Battery Charging |
Charging of Lead Acid Battery |
The chemistry of charge & discharge
The chemical reactions on discharge convert the active components in the battery plates, that is the lead in negative plate, lead peroxide in positive plate, and the sulphuric acid into free electrons and water, and lead sulfates. The chemical reaction of recharge is a reverse process.
The important part is to recharge the battery in such a manner so that the sulfates are eliminated by recombining with water to re-form into acid without loosing the hydrogen and oxygen gasses that make up the water. Loss of water happens due to a secondary reaction known as electrolysis of water and it becomes predominant at a charging of voltage of 2.35 volt per cell and increases further with increase of charging voltage.
The above charge and discharge process is represented by the chemical formula given below.
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Typical charging curve : |
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The curve above shows the four steps involved in Lead Acid battery charging.
The first step is done at constant current where the battery replenishes the maximum energy drawn out in the discharging step, without losing water. The charge replenished can be between 80 to 90 percent and depends on the age of the battery (decreases with age). The cell voltage increases and changed to the next step when the voltage reaches the gassing voltage of 2.35 volts.
The second step is done at constant potential to minimize the water loss. This step can be considered as providing the balance 10 to 20 percent of the charge.
The total of the energy fed to the battery will be extra by some percentage over the energy drawn out to take care of the efficiency of the charging process. It can be between 15 to 25 percent depending on the height of the cells for a battery used for deep cycling application.
The charge provided with these two steps can be termed as ‘service charge’ and can be a charging method used to get battery banks ready between two discharges as done for a traction battery.
The above charge process will leave the cells with a variation of state of charge between cells in battery bank, which is best represented by the specific gravity readings. (No two cells in a battery bank are exactly equal in their characteristics and follow the laws of ‘Natural Variation’. This is a statistical mathematics governing any manufacturing process.)
The variation in specific gravity reading in a battery bank should not go beyond 30 units. (Considering that the cells had an initial variation of 10 units when it was commissioned. Another 20 units get built up from the variation in charge acceptance characteristics of the cells.) The cells need to be equalized on reaching this condition. An easy to follow guidelines is equalizing after a months use. More frequent equalization may become necessary with increasing age of the battery bank.
The third step can be a constant current or combination of a constant current & constant potential charge and is known as ‘equalizing charge’ aimed at bringing all the cells in a same state of charge.
Completion of charging in a vented battery is judged by specific gravity readings. When three readings taken at an hourly interval and corrected for temperature, show no change, it can be considered that the battery is fully charged. Electrolyte level adjustment shall be done while the battery is on charge by addition of Distilled or De-mineralized water.
Temperature correction of specific gravity reading
For every 1 deg C the specific gravity of the acid changes by 0.0007. If the observed reading is at certain temperature above the reference temperature of 27 deg C, the equivalent reading at 27deg C is done by calculating the difference in temperature and multiplying by the factor 0.0007 and then adding to the observed reading. Similarly if the observed reading is at a temperature below 27 deg C, deduct the product of temperature difference and factor from the reading.
The formula for this correction is
SG at 27 deg C = SG reading at T deg C + (T –27) X 0.0007
Important: After every charge cycle, the battery shall be allowed to cool down to near room temperature.
Any charging above gassing voltage leads to higher water loss.
The fourth step is known as float charge, which is a constant potential charging and is done to prevent the self-discharge of electrochemical cells. This is generally done for batteries banks supporting systems, which is normally working on mains supply, during power outages.
The charging method mentioned above works well for flooded & vented cells where lost water by gassing is made up manually or by use of automatic filling devices.
Charging of maintenance-free battery
For maintenance free batteries the charging configuration will be different. A point, which needs to be discussed here, is the depth of discharge. We consider here two variants of discharge.
1. Discharge in stand-by banks, which have shallow discharges required to give back up to loads in case of power outages. The duration can be considered as the time required to put a DG set on. In this type of use, the battery remains connected to the load in parallel with the mains supply. 2. Motive power use in Electric vehicles like forklifts and other battery operated vehicles. Here the battery is meant for deep discharge and can be 80 percent of the rated capacity. The battery is connected and disconnected to chargers after every use cycle.
The charging method defined above is best suited for application in (2).
Battery banks for standby application both (Maintenance-Free and Vented type) are charged by constant potential type charging at a float voltage of 2, 23V per cell. This voltage keeps the cells in a full charged state.
Only if the batteries are discharged very often, so that the float voltage does not create the full charge an equalizing at 2, 33V constant potential is done.
Safety requirement in battery charging.
• Personnel qualified and specially trained for the purpose should operate the lead-acid traction batteries.
• Use only strong, impact-proof acid- and heat-resistant containers for the preparation of electrolyte.
• To avoid skin injury (burns) when preparing electrolyte wear protective goggles, mask and clothing (acid-resistant clothing, rubber gloves and boots). Wash thoroughly with water IMMEDIATELY to remove acid, if spilled on body parts and seek medical help.
• Acid and electrolyte should be transported in acid-resistant containers and their pouring should be done by means of appropriate devices.
• During the charge the cells emit hydrogen and oxygen. Reaching a 4% hydrogen concentration can lead to explosion. That is why the battery room ventilation should provide 4-5 times air volume of the room per hour.
• Never use unprotected flame when checking the cells because of the explosive gas, formed in the cells. It is recommended to use a battery torch.
• Do not use bare metal tools on the battery as they can cause short-circuits and burn-outs.
• Avoid storing foodstuff, water and other beverages. Do not smoke and eat in the battery rooms.
• Wear dielectric gloves and rubber shoes to avoid electric shock.
• The circuit should be open prior to connecting and dis-connecting of the battery to the power source
• When handling the battery use the appropriate lifting appliance only. |
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Copyright © 2009 Microtex Energy P Ltd. All rights reserved.
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Dch |
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Pb + PbO2 + 2 H2SO4 |
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2 PbSO4 + 2 H2O E cell = 2.0V |
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Cha |
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Phase I : |
Bulk charge phase, Constant current and voltage increases |
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Phase II : |
Absorption charge, Constant voltage and current decreases |
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Phase III : |
Equalization charge first at constant current when voltage increases |
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Phase IV : |
Float charge done at constant voltage to prevent self-discharge. |