What is lead-acid Battery?

What is lead-acid battery?

Lead-acid battery (VRLA) is a storage battery whose electrodes are mainly made of lead and its oxides, and the electrolyte is sulfuric acid solution. In the discharge state of a lead-acid battery, the main component of the positive electrode is lead dioxide and the main component of the negative electrode is lead; in the charged state, the main component of the positive and negative electrodes are lead sulfate.

The nominal voltage of a single-cell lead-acid battery is 2.0V, which can be discharged to 1.5V and can be charged to 2.4V; in applications, 6 single-cell lead-acid batteries are often connected in series to form a lead-acid battery with a nominal 12V , There are 24V, 36V, 48V and so on.

The AGM is a newer type sealed lead-acid that uses absorbed glass mats between the plates. It is sealed, maintenance-free and the plates are rigidly mounted to withstand extensive shock and vibration. Nearly all AGM batteries are recombinant, meaning they can recombine 99% of the oxygen and hydrogen. There is almost no water is loss.

The charging voltages are the same as for other lead-acid batteries. Even under severe overcharge conditions, hydrogen emission is below the 4% specified for aircraft and enclosed spaces. The low self-discharge of 1-3% per month allows long storage before recharging. The AGM costs twice that of the flooded version of the same capacity. Because of durability, German high performance cars use AGM batteries in favor of the flooded type.

Discharge
During discharge, the lead dioxide (positive plate) and lead (negative plate) react with the electrolyte of sulfuric acid to create lead sulfate, water and energy.

Charge
During charging, the cycle is reversed: the lead sulfate and water are electro-chemically converted to lead, lead oxide and sulfuric acid by an external electrical charging source.

Many new competitive cell chemistries are being developed to meet the requirements of the auto industry for EV and HEV applications.

Even after 150 years since its invention, improvements are still being made to the lead acid battery and despite its shortcomings and the competition from newer cell chemistries the lead acid battery still retains the lion’s share of the high power battery market.

History of Lead-acid Battery

The lead–acid battery was invented in 1859 by French physicist Gaston Planté and is the earliest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, its ability to supply high surge currents means that the cells have a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in motor vehicles to provide the high current required by starter motors.

As they are inexpensive compared to newer technologies, lead–acid batteries are widely used even when surge current is not important and other designs could provide higher energy densities. In 1999 lead–acid battery sales accounted for 40–45% of the value from batteries sold worldwide (excluding China and Russia), equivalent to a manufacturing market value of about $15 billion. Large-format lead–acid designs are widely used for storage in backup power supplies in cell phone towers, high-availability settings like hospitals, and stand-alone power systems. For these roles, modified versions of the standard cell may be used to improve storage times and reduce maintenance requirements. Gel-cells and absorbed glass-mat batteries are common in these roles, collectively known as VRLA (valve-regulated lead–acid) batteries.

In the charged state, the chemical energy of the battery is stored in the potential difference between the pure lead at the negative side and the PbO2 on the positive side, plus the aqueous sulfuric acid. The electrical energy produced by a discharging lead–acid battery can be attributed to the energy released when the strong chemical bonds of water (H2O) molecules are formed from H+ ions of the acid and O2− ions of PbO2. Conversely, during charging, the battery acts as a water-splitting device.

Lead-acid batteries are composed of a Lead-dioxide cathode, a sponge metallic Lead anode and a Sulphuric acid solution electrolyte. This heavy metal element makes them toxic and improper disposal can be hazardous to the environment.

Advantages of Lead Acid Battery

1. Mature technology
2. Relatively cheap to manufacture and buy (they provide the lowest cost per unit capacity for rechargeable cells)
3. Large current capability
4. Can be made for a variety of applications
5. Tolerant to abuse
6. Tolerant of overcharging
7. Wide range of sizes and specifications available
8. Many producers worldwide

Disadvantages of Lead Acid Battery

1. Fails after a few years use lifespan typically 300 – 500 cycles

2. Cannot always be used in a variety of orientations

3. Corrosive electrolyte (can cause burns to people and corrosion on metalwork)

4. Lead content and electrolyte make the battery environmentally unfriendly

5. Acid needs disposing of with care

6. Not suitable for fast charging

7. Must be stored in charged state once electrolyte introduced

Causes of Lead-acid Battery Failure

•Elevated Temperatures: Anticipated battery life is specified by the manufacturer for batteries installed in an environment at or near the reference temperature of 25°C (77°F). Above this temperature, battery life is reduced. The chief aging mechanism is accelerated corrosion of the positive plates, grid structure, and strap, which increases exponentially as a function of temperature. Elevated temperatures reduce battery life. An increase of 8.3°C (15°F) can reduce lead-acid battery life by 50% or more.

• Repeated Cycling: Repeated cycling from fully charge to fully discharge and back may cause loss of active materials from the positive plates. This reduces battery capacity and its useful life.

• Overcharging: Overcharging by the battery charging system causes excessive gassing and high internal heat. Too much gassing can lead to the removal of active material from the plates. Too much heat can also oxidize the positive plate material and warp the plates.

• Undercharging: A faulty charging system will not maintain the battery at full charge. Severe undercharging allows sulfate on the plates to become hard and impossible to remove by normal charging. The undercharged battery may fail to deliver the required power needed for its application.

• Over discharge: Over discharge leads to hydration. Hydration occurs in a lead-acid battery that is over discharged and not promptly recharged. Hydration results when the lead and lead compounds of the plates dissolve in the water of a discharged cell and form lead hydrate, which is deposited on the separators. When the cell is recharged, multiple internal short circuits occur between the positive and negative plates. Once hydration is evident, the cell is permanently damaged. Hydration is not visible in VRLA cells because the containers are opaque

• Vibration: A battery must be mounted securely. Vibrations can loosen connection, crack the case and damage internal components.

• DC Ripple Current: Excessive DC ripple current might contribute to battery aging. VRLA batteries are extremely susceptible to ripple current since it can lead to cell heating and will accelerate the degradation of cells which are at risk from thermal runaway.

• Improper Storage: Storing wet cells beyond the manufacturer’s recommended duration promotes sulfation, and decreases cell capacity and life.

• Misapplications: Batteries are commonly designed for a specific use. If the battery is not designed for a given application, it might not meet its life or performance expectations.