BATTERY MAINTENANCE

e telephone equipment no matter where it is found whether in a central office an isolated microwave repeater station a dial PBX or a key telephone system in a customer's office depends upon a battery plant for its operation most telephone equipment is powered by direct current converted from alternating current by motor generator sets or rectifiers when the alternating current Supply is interrupted the continued operation of the telephone equipment depends upon the storage battery also the storage battery provides a steady flow of current to care for sudden increases in load the storage battery is an important part of the telephone plant representing a considerable sum of money in plant investment this PBX battery costs about $450 to replace the replacement cost of this battery plant is over $100,000 when a power failure occurs the batteries are the sole source of direct current to keep the telephone plant working until emergency alternating equipment can be operated storage batteries in the telephone plant can be grouped into certain types the open tank type shown here no longer is used in new installations but is still in operation in many offices hard rubber cells have now replaced the older open tank cells this is a glass jar type notice the similar appearance of the latest plastic jar to understand the correct maintenance of storage batteries it is necessary to know their fundamental parts and the theory of their operation a storage battery is a group of two or more storage cells which are referred to as a string the lead acid storage cell which is the type we use is made up of a container or jar with one or more positive plates and one or more negative PL the positive and negative plates are kept apart by separators the Assembly of positive and negative plates with separators is called a plate element the element is immersed in a solution of sulfuric acid and water called the electrolyte it is important to remember that the acid is corrosive to most materials and will burn the skin it is most dangerous to the eyes a positive plate consists of a lead alloy framework or grid lead oxide paste is pushed or pasted into the grid after the initial charge of the cell the lead oxide is converted to lead peroxide a negative plate is similarly constructed but the lead oxide paste is reduced during the initial charge to Pure lead this drawing represents an elementary storage cell composed of a container a positive plate a negative plate and the electrolyte when the cell is fully charged the chemical composition of the positive plate is lead peroxide the negative plate is pure sponge lead and the electrolyte is sulfuric acid and water at maximum specific gravity the charge condition is indicated by the weight of the solution as measured by a hydrometer the hydrometer measures the specific gravity or weight of the electrolyte as compared to the weight of pure water most telephone batteries are designed so that the specific gravity at full charge is about 1.22 commonly known as 1220 as discharge occurs a chemical change takes place the acid of the electrolyte combines with the lead of the negative plates and with the lead peroxide of the positive plates to form lead sulfate on both plates as the battery discharges the electrolyte becomes lighter in weight when the battery is recharged the chemical action is reversed when the battery is fully charged the electrolite returns to its original gravity the open circuit voltage across a cell varies when fully charged the vol voltage is about 2.1 volt when discharging it drops the cell is considered completely discharged at 1.75 volt these are emergency or n cells which are used in most plants and are switched into the circuit to maintain the overall battery voltage when the regular cells are are being discharged when the lead sulfate is completely removed from the plates overcharge occurs water in the solution is changed to explosive of hydrogen gas at the negative plates and oxygen gas at the positive plates this is called gassing note the construction of one type of cell a negative plate a positive plate separator terminal post filling funnel and a gas vent in connection with the gas vent note the explosion proof Dome Construction a baffle diverts the gas to the vent eliminating an accumulation of gas at the top of the cell here is another design for explosion protection a porous ceramic vent acts to disperse the gas as it leaves the cell batteries are operated in different charging routines covered by Bell System practices the charge discharge method generally involves two strings of batteries one being charged while the other is being discharged when using the continuous variable current charge method the charging is fixed at the correct rate to restore the charge lost during Peak loads during the light load period for continuous float operation regulated char charging equipment is used which provides the power necessary for the load plus a small additional amount necessary to maintain the batteries at full charge the newer power plants are predominantly operated at continuous float under this operation it is most important to keep accurate float volage and this is Possible only if float voltmeters and voltage regulating equipment are in accurate adjustment meters should periodically be checked by comparison with the standard meter the taking and recording of readings is an important battery maintenance function note the protective equipment used by the man who will take the hydrometer reading before starting the job static electricity is discharged by touching an intercell connector near the grounded end of the string the hydrometer must be held vertical the electrolyte level in the barrel must not be too low or too high now this will give an accurate reading the maintenance man must always remember that the storage battery can be explosive even with the new explosion proof features voltage readings are even more important than the specific gravity readings as we are looking for differences in vage between cells the battery volage must be stable the records of readings are required for future reference and comparison since the voltage and gravity both vary with cell temperature the readings must be corrected in accordance with the thermometer readings when records show a drop in specific gravity or cell voltage vary excessively or a battery discharge has occurred a boost charge must be given the length of Rec recommended boost charge is specified in the Bell System practices the water level of the cells must be checked at intervals specified and water added if needed note a check is made to see that the float is not St duck cleanliness is always the rule around batteries battery lugs should be checked periodically to ensure tightness the wrenches are taped to prevent a possible short across the terminals lugs should be kept tight but not too tight because of the soft material in the smaller size battery plants if the cells will not hold a charge after boost charging a capacity tester is used in accordance with Bell System practices the rated capacity of any cell is related to the size and number of plates the tester can be set for different battery sizes the principal cause of loss of capacity is low float voltage this allows the cell to remain partially discharged if it remains This Way Long the lead sulfate on the negative plate will not return to Pure lead when charged this is known as sulfation on the other hand a high float bullet will cause excessive sediment to form at the bottom of the jar and cracking and flaking of the positive plate such deterioration also causes a cell to have low capacity and if the sediment level becomes too high will short out the plates here is a low water condition that if left uncorrected will result in deterioration of the exposed plates when water is added to any battery use only approved tap water or distill water common troubles are evidences of deterioration that need remedial action sulfation which is evidenced by low gravity and low voltage readings and found by analyzing the battery records this condition in its earlier stages can be corrected by Boost charging sediment can be seen in a glass jar cell normal amounts of sediment will be found under cycle charging however sediment found in floated batteries indicates incorrect float voltages this illustrates the importance of keeping the regulating equipment in proper adjustment cracked plates are caused by overcharging and shorten the life of the battery this would show up during capacity tests and necessitate premature replacement of the battery these Buckle plates are another example of improper charging this corroded post is the result of improper maintenance post should be kept clean and covered with a layer of petrolum counter EMF cells are a vital part of most storage battery plants the plates are of stainless steel and the solution is sodium hydroxide note the correct water level a layer of oil covers the top of the solution to prevent loss of solution as spray when gasing and to prevent solution contamination by the carbon dioxide in the air counter EMF cells act as a resistance in the circuit and are used to reduce the volage in certain plants during normal operation and in others during boost charge only the value of the counter cell voltage varies slightly with the load as is the case with the storage battery the gas formed in the counter EMF cell is also very explosive a check should be made of the voltage across the counter EMF cell with it in the discharge circuit when the cell readings of the battery are taken when the voltage drop increases to 2.6 volts the solution should be changed to prevent damage to the cell as a review of battery maintenance and operation let's check these important points safe work operations correct float volage keeping of accurate records boost charge as preventive maintenance maintaining correct water level cleanliness at all time battery maintenance is is an important part of the telephone job because power is the heart of the telephone system