Electrolyte of Lead Acid Battery
Electrolyte of lead acid battery cell is a solution of sulfuric acid and distilled water. The specific gravity of pure sulfuric acid is about 1.84 and this pure acid is diluted by distilled water until the specific gravity of the solution becomes 1.2 to 1.23. Although, in some cases specific gravity of diluted sulfuric acid are recommended by manufacturer of the battery depending upon type of battery, season and climatic condition.
Chemical Action of Lead Acid Battery
The battery cells can be recharged by reversing the direction of discharge current, in the battery. This is done by connecting, positive terminal of a DC source with positive terminal of the battery and similarly, negative terminal of the DC source with negative terminal of the battery.
A rectifier type battery charger of suitable capacity is used as DC source for changing battery. Due to charging current (reverse of discharging current) positive plates change to lead peroxide and negative plates change to pure lead. As soon as load is connected across the battery terminals, discharge current starts to flow through the load and battery starts discharging. During discharging process, acidity of the electrolyte solution decreases and lead sulfate deposits on both positive and negative plates. In this process of discharge the amount of water in the electrolyte solution increases that is specific gravity of the electrolyte decreases.
Theoretically, this discharge process continues until the negative and the positive plates contain maximum amount of lead sulfate and at that point both types of plate becomes electrically similar that means there is no potential difference between the electrodes of the cell. But practically, no battery cell is allowed to be discharged up to this point.
The battery cells are allowed to be discharged up to a predetermined minimum cell voltage and specific gravity. A fully charged lead acid battery cell has voltage and specific gravity, 2.2 V and 1.250 respectively, and this cell is normally allowed to be discharged till the corresponding values become 1.8 V and 1.1 respectively.
Maintenance of Lead Acid Battery
If the cells are over charged, the physical property of lead sulfate gradually changes, and it may become obdurate from which it becomes difficult to convert by charging process. Hence, the specific gravity of the electrolyte decreases for which the rate of chemical reaction is hampered.
Sulfated battery cells can easily be recognized by viewing changed color of plates. The color of a sulfated plate becomes lighter and its surface becomes harsh and gritty. Such cells evolve gas prematurely on charge and show a decreased capacity. If the sulphation is allowed for long time, it becomes difficult to rectify the cells. To avoid this situation, it is recommended to charge the lead acid battery cells for prolong time at a low rate of charging current.
There is always a high chance at the terminal connectors of the battery cells to be corroded. Corrosion mainly affects at the bolted connection between cells in a row. This can easily be avoided if the tightness of each bolt is properly checked and rectified and also each nut bolt connection is covered by a thin layer of petroleum jelly. If any of the cells is corroded, it should be replaced immediately.
The specific gravity of the electrolyte may be permanently decreased due to ageing effects. This problem is generally found in old battery cells. This is mainly due to,
- Action of sediment-at the bottom of the cell container.
- Due to loss of acid by spray during charging.
- Inadequate treatment after the removal of short circuit.
- Due to excessive sulphation on the plates.
If the lowering of specific gravity is not due to sulphation or short circuit, concentrated sulfuric acid may be added to restore normal value of specific gravity.
A short circuit may occur between the positive and negative plates either due to treeing or due to buckling of the plates. Treeing is usually due to excessive gassing which tends to loosen the active materials from the plates. The particles of active materials fall into the electrolyte and may accumulate on the negative plates in such a way as to bridge the space between the positive and negative plates. This treeing can be removed by the use of scaling stick made of ebonite. By this stick it is possible to explore the space between these two types of plates of a cell and to remove loose materials or treeing.
If the short circuit is due to buckling of plates, this can be removed by inserting additional separator or by removing and straightening the plates mechanically.
After removal of short circuit, care should be taken to restore the specific gravity of the electrolyte to normal by constant charging by high current.
Maintaining Battery Lead Acid Battery Room
There is a high chance of acid spray and gases during charging of battery. These may pollute the atmosphere surrounding the battery. Hence, ample space and good ventilation is essential inside the battery room. These gases can explode and hence naked flames should not be brought inside the battery room and also smoking is strictly prohibited inside the battery room. There should be at least one exhaust fan of suitable size, fitted in the battery room to keep the atmosphere free from those gases and free from moisture inside the room. The temperature inside the battery room should always be maintained above 10oC. The walls, ceilings, doors, window frames, ventilators, metal parts and other apparatus in the battery room should be painted with anti-acid coating at regular intervals. The electrical wiring inside the room should be in metal conduit and lighting fixtures should be flameproof in construction. All the switching elements including electrical fuses and plug sockets should be installed outside the battery room otherwise there may be a chance of fire hazard initiated from sparking during switching operation. The floor of the room should be well finished preferably by using ceramic tiles. The floor and walls of the room should be cleaned properly in regular intervals.
|There are some safety measures to be taken during handling storage battery in substation|
|1||Do not smoke inside the room.|
|2||Do not bring a flame inside the room.|
|3||Do not generate any spark inside the room.|
|4||Wear plash proof goggles, rubber gloves, while working with the battery.|
|5||During preparing electrolyte always add the concentrated acid little by little to the water.|
|6||Never pour water into the concentrated acid.|
All the above mentioned points should be displayed on the door or any other well visible place of the battery room.
The following rules to be obeyed during operation, control and emergency lighting service of the storage battery.
- Do not allow the battery to stand idle for long time, this may cause inactivation of the battery cells.
- Do not charge the battery with very high rate of current because high rate of charging causes high temperature rise and excessive gassing, resulting in heavy loss of water and sometime overflowing of electrolyte from the battery cells.
- After every complete discharge, the battery should be immediately charged before returning it to its regular floating service. Otherwise there may be a chance of deposition of sulphate film on the plates.
- As already mentioned, the battery cells should be charged gently at normal rate so that there will be no chance of immediate gassing and temperature rise over 40oC. Otherwise there may be damage to the battery cells due to high temperature. During charging of batteries, continuous monitoring is required, if gassing starts and the temperature reaches the said limit, then reduce the rate of charging. If after reducing the charging rate, the temperature is still approaching to the limit, it indicates the completion of charging process because even the normal rate of charging may produce high temperature rise if the battery approaches to the fully charged condition.
- The voltage of each of the battery cells should be checked before conclusion of charging and each of the cells are properly and equally charged and the readings should also be tallied with previous record.
- If the electrolyte level inside the battery cell comes down, it must be filled with distilled water up to the level marked on the cell itself. This is to compensate the loss of water due to evaporation.
- During filling up distilled water in the battery cell, it must be carefully watched that the electrolyte level in the battery cells should not exceed the line marked on it. Otherwise there may be a chance of overflowing of electrolyte during gassing of the battery. High level of electrolyte may also cause softening the sealing compound on the top cover and subsequent leakage in the battery cell.
- The specific gravity should be measured after at least two weeks from topping up to ensure thorough mixing of water in electrolyte.
- The battery should be discharged up to the allowable limit and then it should be overcharged once in 2 to 3 months. The rate of overcharging must be followed as specified by the manufacturer. This operation is very important to maintain the acid storage battery in the active state.
- When specific gravity of electrolyte is measured, it should not be forgotten to correct it for temperature. So that, all hydrometer readings will be referred to same temperature. The hydrometer should be kept clean with distilled water otherwise the hydrometer will cause incorrect readings and as well as spoil the quality of electrolyte. The specific gravity of electrolyte must be kept within 1.180 to 1.240. Low value of specific gravity decreases the capacity of battery and on the other hand high value is harmful for the battery plates.
- The substation battery normally operates under floating mode. At floating mode, a constant voltage from the battery charger is applied across the battery during its normal operation to keep the battery approximately in fully charged condition. In normal condition the battery charger supplies the substation load and also compensate the losses in the battery. But in case of heavy demand during simultaneous operation of many switch gears, the battery and the charger, are combined to supply the demand.
- During normal floating condition the cell voltage, specific gravity and temperature of pilot cells should be measured daily to follow up the condition of battery as a whole. But the same readings of each and every battery cell should be taken at least once in the month to keep monitored the conditions of individual cell.
- The battery should be refilled with fresh electrolyte once in three years interval to maintain the battery in proper operating conditions.
- This is should be noted that, the pilot cell of a battery bank is one arbitrarily chosen battery cell which is used to get the information about overall condition of the battery. But one pilot cell must be fixed for one month and it should be changed in the next month.
- This was a brief description of maintenance of substation battery but it is always preferable to follow the instructions given into the maintenance manual supplied by the manufacturer too.