This test is performed by applying a constant current charge for a time period. (There are other charge methods besides constant current, such as constant voltage and pulse, but constant current is the method specified by battery manufacturers for proper determination of the condition of the cells.) A 40A nickel-cadmium battery is typically charged at 20A for two hours (100 percent main charge) and at 4A for four additional hours (40 percent overcharge) while monitoring the voltage of each of the cells. (The overcharge is required because the charge process is not 100 percent efficient and gets less and less efficient as the cell is charged.)
Nickel-cadmium (vented) cells have a minimum charge voltage requirement of 1.50V with 1.55V to 1.65V being the typical voltages developed by cells that have been deep cycled several times. In addition, cell voltages must never drop and there must not be an appreciable increase on temperature. A drop in cell voltage (50mV) and/or a significant warming (5C) could indicate a cell separator failure. (In the event of a cell separator failure, the cell must be replaced because cells cannot be repaired.)
Automation of cell voltage measurements
Since a cell failure can occur at any time during the charge/discharge test, it is important to be able to monitor the performance of the cells on a continuous basis, but this is not practical unless it is automated.
Automation simplifies the measurement task but more importantly it makes it accurate and timely. And, since measurement automation implies data recording, the test data is automatically archived for retrieval at any time.
Furthermore, once there is cell voltage measurement automation it follows that there can be battery voltage, battery temperature and charge/discharge current measurement as well.
Once battery test data is collected, the next step is to analyze it to determine the condition of each cell individually and of the entire battery. Typical questions are centered on: Initial condition of the battery/cells; as received voltages; first capacity test; first charge and water level. How did the battery/cells do in subsequent tests? Did capacity time improve? Did failing cells recovered? Was the charge voltage acceptable? Did the battery remain cool during charge? Did any of the cells require more than expected water for electrolyte leveling?
With an automated system than produces color coded bar graphs it becomes a lot simpler to observe and determine the condition of the battery. This makes a final pass/fail determination a lot more assertive and it also allows for spotting early failures that can shorten the test time.
With all the data archived for retrieval at any time, it is then possible to easily compare new results with prior ones, thus providing a clear picture of the condition of the battery as it ages.
At this time, when more is required from fewer resources, automation of the battery test process results in accuracy and efficiency in the operation of a battery test facility.
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