1. Capacity discharge test should be run every 5 years and annually after capacity drops to 90 percent and below
2. Specific gravity and electrolyte color should be checked every 5 years (see section 6.8 and 6.9).
6. VENTED NICKEL CADMIUM BATTERY PRINCIPLES
6.1 GENERAL
The principles of operation, maintenance, and care of nickel-cadmium (ni­ cad) batteries are different than those for lead-acid batteries. If a conflict is encountered, use manufacturer’s instructions in precedence to this manual. IEEE-1106—Recommended Practice for Installation, Maintenance, Testing, and Replacement of Vented Nickel-Cadmium Batteries for Stationary Service —may also be referenced for further information. Nickel cadmium cells are resilient to both overcharge and undercharge and may even be recovered if they go into polarity reversal. They may be used in a wide temperature range, from -40 to +140 EF. High temperatures do reduce service life but not as severely as lead cells. With proper maintenance, service life may be expected to be as long as 25 years.
6.2 DESCRIPTION
In a ni-cad cell, both positive and negative plates are similar in construction, consisting of very thin strips of perforated nickel-plated steel screen. Active materials are nickel compounds in positive plates and cadmium compounds in negative. The electrolyte solution is potassium hydroxide within a steel or plastic container. Positive and negative plates are separated by means of hard rubber or plastic. Sheet hard-rubber separators are used to insulate steel containers on the inside. Almost no active materials migrate from the plates, so space between the container and plates is small. Cells are mounted in insulated trays when furnished in steel containers. Containers are separated from each other with insulated buttons to prevent shorting and to provide ventilation between cases. Vent caps are spring-loaded so they remain closed and are only open when electrolyte is being checked.
6.3 CHEMICAL REACTIONS
Charging and discharging nickel-cadmium cells is similar to other storage cells. During discharge, nickel hydrate is removed from positive plates by the reduction of nickel hydroxide (OH) and combines with the cadmium of the negative plates, forming cadmium oxide. This process is the reversible action of nickel and cadmium. On charging, the OH leaves the negative plate and returns to the positive plate. The chemical reactions taking place in a storage cell are:
Battery Discharged:: Negative Plate: 2Ni(OH)2 - 6H2O + 2KOH + 2H2O + CdO
Battery Discharged:: Positive Plate 2Ni(OH)3 - 5H20 + 2KOH + 2H20 + Cd
The net result is the transfer of oxygen from the active material of one plate to that of the other without measurable change of the electrolyte. In the
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electro-chemical reaction within the battery, the electrolyte acts as a carrier for ions and does not change in concentration. Specific gravity remains the same, and does not indicate the state of battery charge.
6.4 UPON INSTALLATION
Make sure all connections are torqued according to the manufacturer’s specifications. Coat the connections with no-oxide grease.
A. Annually
Retorque all connections to the manufacturer’s specifications.
6.5 CHARGING
The density of electrolyte in ni-cad cells does not change appreciably with charge so specific gravity does not indicate state of charge. This state can be estimated when the battery is on float. To hold battery voltage at 1.40 volts per cell, a fully charged battery requires about 1 milliampere (excluding load current) per ampere-hour of capacity. A fully charged 100-ampere-hour battery draws a charging current (0.001 x 100 = 0.1 amp). The battery is not fully charged if charging current is higher.
A more accurate state-of-charge determination may be made as follows. With the battery on float charge, record the ampere output of the charger and the battery terminal voltage. Record the time the above readings were taken. Then place the charger into the high-rate charge or equalize charge position. Record both charger ampere output and battery terminal voltage again. Continue the high-rate charge until the voltage reaches the high-rate maximum voltage for the system. The state of charge may then be determined as follows:
1. The battery is fully charged if the high-rate charge voltage is reached in less than 1 minute and the current output of the charger drops to near the float current measured above. 2. The battery is in need of a charge if the high-rate charge voltage is not reached in 1 minute or less. 3. The battery is in need of a charge if the charging current is greater than the float current after 1 minute or more on high rate. Continue the high- rate equalizing charge until the current is nearly the same as the measured float current. Record the charge on form POM-133C.
A. A. Initial Freshening ChargeInitial Freshening Charge
An initial freshening charge should be given to compensate for self discharge losses during shipment and storage. If the cells have been shipped filled and charged, placing the battery on float will probably be sufficient. If the cells were shipped discharged, they must be charged according to the manufacturer’s instructions as to voltage level and time. Inspect all cells for
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the proper electrolyte level and oil level before charging.
B.. Float Charge
Float voltage is normally maintained at 1.40 to 1.42 volts per cell to avoid gassing. Gassing begins at about 1.47 volts, and charging at this level should be avoided because water consumption becomes excessive. Excessive voltage variations between cells may be caused by excessively low float voltage, which causes the battery to become partially discharged. If so, the charger voltage setting should be increased slightly to within the above limits. The charger voltmeter should be checked against an accurate digital voltmeter at least once a year.
C..Equalizing Charge
An equalizing charge should not be given unless the battery has been discharged to less than 90-percent capacity. Smaller discharges are handled by float charging. A fully discharged ni-cad battery in good condition can be fully recharged in 4 to 10 hours. When an equalizing charge is necessary, 1.52 volts per cell may be used (1.40 volts on a 92-cell bank) if the charger can provide the current. As the battery charges, current decreases and voltage stabilizes at the pre-set charger voltage. The charge should be continued until charging current has leveled off for two consecutive readings, 30 minutes apart. During an equalizing charge, carefully monitor electrolyte temperature; lower charging voltage immediately if 115EF is reached.
6.6 VOLTAGE READINGS
Voltage readings must be taken by an accurate digital voltmeter (see section 1.3).
A.. Each Shift
Check the voltmeter on the control panel to determine if the battery is being charged at the proper voltage. Adjust the voltage if necessary.
BB.. When Taps Are Changed When Taps Are Changed
When taps are changed on power or station-service transformers, check the voltage on the control panel and adjust the charging voltage if necessary.
C..During Initial Charge
Just before terminating the initial or equalizing charge, measure and record the voltage of each cell with an accurate digital voltmeter. Choose a pilot cell from one with the lowest voltage for readings in the coming year. Change this cell each year.
D.. Monthly
Check the accuracy of the panel voltmeter against the digital voltmeter by measuring the battery terminal voltage. Adjust the panel voltmeter if necessary.
E.. Quarterly
Read the pilot cell voltage and the overall battery terminal voltage while on float. Read the voltages with the digital voltmeter and record them on form POM-133C.