Charging the lead-acid battery
The charge algorithm for lead-acid batteries is similar to lithium-ion but differs from nickel-based chemistries in that voltage rather than current limiting is used. The charge time of a sealed lead-acid battery is 12-16 hours (up to 36 hours for larger capacity batteries). With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less. Lead-acid cannot be fully charged as quickly as nickel or lithium-based systems.
It takes about 5 times as long to recharge a lead-acid battery to the same level as it does to discharge. On nickel-based batteries, this ratio is 1:1, and roughly 1:2 on lithium-ion.
A multi-stage charger first applies a constant current charge, raising the cell voltage to a preset voltage (Stage 1 in Figure 1). Stage 1 takes about 5 hours and the battery is charged to 70%. During the topping charge in Stage 2 that follows, the charge current is gradually reduced as the cell is being saturated. The topping charge takes another 5 hours and is essential for the well being of the battery. If omitted, the battery would eventually lose the ability to accept a full charge. Full charge is attained after the voltage has reached the threshold and the current has dropped to 3% of the rated current or has leveled off. The final Stage 3 is the float charge, which compensates for the self-discharge.
Figure 1: Charge stages of a lead-acid battery. The battery charges at a constant current to a set voltage threshold (Stage 1). As the battery saturates, the current drops (Stage 2). The float charge compensates for the self-discharge (Stage 3). 
Correct settings of the voltage limits are critical and range from 2.30V to 2.45V. Setting the voltage limit is a compromise. On one end, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate. A continually over-saturated condition at the other end, however, would cause grid corrosion on the positive plate. It also promotes gassing, which results in venting and loss of electrolyte.
The voltage limit shifts with temperature. A higher temperature requires slightly lower voltages and vice versa. Chargers that are exposed to large temperature fluctuations should be equipped with temperature sensors to to adjust the charge voltage for optimum charge. Figure 2 compares the advantages and limitations of various peak voltage settings.
Figure 2: Effects of charge voltage on a small lead-acid battery (SLA).
Cylindrical lead-acid cells have higher voltage settings but are lower for VRLA and car batteries. 
The battery cannot remain at the peak voltage for too long; the maximum allowable time is 48 hours. When reaching full charge, the voltage must be lowered to maintain the battery at between 2.25 and 2.27V/cell. Manufacturers of large lead-acid batteries recommend a float charge of 2.25V at 25°C.
Car batteries and valve-regulated-lead-acid batteries (VRLA) are typically charged to between 2.26 and 2.36V/cell. At 2.37V, most lead-acid batteries start to gas, causing loss of electrolyte and possible temperature increases. The exceptions are small sealed lead acid batteries (SLA), which can be charged to 2.50V/cell without adverse side effect.
The cylindrical Cyclone by Hawker requires a very high peak voltage of 2.60V/cell. Failing to apply the recommended voltage threshold causes a gradual decrease in capacity due to sulfation. Follow manufacturer's recommended settings on these lead-acid variations.
Large VRLA batteries are often charged with a float-charge current to 2.25V/cell. A full charge may take several days. It is interesting to observe that the current in float charge mode gradually increases as the battery ages in standby mode. The reasons may be electrical cell leakages and a reduction in chemical efficiency.
Aging affects each cell differently. Since the cells are connected in series, controlling the individual cell voltages during charge is virtually impossible. Even if the correct overall voltage is applied, a weak cell will generate its own voltage level and intensify the condition further.
Much has been said about pulse charging lead-acid batteries. Some experts believe there is a benefit in reduced cell corrosion but manufacturers and service technicians are not in full agreement on the effectiveness. There are also disagreements on the 'equalizing charge'. An equalizing charge raises the battery voltage for several hours above that specified by the manufacturer. Although beneficial in reversing sulfation, the side effects are elevated temperature, gassing and loss of electrolyte if the service is not administered correctly. A periodic discharge of about 10% is said to benefit the battery but little conclusive evidence is available.
Lead-acid batteries must always be stored in a charged state. A topping charge should be applied every six months to avoid the voltage from dropping below 2.10V/cell on an SLA. Prolonged storage below the critical voltage causes sulfation, a condition that is difficult to reverse. (See also: "How to restore and prolong lead-acid batteries")
Charging lead-acid batteries with a power supply
Lead-acid batteries can be charged manually with a commercial power supply featuring voltage regulation and current limiting. Calculate the charge voltage according to the number of cells and desired voltage limit. Charging a 12-volt battery (6 cells) at a cell voltage limit of 2.40V, for example, would require a voltage setting of 14.40V.
The charge current for small lead-acid batteries should be set between 10% and 30% of the rated capacity (30% of a 2Ah battery would be 600mA). Larger batteries, such as those used in the automotive industry, are generally charged at lower current ratings. Cells constructed of a non-antimonial lead grid material allow higher charge currents but have a lower capacity. The cylindrical Cyclone is sealed and can sustain a pressure of up to 3.5 Bar (50 psi). A pressurized cell assists in the recombination of gases.
Observe the battery temperature, voltage and current during charge. Charge only at ambient temperatures and in a ventilated room. Once the battery is fully charged and the current has dropped to 3% of the rated current, the charge is completed. A good car battery will drop to about 40mA when fully charged; a bad battery may not fall below 100mA.
After full charge, remove the battery from the charger. If float charge is needed for operational readiness, lower the charge voltage to about 13.50V (2.25V/cell). Most chargers perform this function automatically. The float charge can be applied for an unlimited time.
State-of-charge reading based on terminal voltage
The state-of-charge of a lead-acid battery can, to a certain extent, be estimated by measuring the open terminal voltage. Prior to measuring, the battery must have rested for 4-8 hours after charge or discharge and resided at a steady room temperature. A cold battery would show slightly higher voltages and a hot battery would be lower. Plate additions of calcium and antimony will also vary the open terminal voltage with calcium being a little higher than antimony. Furthermore, AGM has a higher voltage plateau than the flooded lead acid and the readings on Figure 3 may not apply for AGM systems. Due to surface charge, a brief charge will raise the terminal voltage and provide inflated state-of-charge reading. For example, a 30 minute charge could wrongly indicate 100% SoC if no rest is applied.
With sufficient rest and stable temperature, voltage measurements provide an amazingly accurate SoC estimation for lead acid batteries. It is important that the battery is free of polarization. If connected in a system, such as in a car, there are steady auxiliary loads, not to mention frequent starting and driving.
Figure 3: BCI standard for SoC estimation of a 12V flooded lead acid car battery. Test the battery at room temperature. Allow 4-8 hour of rest after charge or discharge.
Courtesy of BCI
open circuit voltage state of charge in 96
12.65V 100%
12.45V 75%
12.24V 50%
12.06V 25%
11.89V or less Discharged
Note: The BCI readings apply to flooded batteries with antimony doping. Calcium will raise the voltage by 5 - 8%. Calcium is commonly used for maintenance-free lead acid batteries.
After charge or discharge, allow the battery to rest for a minimum of eight hours before assessing the state-of-charge by measuring the terminal voltage.
Battery as a buffer
While dwelling on float-charge, an external load can be connected to a lead-acid battery. In such a case, the battery acts as a buffer. Micro-towers on cell sites work this way. During off-peak periods, the batteries get fully charged. On peak traffic times, the load exceeds the net supply provided by the rectifier (charger) and the battery supplies the extra energy. A car battery works in a similar way.
When configuring a battery as a buffer, make certain that the battery has the opportunity to fully charge between loads. The net charge must be greater than what is drawn from the battery. Some chargers switch to fast charge after a deep discharge, others simply use the float charge to recharge. Allow up to 48 hours to fully recharge on float charge. Deep discharges should be avoided if possible. Assure that the float charge voltage is set correctly.
