Two principal types of lead—acid batteries are in use today: Plante and Faure. Plante invented the lead—acid battery about 100 years ago using positive and negative plates of pure-lead sheets. Today batteries that have pure lead in their positive plates are called Plante types. The original Plante battery was expensive because the lead peroxide, which is the active material of the positive plate, was extremely difficult to form on the surface of the positive plate. Some time later, Faure invented an eco­nomical method of pasting lead peroxide on the positive plates and lead oxide on the negative plates. This is the type of construction used today.

Faure batteries can be either lead—antimony or lead — calcium alloy plate grid types. The names are derived from the hardening alloy material used in the manufacture of the plate grids. Of the many types of batteries available, the lead—antimony and lead—calcium Faure types are the most common in noninterruptible power supplies. A lead — calcium battery costs up to 15% more than a lead — antimony battery; however, the lead—calcium battery offers several advantages over the lead—antimony battery. The lead—calcium battery requires much less water replace­ment and therefore generates a proportionately smaller amount of hydrogen gas during the recharge cycle. The floating voltage is less critical, 2.17 to 2.25 volts per cell (compared to 2.15 to 2.17 volts per cell for the lead — antimony battery). If the lead—calcium batteries are float — charged between 2.20 and 2.25 volts per cell, they are reported to never require an equalizing charge. These advantages must be balanced against the higher initial cost of the lead—calcium battery since satisfactory operation for equal lifetimes can be expected from both types of batteries when properly maintained.

The current produced by a lead—acid battery results from the chemical reaction of dilute sulfuric acid on the active materials in the plates. Lead dioxide reacts with the sulfuric acid at the anode to produce a positive charge. At

’The maximum flux in a transformer is inversely proportional to the form factor (root mean square/average) for the input voltage. Since the form factors are 1.00 and 1.11 for square-wave and sinusoidal inputs (of the same frequency), respectively, there is an 11% additional swing in flux for the square-wave input. In view of the fact that this increase would increase the total core loss, it could lead to exceeding the rated temperature rise within the transformer at full load.

the cathode, metallic lead reacts with the acid to produce a negative charge. During the discharge process, sulfuric acid is consumed and replaced by a corresponding amount of water. During charging the process is reversed, acid being formed at the plates with a corresponding consumption of water and generation of oxygen and hydrogen gas.

As the discharge of a lead—acid battery progresses, the water formed is absorbed into the electrolyte, resulting in a reduction of the specific gravity of the acid. The battery open-circuit voltage depends on the concentration of the acid in contact with the active plate materials. The-voltage available for useful work is the voltage across the battery terminals during discharge. This latter voltage is equal to the sum of the internal cell voltages minus the drop due to the internal resistance of cells. The reduction of acid concentration as the cell discharges is accompanied by an increase in internal resistance; the increase is gradual at first and then rapid as the cell approaches full discharge. The internal resistance may increase by as much as a factor of 2 to 3 on approaching full discharge. The internal resistance of a fully charged battery is so small that it has little effect on the terminal voltage except when high discharge rates are encountered. During discharge lead sulfate accumulates on the plates. Lead sulfate is a nonconductor and has a tendency to block the pores of the plates, thereby impeding the chemical reaction. Sulfate is a contributing factor in the reduction of the terminal voltage during discharge.

When the battery is discharged at low rates, the formation of water and lead sulfate proceeds slowly, allowing the acid in the electrolyte to be readily absorbed into the pores of the plates and resulting in a gradual decrease of the terminal voltage. When the battery is discharged at a very high rate, the depletion of acid at the plates takes place so rapidly that the rate of acid replace­ment cannot keep pace, which causes a greater reduction in terminal voltage. The increased depletion of acid at the plates on high discharge is the primary reason that batteries have a lower ampere-hour capacity at high discharge current rates.

The capacity of the lead—acid battery is reduced as the ambient temperature decreases. The amount of reduction also depends on the rate of discharge and cell design. The major reasons for the reduction in capacity with decreasing temperature are the increased electrolyte viscosity, which impedes the diffusion of the acid at the plates, and the higher internal cell resistance caused by increased resistance of the electrolyte. The common practice is to refer to the capacity of a battery at reduced temperatures as a percentage of its capacity at 77 F.

Batteries are rated on the basis of ampere-hours, which means the amperes that the battery will deliver for a given time at a specified temperature to reach a specified “final” voltage. The standard battery terminal voltages commonly used are 24, 48, 120, and 240 volts. These correspond to 12, 24, 60, and 120 cells, respectively, at a nominal 2 volts per cell. Faure batteries are float-charged at a cell voltage ranging from 2.15 to 2.25 volts, depending on the type of cell. Determining the size or ampere-hour capacity of a battery involves a detailed procedure that is beyond the scope of this test. (Details of the methods are given in the bibliography at the end of this chapter ) Sizing of the bat­tery can be done by the design engineer, who is aware of the battery limitations, or by the manufacturer or supplier of the nonmterruptible supply package In general, the battery must have sufficient ampere-hour capacity to carry momentary loads plus continuous or basic loads for a specified length of time before reaching its final voltage, commonly referenced at 1.75 volts per cell

A lead—acid battery must receive the correct charge to give optimum performance and life. It is difficult and impractical to obtain this precisely on every charge or under floating operation. Lead—acid batteries do not need a full charge on every recharge to obtain satisfactory opera­tion. For long life, however, they must be brought to the fully charged condition periodically, the period depending on the degree and frequency of discharge On daily or frequent recharges, it is common practice to charge slightly short of a fully charged condition, a complete charge must be made every 1 to 3 months This complete charge is commonly referred to as an “equalizing charge ” Most modern battery-charging equipment is designed to effect a periodic equalizing charge.

The equalizing charge is intended to be sufficient to equalize any minor differences among the cells and should be continued until each cell of the battery reaches maximum voltage and specific gravity. To achieve this state requires manual attention and is, therefore, impractical However, the same effect is obtained by giving the entire battery an additional amount of charge for a limited period, which is not harmful to the battery. Most charging is controlled by an automatic timing switch incorporated in the charging equipment The most practical means of giving an equalizing charge is to set the time switch for an additional period of time For batteries in normal full­floating service, which is the most common service in standby power systems, a common practice is to raise the floating voltage above its normal value by about 5 to 10% for a period of 8 to 24 hr, depending on the type of battery and application. It is important to ensure that any nonmterruptible loads on the battery during the equalizing charge are rated for operation at the higher voltage levels

The efficiency of a battery may be expressed as ampere-hour efficiency, voltage efficiency, or watt-hour efficiency. Ampere-hour efficiency is the ratio of the number of ampere-hours a battery yields on discharge to the number of ampere-hours required to fully recharge the battery. A typical lead—antimony battery requires a re­charge in ampere-hours about 10% greater than the previous discharge, and the ampere-hour efficiency is 91%. Voltage efficiency is similarly defined as the ratio of discharge voltage to charge voltage. For the same typical lead — antimony battery, the average voltage on charge is approxi­mately 17% higher than on discharge, the voltage efficiency is 85%. The product of ampere-hour and voltage efficiencies gives a watt-hour, or total, efficiency. For the lead- antimony battery, the watt-hour efficiency is 0.91 X 0.85, or 77% This is a representative value for such batteries. (The watt-hour efficiency is slightly higher for a typical lead—calcium battery.)

The Faure battery is the most reliable single component used in nonmterruptible power systems today. Properly designed, battery-supported systems afford the optimum in overall reliability.