a string, all cells receive the same charge current and controlling individual cell voltages as each reaches full capacity is almost impossible. Weak cells may go into overcharge while strong cells remain in a starved state. A float current that is too high for the faded cell might sulfate the strong neighbor due to undercharge. Cell balancing devices are available compensate for the differences in voltages caused by cell imbalance. Ripple voltage also causes a problem with large stationary batteries. A voltage peak constitutes an overcharge, causing hydrogen evolution, while the valley induces a brief discharge that creates a starved state resulting in electrolyte depletion. Manufacturers limit the ripple on the charge voltage to 5 percent. Much has been said about pulse charging of lead acid batteries to reduce sulfation. The results are inconclusive and manufacturers as well as service technicians are divided on the benefit. If sulfation could be measured and the right amount of pulsing applied, then the remedy could be beneficial however giving a cure without knowing the underlying side effects can be harmful to the battery. Most stationary batteries are kept on float charge and this works reasonably well. Another method is the hysteresis charge that disconnects the float current when the battery goes to standby mode. The battery is essentially put in storage and is only borrowed from time to time to apply a topping charge to replenish lost energy due to self discharge, or when a load is applied. This mode works well for installations that do not draw a load when on standby. Lead acid batteries must always be stored in a charged state. A topping charge should be applied every 6 months to prevent the voltage from dropping below 2. Vcell and causing the battery to sulfate. With AGM, these requirements can be relaxed. Measuring the open circuit voltage OCV while in storage provides a reliable indication as to the state of charge of the battery. A cell voltage of 2. V at room temperature reveals a charge of about 9. Such a battery is in good condition and needs only a brief full charge prior to use. See also BU 9. 03 How to Measure State of charge. Observe the storage temperature when measuring the open circuit voltage. A cool battery lowers the voltage slightly and a warm one increases it. Using OCV to estimate state of charge works best when the battery has rested for a few hours, because a charge or discharge agitates the battery and distorts the voltage. Some buyers do not accept shipments of new batteries if the OCV at incoming inspection is below 2. V per cell. A low voltage suggests a partial charge due to long storage or a high self discharge caused by a micro short.