Lead-Acid Storage BatteriesDOE-HDBK-1084-95MAINTENANCERev. 0Page 33BatteriesOverdischarge may cause difficulties in recharging the cell by increasing the battery'sinternal resistance. Also, overdischarging may cause lead to be precipitated in theseparator and cause a short in the cell or between cells.MaintainingElectrolyteLevelsDuring normal operation, water is lost from a flooded lead-acid battery as a result ofevaporation and electrolysis into hydrogen and oxygen, which escape into theatmosphere. One Faraday of overcharge will result in a loss of about 18 g of water. Evaporation is a relatively small part of the loss except in very hot, dry climates. With afully charged battery, electrolysis consumes water at a rate of 0.336 cm per ampere-3hour overcharge. A 5000-Ah battery overcharged 10% can thus lose 16.8 cm , or3about 0.3%, of its water each cycle. It is important that the electrolyte be maintained atthe proper level in the battery. The electrolyte not only serves as the ionic conductor,but is a major factor in the transfer of heat from the plates. If the electrolyte is belowthe plate level, then an area of the plate is not electrochemically efficient; this causes aconcentration of heat in other parts of the battery. Periodic checking of waterconsumption can also serve as a rough check on charging efficiency and may warn whenadjustment of the charger is required.Since replacing water can be a major maintenance cost, water loss can be reduced bycontrolling the amount of overcharge and by using hydrogen and oxygen recombiningdevices in each cell where possible. Addition of water is best accomplished afterrecharge and before an equalization charge. Water is added at the end of the charge toreach the high level line. Gassing during overcharge will stir the water into the aciduniformly. In freezing weather, water should not be added without mixing as it mayfreeze before gassing occurs. Only distilled water should be added to batteries. Although demineralized or tap water may be approved for some batteries, the low costof distilled water makes it the best choice. Automatic watering devices and reliabilitytesting can reduce maintenance labor costs further. Overfilling must be avoided becausethe resultant overflow of acid electrolyte will cause tray corrosion, ground paths, andloss of cell capacity. Although distilled water is no longer specified by most batterymanufacturers, good quality water, low in minerals and heavy metal ions such as iron,will help prolong battery life.CleaningKeeping the battery clean will minimize corrosion of cell post connectors and steel traysand avoid expensive repairs. Batteries commonly pick up dry dirt which can be readilyblown off or brushed away. This dirt should be removed before moisture makes it aconductor of stray currents. The top of the battery can become wet with electrolyte anytime a cell is overfilled. The acid in this electrolyte does not evaporate and should beneutralized by washing the battery with a solution of baking soda and hot water,
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