Conventional Lead Acid Cells for TL applications are governed by IS 6848. Type of cells in use for train lighting and coach air-conditioning are as under :-
Principle of Operation :
In a charged lead acid cell positive active material consists of lead peroxide (PbO2) and the negative of spongy lead (Pb). Dilute sulphuric acid (H2SO4 + H2O) serves as electrolyte. The overall reactions inside the cell during discharge and charge are represented most -conveniently by a reversible equation as follows :-
PbO2 + Pb + 2H2SO4 <=> 2PbSO4 + 2H2O
During discharge, the lead peroxide on the positive plates as well as the spongy lead on the negative plates are converted into lead sulphate (PbSO4). In this process, sulfuric acid (H2SO4) is consumed and water (H2O) is formed. Consequently, the specific gravity of the electrolyte falls, the extent of fall being proportional to the ampere-hours taken out. The process causes at first a slow, and then a faster voltage drop, until a permissible lower limit (final discharge voltage) is reached, which depends on the rate of discharge current. The amount of ampere-hours (constant current x time) taken out is called the capacity of the cell at this rate.
The chemical process during charge is the reverse of that during discharge. The lead-sulphate on the positive plates is reconverted into lead peroxide and the lead sulphate in the negative plates into spongy lead. Sulfuric acid is formed and the water consumed. The specific gravity of the electrolyte rises. There is at first a slow, later a faster rise of cell voltage. From 2.4 volts on wards gassing sets in due to a strong decomposition of water into hydrogen and oxygen. Violent gassing is injurious to the plate material. So after reaching this gassing voltage the rate of the charging current must be limited to within safe permissible values.
The nominal voltage of a lead acid cell is 2.0 volts. The true open circuit voltage, however, is dependent on the specific gravity of the electrolyte and varies between 2.02 to 2.05 volts. During discharge the voltage depends on the rate of the discharge current.
Constructional Features Main components of lead acid cell are :-
a) Positive Plates - Usually tubular construction is adopted. Positive plates are made up of a number of tubes which contain active materials. Tubes have a large number of minute pores which allow the electrolyte to pass through pores freely, while preventing any loss of active material.
b) Negative Plates - Usually consist of a lead grid into which active material is pressed. The grids are designed to retain the active material in position.
c) Separators - Synthetic separators are used between positive and negative plates. The separators allow good diffusion of electrolyte.
d) Container - is made of hard rubber with high insulating strength to resist acids.
e) Cell cover - is also made of hard rubber, resistant to acid having vent and level indicator holes.
Accessories: The following are few accessories of a lead acid cell -
b) Float guide,
c) Vent plug.
a) Float : The float stem will have markings to indicate the lowest and highest electrolyte levels permissible. It should be ensured that the electrolyte level is maintained in service by adding pure distilled / de-mineralised water to IS:1069.
b) Float guide : The float guide is of removable and anti splash type and facilitates unrestricted vertical movement of float stem. During maintenance it is essential to ensure that the float assembly is designed to prevent acid splash in service (IS:6848). Any defective float guides/assemblies should be replaced promptly.
c) Vent plug : This is of the anti-splash type with more than one exit hole. This will allow the gases to escape freely but effectively prevent acid particles or spray from coming out. It should be ensured that the vent plug is tightened fully whenever the plug is opened during maintenance for checking specific gravity of cell or after topping up electrolyte level with distilled water.
It is necessary to procure all these accessories exactly to the approved design of the original manufacturers since any defective supply is likely to affect the life of cell adversely.
External accessories for cells are inter cell connectors, end cell connectors and fasteners. These accessories are governed by IS: 6848.
Water intended for storage batteries should conform to IS: 1069 and should be added to bring the level of electrolyte to approximately the correct height just before the charge or during early part of the charge so that gassing will thoroughly mix it with electrolyte. The salient requirements of the battery grade water are as follows
Appearance - The same shall be clear, colorless, odorless and free from suspended impurities.
1. pH. 6.5 to 7.0
2. Non-volatile residue. maximum 0.001 %
3. Chloride as Cl maximum 0.001 %
4. Ammonia as NH3 maximum 0.001 %
5. Heavy metals NIL
6. Calcium NIL
7. Manganese NIL
8. Oxidised matters - To pass 'KMNO4' test
9. Electrical conductivity at 27 ± 2 degree C in micro mhos/cm maximum value 10000
It should be noted that testing of mere PH value alone would not suffice to assess the quality of water used for replenishing in batteries. It is therefore necessary to undertake periodic chemical analysis at least once in 3 months and keep a record of these results.
In the early days, water intended for storage batteries used to be obtained by coal fired steam boilers or electrical distilled water plants. Demineralising plants are now available in the market and no electrical energy or fuel is required to operate this plant. The inlet water is connected to the plant and the treated water is obtained at the outlet after passing through chemicals provided for the purpose. A continuous monitoring PH meter is also provided in the plant. The whole unit occupies lesser space, is compact and neat. Requirement can be picked up from a wide range of capacities available in the market.
Hydrometer is used to ascertain the specific gravity of electrolyte in a lead acid cell. The specific gravity is the relative weight or density of the electrolyte as compared with a similar volume of pure water. The specific gravity of a cell should be maintained at the value given by the manufacturer in the fully charged condition. This value for fully charged cells at 27 Degree C shall be between 1,210 and 1,220 for cells up to 525 Ah capacity and between 1.245 to 1.255 for cells over 525 Ah capacity as per IS:6848.
Voltmeter is used for taking the individual voltage of cells and the battery as a whole. This voltmeter shall preferably be of a dry cell operated digital type with a range of D.C. from 0 to 200 V.
The rating assigned to the cell or battery is the capacity expressed in ampere-hours (after correction to 27 degrees C) stated by the manufacturer to be obtainable when the cell or battery is discharged at the 10 Hr. rate to the end voltage of 1.80 V per cell.
The present and most efficient procedure for the maintenance of lead acid cells is to carry out through overhauls, repairs, rigid tests and quality control during POH work of TL / AC coaches in the workshops. The work in the maintenance depots is confined only to regular and systematic examination, occasional topping up of cells and charging whenever needed.
Train lighting batteries of coaches by the very nature of service conditions cannot be expected to have steady rate of charge/discharge. They are often left to idle for long duration or charged at higher rates. Such strenuous service of these cells therefore calls for systematic and thorough examination while in service, prompt remedial measures of defects/replacement of cells and quality POH work in Shops to achieve the expected life without any loss of efficiency below 80 %.
Running maintenance of storage batteries falls under four categories :-
1. Trip examination,
2. Fortnightly examination,
3. Quarterly examination,
4. Intermediate overhaul.
Trip Examination :
To know the condition of cells during 'Trip Examination' some cells in a battery are treated as 'Pilot' cells. On arrival of train in the maintenance line, disconnection shall be done at inter vehicle connections, Recording of the specific gravity of' ‘Pilot’ cells in each battery shall be done, Pilot cells in coaches should be changed every month and. Marking of Pilot cells should be made as described. Cells are usually provided either in 2 battery boxes of 6 cells each or 1 battery box of 12 cells in coaches with DC-.24 V system. Cell number for marking pilot cells should be reckoned from left to right while facing the battery box. On receipt of coach from POH, the 1st and 12th cells should be marked 'P' in chalk indicating as Pilot cells. This should be changed to 2nd and 11 th after a month, 3rd and 10th in the next month, and so on in the subsequent months. After the cycle is completed, the same cycle should be repeated. The idea of changing the Pilot cells is to ensure that true condition of the battery is reflected, till the cells are sent for next POH and to take prompt remedial action in case of defects. In case of conventional coaches working on DC 110 V system there are two boxes with 28 cells in each.
FORTNIGHTLY EXAMINATION :
In addition to the instructions contained under "Trip Examination" the following works shall be carried out.
In addition to the instructions contained under "Fortnightly Examination" give an equalising charge as given below :
1. Switch OFF load. Charge the cells at 50% of normal rate of charge i.e. at l/20th of the rated capacity of cells. Record hourly cell voltage and specific gravity readings.
2. Terminate charging when 3 successive readings are constant. Record specific gravity and no load voltage of each cell 15 minutes after terminating charge.
Specific gravity should be between 1.210 and 1.220 for cells upto 525 Ah The voltage should not be less than 2.1 V. If there is wide variation in the specific
gravity and voltage readings, such cells have to be sent to shops for treatment.
Instructions given under "Quarterly Examination" should be followed.
Facilities required in major depots for battery maintenance.
INITIAL FILLING AND FIRST CHARGE
The cells are supplied in dry uncharged condition. These require diluted battery grade Sulphuric Acid of Specific Gravity as laid down by the manufacturers (1.18 - 1.22), corrected up to 27 degree C, as electrolyte for Initial Filling. This can be prepared by mixing concentrated battery grade Sulphuric Acid (as per IS:266) of sp. gravity. 1.835 with Water of approved quality approved quality (as per IS; 1069). It is important that the acid and the water should preferably be free from harmful impurities like Iron, Arsenic, Ammonia, Nitrates and Chlorides, but in any case below the specified limits as per IS:266 & IS: 1069.
The diluting and mixing of 1.835 acid, should preferably be done in Lead Lined tanks. However, this may be done in Ebonite boxes or Polythene tanks if adequate precautions are taken to regulate the rate of the acid addition to a safe level, which does not generate excessive heat.
Take the estimated quantity of distilled or de-ionised water in the tank and to this go on adding the estimated quantity of concentrated acid at a slow rate, while keeping the mix well stirred, say with a plastic or wooden ladle. After complete mixing allow the acid to cool down to the ambient temperature. It may be noted that while preparing dilute acids, concentrated acid should always be poured in to water and never water in to acid.
Do not allow the acid to come in contact with skin, clothing or any other material which it might damage. If some acid should, however, get spilled on the skin, rinse promptly with clear water and wash with soap. Bicarbonate of soda solution (1/2 kg. to 5 ltrs of water) will neutralise the acid spilled on clothing or other materials. Apply until bubbling stops and then rinse with clear water.
In our country, the standard temperature for measuring sp. gravity of any electrolyte is 27 degree C. As such, if the electrolyte temperature differs from this reference temperature while taking sp. gravity readings with a hydrometer, the readings require correction. For every 10 degree C above 27 degree C, add 0.007 or seven points to the sp. gr. reading on the hydrometer and for every 10 degree C below 27 degree C, subtract 0.007 or seven points from the readings.
Remove the vent / filler plugs and fill the cells, with the previously prepared and cooled electrolyte, till the lower marking on the float indicator stem just appears above the float plug.
The approximate quantity and sp. gravity of the electrolyte for initial fillings are given in Table II.
After filling, allow the cells to rest for a period of around 16 - 24 hours.
During the rest period there will be some fall in the level of electrolyte. Restore this with some more electrolyte, before putting the cells on first charge,
Now the cells are ready for first charge.
FIRST CHARGE :
The recommended first charge current is given in Table II.
Select a D.C. source of 50% higher voltage and current capacities as compared to the battery voltage and maximum current requirement. Connect the positive of the source to the positive of the cell battery as marked on the terminals and negative of the source to the negative of the cell / battery also as marked on the terminals.
Now charge the cells at the specified rate for 80/100 hours as indicated in Table II. 126.96.36.199 During the charging it is not advisable to allow the temperature of the electrolyte to exceed 50 degree C. So, should it cross 45 degree C, reduce the charging rate to half the value and increase time proportionately. If the temperature continues to rise towards 50 degree C, stop charging immediately, and recommence only after the electrolyte has cooled down below 50 degree C. The total charge input should equal Time x I (where I is the specified charging current).
While charging, there will be some fall in the level of electrolyte due to loss of water by gassing. Restore this at intervals, say 24 hrs. by adding required quantity of approved quality of water into the cells.
It is necessary to start adjusting the sp. gravity of electrolyte to 1.215 ± 0.005 (with RPg-800, 1.250 ± 0.005) corrected to 27 degree C, at about 10-hours prior to the completion of charge; so that the adjustment is complete before the completion of charge. If the sp. gr. is higher than specified, withdraw some electrolyte from the cells and replace with equal quantity of water. Charge for about one more hour. Check the sp. gravity and repeat, if necessary. If the sp. gr. is lower, withdraw some (say 100 ml) electrolyte and replace with concentrated acid of sp, gr. 1.400. Charge for about 15 minutes. Check the sp. gravity. Repeat, if necessary.
NOTE : After adjustment of the sp. gravity of the electrolyte the cells must be gassing freely for a minimum period of two hours of charging. This helps in proper mixing of the electrolyte
After standing on open circuit for neither less than 12 hours nor more than 24 hours from the completion of a full charge, the battery shall be discharged through a suitable resistance at a constant current I = 0.10 x C10 amperes, and the discharge shall be stopped when the closed circuit voltage across the battery terminals fall to 1 .80 volts per cell (Refer IS:6848-1979).
3.6.8 The battery shall be charged at the normal charging
4FEATURES AFFECTING LIFE OF LEAD ACTD BATTERIES :
Life obtained on Railways in case of conventional lead acid cells and batteries is not very encouraging and varies from 3 to 4 years. Life of lead acid cells is affected due to the following features :-
a) Necessity for frequent topping up cells:
There are practical constraints in frequent topping up of cells in rake considering the unhygienic surroundings, enormous quantities of distilled /demineralised water required, manpower requirements for completion of topping up process in limited time, difficulties in attending cells in rear row, spilling of electrolyte. Lapses in topping up the cell, however, seriously affect the life and performance of cells.
b) Leakages of electrolyte on lid and on container body:
During transit/handling/storage some cells develop fine cracks in container body and result in leakages in services later on. Cases of spillage of electrolyte
while topping up the cells also occur. These leakages/spillage result in undesirable leakage currents and even self discharge of cells.
c) Failure of one cell in Monoblock unit: Cases of failure of one cell in 3 cell Monoblock unit have been occurring frequently.
d) Undercharging/Overcharging :
In case of unforeseen detentions, failure of regulator/alternator, undercharging occurs. In day time, in winter, or in SLRs, the cells are likely to get overcharged if voltage settings in the regulator is not properly adjusted. These features affect the life of battery.
LOW MAINTENANCE BATTERIES
In the low maintenance version, modifications have been made in the chemical composition of grid-structure of plates to reduce water losses in service. While grid structure of conventional lead acid battery contains antimony more than 3.5%, in that of Low maintenance version has a lower antimony content of 1.8 to 3.5 %. Reduction of antimony content helps in reduction in loss of water in the electrolyte in service. Antimony is added to give strength to lead spines. Selenium is added to compensate the reduction of antimony content. These batteries should not require topping up earlier than 9 months, as laid down in RDSO Specification No.EL/TL/55 (Revision 'B'). The batteries are provided with microporous vent-cum-filling plug, which allows free escape of gases evolved during service but does not allow electrolyte to come to surface of lid. A sealed float guide is provided to reduce the water loss. These batteries have been provided on some ac coaches.
VALVE REGULATED LEAD ACID (VRLA) BATTERIES / SMF BATTERIES
To overcome problems of frequent topping up, and leakage of electrolyte, sealed maintenance free lead acid batteries, termed as SMF (VRLA) batteries have been developed and are now used in most of the ac coaches. These batteries are governed by RDSO specifications EL/TL/59. Electrolyte in these batteries is in immobilised form and these can be used in any position - horizontal or vertical. The batteries are supplied by manufacturers duly charged and no initial charging is required. Such a battery requires no topping up and maintenance except periodic cleaning of terminals. It has self sealing vent plug which normally does not open out in service.
Valve Regulated Lead Acid (VRLA) Batteries
These batteries are also called Sealed Maintenance Free (SMF) Batteries
Safety Valve :When the internal pressure increases abnormally, the safety valve opens to release gas from the cell to restore the normal pressure.
Flame Arresting Vent Plug : Provides with the explosion-proof filter constructed of aluminium oxide.
Container & Lid : Made of Polypropylene Co-polymer.
Positive Plate :With lead-calcium-tin alloy grid providing lower corrosion and less self-discharge rates.
Separator : Made of high Absorbent Glass Mat woven with excellent porosity (AGM type).
Negative Plate : With lead-calcium-tin alloy grid providing lower corrosion and less self-discharge rates.
Electrolyte : Dilute sulphuric acid without any impurity.
The charge and discharge reaction of the lead acid battery can be expressed by the following equation :
Pb02 + 2H2S04 + Pb Charge PbSO4 + 2H2O + PbSO4
In a conventional flooded battery, towards the end of charge major part of the energy supplied by charging is dissipated by electrolysing the water in the electrolyte generating Oxygen at the positive plate and Hydrogen at negative plate. These gases are lost in a flooded system through the vent holes causing steady depletion of water and therefore requiring periodic topping up. In a VRLA system the design is such that negative plates are never fullycharged-even when the positive plate is fully charged and hence almost no Hydrogen gas generates from the negative plate although Oxygen is generated from positive plate. This Oxygen gas generated at the positive plate migrates towards the negative plate and reacts with the freshly formed spongy lead and turns into lead monoxide. The lead monoxide in turn reacts with the Sulphuric Acid to turn into lead Sulphate resulting in the negative plate to be partially discharged. To summarize the Oxygen evolved at the positive plate is absorbed by the negative plate without being released to the outside. The negative plates being always in a state of partial discharge never generate Hydrogen. This completely prevents loss of water.
Some features of VRLA Batteries :
The Pure Lead-Tin range offers the customer the highest energy density of any lead acid battery anywhere. The battery is constructed around a complex thin plate, pure lead-tin grid which packages more power in a smaller space. The plates being made of high purity lead last longer, offering excellent life. The proven benefits o this superior technology are high performance, quick recharge capability, high energy density and a long service life. The 6V & 12V monoblocks are available in capacities ranging from 12Ah to 150Ah.
• Maintenance-free and spill-proof. This enables flexible mounting
• Wide operating temperature range (-40C to +50C)
• High energy density (gravimetric and volumetric)
• Good charge retention leading to long storage life
• Low internal resistance ensures quick recharge
• Excellent high rate capability permits use of smaller capacity batteries
• Superior raw materials for good performance and life
• Excellent deep discharge recovery characteristics
• UL recognized plastic components