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Battery Knowledge Base -- Classification, Capacity And Discharging

Classification of batteries

Batteries are usually divided into two broad classes:

Primary batteries irreversibly transform chemical energy to electrical energy. Once the initial supply of reactants is exhausted, energy cannot be readily restored to the battery by electrical means.

Secondary batteries can have the chemical reactions reversed by supplying electrical energy to the cell, restoring their original composition.

Historically, some types of primary batteries used, for example, for telegraph circuits, were restored to operation by replacing the components of the battery consumed by the chemical reaction. Secondary batteries are not indefinitely rechargeable due to dissipation of the active materials, loss of electrolyte, and internal corrosion.

Battery capacity and discharging

The more electrolyte and electrode material in the cell, the greater the capacity of the cell. Thus a tiny cell has much less capacity than a much larger cell, even if both rely on the same chemical reactions (e.g. alkaline cells), which produce the same terminal voltage.

Because of the chemical reactions within the cells, the capacity of a battery depends on the discharge conditions such as the magnitude of the current, the duration of the current, the allowable terminal voltage of the battery, temperature, and other factors.

The available capacity of a battery depends upon the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected. Therefore, a battery rated at 100 A·h will deliver 5 A over a 20 hour period, but if it is instead discharged at 50 A, it will run out of charge before the theoretically expected 2 hours. For this reason, a battery capacity rating is always related to an expected discharge duration, such as 15 minutes, 8 hours, 20 hours or others.

The relationship between current, discharge time, and capacity for a lead acid battery is expressed by Peukert's law. The efficiency of a battery is different at different discharge rates. When discharging at low rate, the battery's energy is delivered more efficiently than at higher discharge rates.

Battery manufacturers use a standard method to rate their batteries. The battery is discharged at a constant rate of current over a fixed period of time, such as 10 hours or 20 hours, down to a predetermined terminal voltage per cell. So a 100 ampere-hour battery is rated to provide 5 A for 20 hours at room temperature.

In general, the higher the ampere-hour rating, the longer the battery will last for a certain load. Installing batteries with different A·h ratings will not affect the operation of a device rated for a specific voltage.