Invented by the French physician Gaston Planté in 1859, lead acid batteries were the first rechargeable batteries for commercial use. In spite of the fact that it is one of the oldest types of batteries, lead acid batteries continue to be in wide use today, for various reasons. Lead acid batteries are very dependable and much cheaper with respect to the cost-per-watt. Very few types of batteries can deliver bulk power as cheaply as lead acid batteries, and this makes the battery cost-effective for auto-mobiles, uninterrupted power supplies (UPS), golf cars, and forklifts.
The first sealed, or maintenance-free, lead acid emerge in the mid-1970s. The engineers argued that the term “sealed lead acid” is a misnomer because no lead acid battery can be totally sealed. This is true and battery designers added a valve to control venting of gases during stressful charge and rapid discharge. Rather than submerging the plates in a liquid, the electrolyte is impregnated into a moistened separator, a design that resembles nickel- and lithium-bases system. This enables to operate the battery in any physical orientation without leakage. Driven by these advantages, several types of sealed lead acid have emerged and the most common aregel, also known as valve-regulated lead acid (VRLA), and absorbent glass mat (AGM).
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  constant-current charge,  topping charge and  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. The diagram below illustrates these three stages.
The battery is fully charged when the current drops to a pre-determined level or levels out in stage 2. The float voltage must be reduced at full charge.
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.
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. Not all chargers have the float charge feature. If your charger stays on topping charge and does not drop below 2.30V/cell, remove the charge after 48 hours of charge. 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).