Lead acid charging uses a voltage-based algorithm that is similar to lithium-ion. The charge time of a sealed lead acid battery is 12–16 hours, up to 36–48 hours for large stationary batteries. With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less; however, the topping charge may not be complete. Lead acid is sluggish and cannot be charged as quickly as other battery 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 chargeapplies 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. Figure 4-4 illustrates these three stages.
During the constant-current charge, the battery charges to 70 percent in 5–8 hours; the remaining 30 percent is filled with the slower topping charge that lasts another 7–10 hours. 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 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.
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, and full charge is reached when the current decreases to the three percent level of the rated current. A battery with high leakage may never attain this low saturation current, and a plateau timer takes over to initialize the charge termination.
The correct setting of the charge voltage is critical and ranges from 2.30 to 2.45V 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, an over-saturated condition causes grid corrosion on the positive plate and induces gassing.
To make “dancing on the head of a needle” more difficult, the battery voltage shifts with temperature. Warmer surroundings require slightly lower voltage thresholds and a cold ambient prefers a higher level. Chargers exposed to temperature fluctuations should include temperature sensors to adjust the charge voltage for optimum charge efficiency. If this is not possible, it is better to choose a lower voltage for safety reasons. Table 4-5 compares the advantages and limitations of various peak voltage settings.
Table 4-5: 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 48 hours and must be reduced to the float voltage level. This is especially critical for sealed systems because these systems are less able to tolerate overcharge than the flooded type. Charging beyond what the battery can take turns the redundant energy into heat and the battery begins to gas. The recommended float voltage of most low-pressure lead acid batteries is 2.25 to 2.27V/cell. (Large stationary batteries float at 2.25V at 25°C (77°F.) Manufacturers recommend lowering the float charge at ambient temperatures above 29°C (85°F).
Not all chargers feature float charge. If your charger stays on topping charge and does not drop below 2.30V/cell, remove the charge after 48 hours of charge.
Aging batteries pose a challenge when setting the optimal float charge voltage because each cell has its own age-related condition. Weak cells may go into hydrogen evolution as part of overcharge early on, while the stronger ones undergo oxygen recombination in an almost starved state. Connected in a string, all cells receive the same charge current and controlling individual cell voltages is almost impossible. A float current that is too high for the faded cell might starve the strong neighbor and cause sulfation due to undercharge. Companies have developed cell-balancing devices, which are placed on the battery and compensate the differences in cell voltages that occur as a result of cell
Much has been said about pulse charging of lead acid batteries. There are apparent advantages in reducing sulfation; however, manufacturers and service technicians are divided on the benefits, and the results are inconclusive. If sulfation could be measured with accuracy and the pulses applied as a corrective service, then the remedy could be beneficial. Assumptions without knowing the underlying results can be harmful.
Lead acid batteries must always be stored in a charged state. A topping charge should be applied every six months to prevent the voltage from dropping below 2.10V/cell. With AGM, these requirements can be somewhat relaxed.
Measuring the open circuit voltage (OCV) while in storage provides a reliable indication as to the state-of-charge of the battery. A voltage of 2.10V at room temperature reveals a charge of about 90 percent. Such a battery is in good condition and needs only a brief full charge prior to use. If the voltage drops below 2.10V, the battery must be charged to prevent sulfation. 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 2.10V per cell. A low voltage suggests partial charge due to long storage or a high self-discharge induced by a possible micro-short. Battery users have indeed found that a pack arriving at a lower than specified voltage has a higher failure rate than the others. Although in-house service can often bring such batteries to full performance, the time and equipment required adds to operational costs. (Please note that the 2.10V/cell acceptance threshold does not apply to all lead acid types.)
Simple Guidelines for Charging Lead Acid Batteries
• Charge in a well-ventilated area. Hydrogen gas generated during charging is explosive.
• Choose the appropriate charge program for flooded, gel and AGM batteries. Check manufacturer’s specifications on recommended voltage thresholds.
• Charge lead acid batteries after each use to prevent sulfation. Do not store on low charge.
• The plates of flooded batteries must always be fully submerged in electrolyte. Fill battery with distilled or de-ionized water to cover the plates if low. Tap water may be acceptable in some regions. Never add electrolyte.
• Fill water level to designated level after charging. Overfilling when the battery is empty can cause acid spillage.
• Formation of gas bubbles in a flooded lead acid indicates that the battery is reaching full state-of-charge (hydrogen on negative plate and oxygen on positive plate).
• Reduce float charge if the ambient temperature is higher than 29°C (85°F).
• Do not allow a lead acid to freeze. An empty battery freezes sooner than one that is fully charged. Never charge a frozen battery.
• Do not charge at temperatures above 49°C (120°F).
