Electrochemical cells transform chemical energy into electrical energy. There are two types, primary and secondary.
In a primary cell, this transformation is not reversible, and the cell is discarded at the end of its life. In the secondary cell, the transformation is reversible, and it can be re-charged.
There are two types of secondary cell, wet and dry. In automotive use, the usual main-storage device is the wet cell of a lead acid battery. It has two plates of dissimilar materials immersed in an electrolyte - a solution that conducts electricity by using ions.
The accepted, or nominal, voltage of a cell does not depend on the size of the cell, however, current capacity does. The surface area of the plates in a cell determines its current capacity. In a lead acid battery, the plates are assembled so there is always 1 extra negative plate. The plates are close to each other but do not touch, which would cause a short circuit.
The nominal voltage of a cell is 2 volts. Cells connected in series make a battery, and the number of cells determines its nominal voltage. The cells are sealed from each other and filled with dilute sulphuric acid. The battery case is usually plastic or hard rubber.
One set of plates is connected to the negative side of a DC source, the other to the positive side. Direct current is applied to the plates, changing them chemically, until the battery is ready for service.
Battery capacity
The capacity of a battery to store charge is often expressed in ampere hours (1 A·h = 3600 coulombs). If a battery can provide one ampere (1 A) of current (flow) for one hour, it has a real-world capacity of 1 A·h. If it can provide 1 A for 100 hours, its capacity is 100 A·h. Likewise, 20 A for 2 hours equals 40 A·h capacity. But...
While a battery that can deliver 10 A for 10 hours can be said to have a capacity of 100 A·h, that is not how the rating is determined by the manufacturers. A 100 A·h rated battery most likely will not deliver 10 A for 10 hours. Battery manufacturers use a standard method to determine how to rate their batteries. Their rating is based on tests performed over 20 hours with a discharge rate of 1/20 (5%) of the expected capacity of the battery an hour. So a 100 ampere-hour battery is rated to provide 5 A for 20 hours. The efficiency of a battery is different at different discharge rates. When discharging at 5% an hour, the battery's energy is delivered more efficiently than at higher discharge rates.
To calculate the 5% discharge rate of a battery, take the manufacturer's ampere-hour rating and divide it by 20.
Battery explosion
Under extreme conditions, certain types of batteries can explode violently. A battery explosion is usually caused by the misuse or malfunction of a battery (such as the recharging of a non-rechargeable battery or shorting a car battery).
With car batteries, explosions are most likely to occur when a short circuit generates currents of very high magnitude. A short circuit malfunction in a battery placed in parallel with other batteries ("jumped") can cause its neighbor to discharge its maximum current into the faulty cell, leading to overheating and possible explosion. In addition, car batteries liberate hydrogen when they are overcharged even slightly (because of hydrolysis of the water in the electrolyte). Normally the amount of overcharging is very small and so is the amount of highly explosive gas developed, and the light gas dissipates very quickly. However, when "jumping" a car battery, the high current can cause the rapid release of large volumes of hydrogen, which could be ignited by a spark nearby (for example, when removing the jumper cables).
When a non-rechargeable battery is recharged at a high rate, an explosive gas mixture of hydrogen and oxygen may be produced faster than it can escape from within the walls of the battery, leading to pressure build-up and a possible explosion. In extreme cases, the battery acid may spray violently from the casing of the battery and cause injury.
Additionally, disposing of a battery in fire may cause an explosion as steam builds up within the sealed case of the battery.
Overcharging, which is charging a battery beyond its electrical capacity, can also lead to a battery explosion, leakage, or irreversible damage to the battery. It may also cause damage to the charger or device in which the overcharged battery is later used.
Source: CDX Global & Wikipedia - en.wikipedia.org
