The lead-acid battery is a rechargeable battery first invented in 1859 by French physicist Gaston Plante. It was the first ever rechargeable battery. Compared to modern rechargeable batteries, lead-acid batteries have a relatively low energy density. Nevertheless, their ability to deliver high inrush currents means that the batteries have a relatively large power-to-weight ratio. These characteristics, along with their low cost, make them attractive in motor vehicles, providing the high currents required to start motors.

Because they are inexpensive compared to newer technologies, lead-acid batteries are widely used even when inrush current is not important and other designs can provide higher energy densities. 1999 sales of lead-acid batteries accounted for 40-50% of global (excluding China and Russia) battery sales, equivalent to a manufacturing market worth about $15 billion. [8] Large-size lead-acid designs are widely used for storing backup power for cell towers, high-availability environments such as hospitals, and stand-alone power systems. For these roles, modified versions of standard cells can be used to shorten storage times and reduce maintenance requirements. Gel cell and absorbent glass mat cells are common in these roles and are collectively referred to as VRLA (Valve Regulated Lead Acid) batteries.

In the charged state, the chemical energy of the cell is stored in the potential difference between pure lead at the negative terminal and PbO 2 at the positive terminal, plus an aqueous solution of sulfuric acid. The electrical energy generated by the discharged lead-acid battery can be attributed to the energy released when the strong chemical bond of the water (H 2 O) molecule is formed by the H + ions of the acid and the O 2- ions of PbO 2 [9] In contrast, during charging, the battery acts as a water decomposer.

Applications

Most of the world's lead-acid batteries are automotive starting, lighting, and ignition (SLI) batteries, with an estimated 320 million shipped in 1999. [8] In 1992, about 3 million tons of lead were used to make batteries.

Wet-cell backup (stationary) batteries designed for deep discharge are commonly used for large backup power supplies in telephone and computer centers, grid storage, and off-grid home power systems. [23] Lead-acid batteries are used for emergency lighting and to power sump pumps in the event of a power outage.

Traction (propulsion) batteries are used in golf carts and other battery electric vehicles. Large lead-acid batteries are also used to power motors in diesel-electric (conventional) submarines underwater and as an emergency power source for nuclear submarines. Valve-regulated lead-acid batteries do not spill their electrolyte. They are used for backup power for alarms and small computer systems (especially uninterruptible power supplies; UPS) as well as electric scooters, electric wheelchairs, electric bicycles, marine applications, battery electric or mini-hybrid cars and motorcycles. Many electric forklifts use lead-acid batteries, where the weight is used as part of the counterweight. Lead-acid batteries are used to provide filament (heater) voltage, commonly 2 V in early vacuum tube (valve) radio receivers.

Portable batteries for miners' cap lamps headlamps usually have two or three cells.

Cycle

Starting batteries

Lead-acid batteries designed to start automobile engines are not designed for deep discharge. They have a large number of thin plates designed for maximum surface area and therefore can provide maximum current output and are easily damaged by deep discharge. Repeated deep discharges will result in capacity loss and eventually premature failure as the electrodes break down due to the mechanical stresses generated by cycling. Starter batteries that are kept in a continuous float state will suffer from electrode corrosion, which will also lead to premature failure. Therefore, starter batteries should be kept open-circuit, but should be charged periodically (at least every two weeks) to prevent sulfation.

Starter batteries are lighter in weight than deep cycle batteries of the same size because the thinner and lighter plates do not extend all the way to the bottom of the battery case. This allows loose decomposition material to fall off the plates and collect at the bottom of the cell, thus extending the life of the cell. If enough of this loose debris rises, it can contact the bottom of the plates and cause the cell to fail, resulting in a loss of cell voltage and capacity.

Deep Cycle Batteries

Specially designed deep cycle batteries are more resistant to degradation from cycling and are suitable for applications where the batteries are discharged periodically, such as photovoltaic systems, electric vehicles (forklifts, golf carts, electric cars, etc.) and uninterruptible power supplies. These batteries have thicker plates that provide less peak current but can withstand frequent discharges.

Some batteries are designed as a compromise between a starter (high current) and a deep cycle. They have a higher degree of discharge compared to automotive batteries, but not as much as deep cycle batteries. They may be called "marine/caravan" batteries or "leisure batteries".

Fast and Slow Charging and Discharging

The capacity of a lead-acid battery is not a fixed value, but varies according to its discharge rate. The empirical relationship between discharge rate and capacity is called Peukert's Law.

When a battery is charged or discharged, only the reactive chemicals located at the interface between the electrodes and the electrolyte are initially affected. Over time, the charge in the chemicals stored at the interface, often referred to as "interfacial charge" or "surface charge," spreads through the entire volume of active material by diffusion of these chemicals.

Consider a fully discharged battery (e.g., what happens when the car lights are turned on overnight, with a current draw of about 6 amps). If it is subsequently charged quickly for only a few minutes, the cell plates are charged only near the interface between the plates and the electrolyte. In this case, the cell voltage may rise to a value close to the charger voltage; this results in a significant reduction in charging current. After a few hours, this interfacial charge will spread to the volume of the electrodes and electrolyte; this will cause the interface charge to be so low that it may not be sufficient to start the car. As long as the charge voltage remains below the bleed voltage (about 14.4 volts in a normal lead-acid battery), the battery is unlikely to be damaged and the battery should return to its nominal charge state in time.

Explosion Hazard

Overcharging can lead to electrolysis, which releases hydrogen and oxygen. This process is known as "outgassing". Wet cells have open vents to release any gas produced, and VRLA cells rely on valves installed on each cell. Catalytic caps can be used in liquid-rich cells to recombine hydrogen and oxygen, and VRLA cells will normally recombine any hydrogen and oxygen produced inside the cell, but failure or overheating may cause a build-up of gas. If this occurs (e.g., during overcharging), the valve will vent the gas and normalize the pressure, resulting in the characteristic sour taste. However, valves can fail, such as fouling and debris buildup, which can lead to pressure buildup.

The accumulated hydrogen and oxygen can sometimes ignite in an internal explosion. The force of the explosion can cause the battery casing to burst or cause its top to fly off, ejecting acid and casing debris. An explosion in one cell may ignite any combustible gas mixture in the remaining cells. Similarly, connecting or disconnecting a closed circuit (such as a load or charger) from the battery terminals in a poorly ventilated area may also result in a spark and explosion if any gas is vented from the battery.

Individual cells in a battery can also short-circuit, leading to an explosion.

The cells in VRLA batteries typically expand when the internal pressure rises, thus warning the user and mechanic. Deformation varies from cell to cell and is greatest at the ends where the walls are not supported by other cells. Such over-pressurized cells should be carefully isolated and discarded. Personnel working near cells that pose a risk of explosion should wear face shields, coveralls, and gloves to protect their eyes and exposed skin from burns caused by acid spray and fire. Using goggles instead of a face shield can sacrifice safety by exposing the face to possible flying acid, casing or battery debris, and heat from a potential explosion.