3 Important Characteristics of a Storage Battery | Electricity

There are three important characteristics of an accumulator (or storage battery) are: 1. Voltage 2. Capacity 3. Efficiency.

Characteristic # 1. Voltage:

Average emf of cell is approximately 2.0 volts. The value of emf of a cell does not remain constant but varies with the change in specific gravity of electrolyte, temperature and the length of time since it was last charged.

The emf of the cell increases with the increase in specific gravity of the electrolyte and vice versa but increase in specific gravity of the electrolyte also causes increase in internal resistance of the cell; therefore, its value should not go beyond 1.22. Best results are obtained with the electrolyte of specific gravity 1.21.

The emf of the cell, though not much, slightly increases with the increase in temperature.

The terminal voltage of a battery is higher during charge than that during discharge due to the following reasons.

The internal voltage developed by chemical action, depends on the strength of electrolyte and increases slightly as the acid becomes stronger and the concentration of electrolyte increases due to formation of H₂SO₄ during charging and decreases due to formation of water during discharging.

Moreover, since acid is formed in the pores of active material during charging and water is formed during discharging, and since it takes time for the acid or water to diffuse out, it follows that the strength of the electrolyte that is in actual contact with the active material is considerably greater during charging than the average strength of the electrolyte (acid) while during discharging it is considerably less than the average. Therefore, emf of the cell is greater during charging than that during discharging.

The terminal voltage of a battery is equal to E + Ir while charging and to E – Ir while discharging where I is the charging or discharging current and r is the internal resistance of the cell.

Characteristic # 2. Capacity:

The ability of an accumulator to last and give current is called rated output or the capacity. While the voltage of a cell is determined by its chemistry, the capacity of a cell is infinitely variable.

The capacity of a cell is essentially the number of electrons that can be obtained from it. Since the current is the number of electrons per unit time, cell capacity is the integration of current supplied by the cell over time. The capacity of the cell is, therefore, expressed in ampere-hours (A -h) and is equal to the product of the specified discharge current in amperes multiplied by the number of hours before the cell discharges to the specified extent. Thus, a rated output (or capacity) of 10 ampere-hours means that one ampere current can be drawn for 10 hours, or half an ampere current for 20 hours.

Thus the capacity of a battery may be defined as the useful quantity of electricity that can be drawn from a battery at the specified discharge rate before it falls to the specified value of voltage, which is equal to 1.75 V multiplied by the number of cells. The capacity of battery depends upon several factors, principal among which are area of plate surface; quantity, arrangement and porosity of the active material used in the manufacture of the plates; quantity and specific gravity of the electrolyte used; and the porosity of the separators. Rate of discharge and temperature also play important role.

The capacity of the cell increases with the increase in plate surface area. A rough rule for estimating the capacity of a battery is the surface area of positive plates in mm² multiplied by the number of such plates and divided by 1,000. For example the capacity of a battery having 5 positive plates each of dimensions of 100 mm and 50 mm, will be (100 × 50 × 5)/1,000 i.e., 25 Ah.

Since electricity is produced by chemical action taking place within the cells, the capacity of the battery depends directly upon the kind and amount of active material used. Theoretically, roughly 4 grams of metallic lead on either element is required to be reduced to sponge lead or to lead peroxide to produce one ampere-hour of electricity. In practice, from four to six times this amount is required.

The reason for this is that it is impossible to reduce all the active material, to bring every particle in contact with the electrolyte, or to cause every part to be penetrated by the current. Experiment shows that from 15 to 22 grams of sponge lead, and from 16 to 24 grams of metallic lead converted into PbO₂, are required on their respective elements to produce a discharge of one ampere-hour at ordinary commercial rates.

The capacity of the cell depends on the concentration or the specific gravity of the electrolyte as it affects the internal resistance and vigourisity of chemical reaction taking place in a cell. It increases with the increase in specific gravity of the electrolyte.

At a particular temperature, the capacity of the cell depends on its rate of discharging. For instance, a 100 ampere-hour battery capable of giving a continuous discharge of 10 A for 10 hours should theoretically give a discharge of 20 A continuously for 5 hours or 50 A for 2 hours or 100 A for one hour, but in reality, the ampere-hour capacity decreases with the increase in discharging rate.

With the increase in the rate of discharge, voltage of the cell falls more rapidly due to internal resistance of the cell; the chemical reactions become brisk and so weaken the plates and reduce the capacity of the cell. If the cell is discharged too rapidly it may break the plates, and in case of pasted plates, a very sudden discharge will dislodge the paste. The capacities of chloride tubular stationary lead- acid cells at various rates of discharge, expressed as a percentage of the ampere-hours available at the 10 hour rate.

The capacity of a battery increases with the increase in temperature because at high temperature, the chemical reactions taking place within the cell become more vigorous, the acid resistance is reduced and diffusion of electrolyte is improved. However, at high temperature, the paste gets rapidly converted into lead sulphate which is always accompanied by expansion of paste particularly at positive plates causing in buckling and cracking of the grid.

At high temperature the antimony-lead alloy grid, terminal posts and wooden separators are also attacked by the acid. So, it will not be advisable to operate lead-acid batteries beyond temperature of 40°C. With the fall in temperature, the chemical reactions become slow, cell internal resistance increases and diffusion of electrolyte becomes poor.

Consequently, the capacity of the cell decreases with the fall in temperature till at freezing point (-35°C at specific gravity of 1.22 of electrolyte) the capacity is reduced to zero even though the battery otherwise be fully charged.

Characteristic # 3. Efficiency:

The efficiency of the cell can be given in two ways, enumerated and explained below:

a. The Quantity or Ampere-Hour (A-H) Efficiency:

Since variations in terminal potential of the cell during charge and discharge are not taken into account while working out this efficiency and terminal potential of the cell during charge is higher than that during discharge, therefore, quantity efficiency is always higher than energy efficiency, in which variations of terminal potential of the cell are taken into account.

As generally efficiency is defined as the ratio of output to the input, similarly quantity efficiency or ampere-hour efficiency is defined as the ratio of ampere-hours of discharge and ampere-hours of charge.

The quantity efficiency of the lead acid cell varies from 90 to 95%. It would be 100 per cent if it were not for the gassing on charge, which represents a non-reversible chemical reaction.

If the charging is discontinued each time as soon as the gassing becomes appreciable, the ampere-hour efficiency will be nearly 100 per cent but ampere-hour capacity will be reduced and it is advisable to give the battery a full charge from time to time in order to avoid deterioration of the otherwise unused lead sulphate. The quantity efficiency also decreases due to self-discharge of the plates caused by local reactions and because of current leakage caused by faulty insulation between the cells and the battery.

b. Energy or Watt-Hour Efficiency:

Energy efficiency is defined as the ratio of energy delivered in watt-hours by the cell during discharge and the energy drawn in watt-hours during charge.

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Operation at low rate of charge and discharge and at reduced ampere-hour capacity both tend to raise the watt-hour efficiency. Actual watt-hour or energy efficiency obtained in practice ranges from about 75 to 85 per cent.