Battery testing of aircraft nickel-cadmium batteries is a complex process but it doesn’t have to be complicated. The tests in themselves aren’t complicated but require strict attention to detail to ensure that the process will be accurate and efficient. In addition, for a successful performance, the process and equipment must result in accuracy and efficiency in the operation of the battery testing facility.
Types of tests
Battery testing focuses on two basic types of tests: charge acceptance and capacity.
Charge acceptance is the ability of the battery to convert input current into charge in the plates, as evidenced by voltages developed across each cell terminal and the overall battery voltage, each one within limits established by the manufacturers.
Capacity is the ability of the battery to deliver the rated current for a minimum time while the overall battery voltage and that of each individual cell remains above minimum set voltages.
Battery performance testing requires exacting conditions of current, voltage, time, and temperature, hence, establishing the need for precision instrumentations.
The most important performance characteristic of the charger-analyzer for nickel-cadmium (NiCd) batteries is constant current because this is the best way to determine charge acceptance and capacity. Although other charge methods are mentioned by battery manufacturers, the only method that is fully described is constant current, hence, the method of choice.
Most NiCd batteries require a two-step constant current charging process. The first, called Main, is used to replenish the charge taken out during the capacity test and the second, called Topping, is to complete the charge and to test that each cell reaches the proper charge voltage. In addition, at the end of the topping charge period, the level of the electrolyte is verified and distilled water added as needed.
The equipment must also monitor the temperature of the battery during the charge process for nickel-cadmium cells must not develop any appreciable temperature increase.
Additional characteristics required of charger-analyzers include intuitive and user-friendly programming of the test parameters such as current, time, and battery voltage/number of cells, all with the necessary accuracy and stability for the validity of the tests.
When controls are easy to understand and use, training time is reduced and the chances of erroneous settings are also reduced. In addition, the equipment must be intrinsically safe for the protection of the operator, the battery, and the instrument itself.
Testing procedures are well defined by manufacturers in their manuals and individual battery data sheets, so the only special requirement is the selection of the equipment to properly meet the test requirements.
Note that failing to observe proper testing requirements can and will result in premature failures and/or unnecessary replacement of batteries and cells, both having a significant impact in the cost of battery maintenance and in the operation of the aircraft. Improper battery maintenance can easily result in costly AOG situations and scary in-flight battery emergencies.
Battery testing is laborious because of the many steps that are required. From the readings taken to determine the “as received” condition, to the multiple cycles of charge and discharge required to restore the capacity of the cells, there is a requirement for a multiplicity of measurements and record keeping.
The most involved part of the testing is the multiple measurements that must be performed. For a typical 20-cell aircraft nickel-cadmium battery this means 20 measurements of cell voltages in addition to the total battery voltage for every step in the test process.
Although readings at the end of the charge and discharge tests constitute the bottom line for pass/fail determination, intermediate readings are useful because they provide early warnings of impending failures and/or they provide historical information that can be used to explain cell behavior.