Maintenance and Care of Batteries for Switchgear Applications
The monitoring and maintenance of batteries for switchgear applications is very important from the point of view of service reliability. The consequences of electrical failures are catastrophic in cases where no control power is avail- able for tripping the circuit breakers. Proper maintenance is the key to depend- able battery operation. The reader is referred to IEEE standard 450-2002, “IEEE recommended practice for maintenance, testing, and replacement of vented lead–acid batteries for stationary applications” for maintenance and testing procedures that can be used to care for these batteries. The following is a summary of battery maintenance procedures derived from the above referenced standard that are offered as a guide to ensure long life and depend- able service. Refer to Section 8.9 for detailed inspection checklist on batteries installed for UPS applications since there are many similarities common to both of these battery systems.
Inspections
Periodic inspection can provide information on the battery conditions and its state of health. All inspections should be made under normal float conditions. Inspection should be made at least once a month (and more frequently, such as weekly depending on service), and should include the following checks:
• Float voltage at battery terminals
• Charger output current and voltage
• General appearance and cleanliness of the battery
• Electrolyte levels, cracks in jars, and leakage of electrolyte
• Evidence of corrosion at terminals, connectors, racks, or cabinets
• Voltage, specific gravity, and electrolyte temperature of pilot cells
• Ambient temperature and ventilation
• Unintentional battery grounds
The monthly inspection should be augmented once every quarter with the following checks:
• Voltage of each cell and total battery terminal voltage
• Specific gravity of 10% of the cells of the battery
• Temperature of electrolyte of 10% of the cells of the battery
The yearly inspection should include the following checks:
• Detail visual inspection of each cell to determine its condition
• Specific gravity, voltage, electrolyte level, and temperature of each cell of the battery
• Contact resistance of cell-to-cell and terminal connections
• Impedance measurements of the battery cells
• Structural integrity of the battery rack and cabinet
Equalizing Charge
The station batteries are sized in terms of their discharge capacity, which is usually stated in ampere-hours. The ampere-hours are based on supply current during an 8 or 4 h period with electrolyte temperature at 25°C. To maintain a constant voltage at the battery terminals, the charger is con- nected in parallel with the battery and the load circuits. The purpose of the float charge voltage is to prevent the internal discharge of the battery. The practical float voltages are listed in terms of volts per cell (VPC). Following are the VPC values for the various types of batteries:
• Nickel–cadmium: 1.4–1.42 VPC
• Lead–calcium: 2.17–2.25 VPC
• Lead–antimony: 2.15–2.17 VPC
• Plante: 2.17–2.19 VPC
When the battery is equipped with a constant voltage charger, it is automatically charged after an emergency discharge. In the case of lead–acid batteries, a periodic equalizing charge is required when the specific gravity, corrected for temperature, of an individual cell falls below the manufacturer’s lower limit (or below its full-charge value by 0.001), or the individual cell float voltage(s) deviate from the average value by ±0.05 V (typical value of lead–antimony cells is ±0.03 V). As an alternative, when an individual cell corrected for tem- perature is below 2.13 V (typical for nominal 1.215 specific gravity cells), equalizing charge to the entire battery should be initiated immediately.
However, it is often more convenient to apply the equalizing charge to the individual cell if there is one or few cells out of limit. The frequency for equalizing charging varies, but is usually from a minimum of 3 months to 1 year. Also, an equalizing charge should be given to a battery after the addition of water to the battery. Different types of batteries require different lengths of time for the equalizing charge. The length of time is a function of cell tempera- tures. A normal electrolyte value for specific gravity based on a temperature of 77°F is taken to be 1.15 for a fully charged lead–acid battery. Battery perfor- mance is affected by electrolyte temperature. Generally, for every 3°F below 77°F, the battery performance can be evaluated by subtracting 0.001 from the specific gravity. Similarly, for every 3°F above 77°F, add 0.001 to the specific gravity. Specific gravity readings may not accurately indicate state of battery charge following discharge or following addition of water. The most accurate indicator of return to full charge is stabilized charging or float current.
Battery Tests
The following tests are performed to
• Determine whether the battery meets its specifications or the manufacturer’s rating or both
• Periodically determine whether the performance of the battery, as found, is within acceptable limits
• Determine whether the battery, as found, meets the design requirements of the system to which it is connected, i.e., whether it has the capacity and capability to power the loads connected to it
Acceptance Test
This is the test to determine whether the battery meets a specific discharge rate and duration in accordance with manufacturer’s ratings. This test is nor- mally made at the factory or upon initial installation.
Performance Test
In IEEE standard 450-2002, it is recommended that the battery capacity test should be made within the first 2 years and followed by a test interval not to be greater than 25% of the expected service life. Assuming a service life of 25 years for a vented cell battery, then this test should be conducted every 5 years or less until the battery shows signs of degradation. Similarly, in IEEE standard 1188-2005 it is recommended that the performance test interval for VRLA batteries should not be greater than 25% of the expected service life or 2 years, whichever is less. Further, annual performance test should be con- ducted on any battery that shows signs of degradation or has reached 85% of its service life. Degradation is indicated if the battery capacity falls more than 10% from its capacity on the previous performance test, or 90% of the manufacturer’s rating.