Just like any other device, batteries need to be tested to determine if they are airworthy. That is, to determine if they will perform properly when required under normal conditions and more so when needed in emergency situations.
While on the ground, if the battery is not capable of performing it could simply result in an annoying and expensive AOG situation but, if there is a problem in flight and the batteries do not perform, this can now become a serious emergency. Will the batteries provide the power needed to start the APU? Will the batteries be able to power all required avionics and allow for a safe touchdown? (n 2008 a Qantas 747 lost electrical power while on approach to Bangkok and the pilot had to rely exclusively on battery power. This is as real as it gets.)
The problem with battery testing is that a simple voltmeter measurement will not provide information on how the battery will perform. Yes, the voltmeter reading can give some idea on the state of charge (more so on lead-acid batteries than on nickel-cadmium) but the voltmeter reading cannot be used to determine if the battery will deliver the rated current for the specified duration.
Therefore, bench testing is needed to determine capacity and charge acceptance as specified by the manufacturers of the batteries.
Capacity is the most basic and most important of the tests. A “Capacity Test” is performed by discharging the battery and verifying that the battery voltage will remain above a minimum voltage in the specified period of time.
Charge acceptance testing is also important. It is not enough to simply pump current into the battery. Each of the cells and the total battery must exhibit a proper response to the current. This means that the voltage must rise properly, that voltages do not fall, and that there is no increase in temperature.
The procedure for battery testing is straightforward but it is by no means simple. It is possible, however, to simplify the process by having a disciplined methodology, equipment that will allow an accurate and simplified performance and a systematic method to determine pass-fail conditions.
Capacity is the most basic and most important test. It goes along with the rating of the battery. A 40A nickel-cadmium battery is expected to deliver 40A, for one hour, with the voltage of each cell remaining above 1V.
This test is performed by applying a constant current load equal to the battery A-Hr rating, while monitoring the voltage of the battery and volage of each of the cells. (Constant current discharge is the basic test form. In some cases a resistive load may be specified by the manufacturer but this is seen more in small battery packs (emergency power supplies).) The battery will pass the capacity test if at the end of one hour the battery voltage is above the minimum (20 volts for a 20 cell battery) and if each of the cells is above 1 volt. (One hour is the typical time limit. Each battery manufacturer and/or appliance manufacturer may specify other time limits, usually shorter.)
The difficulty here is the monitoring of the cells. There are 20 cells in a typical battery and each one needs to be measured before, during and at the end of the test. If this is done manually (measure and write) there are many opportunities to take a wrong reading, skip a cell, etc.
In addition, measuring 20 cells manually means that there will be a considerable time difference between the reading of the first cell and the reading of the last cell. This is aggravated by the fact that the voltage of nickel-cadmium cells fall rather rapidly at the end of their useful capacity. In a battery with marginal capacity, the first cells would pass but the last ones would not. And even on a “good” battery the difference in the readings will result in an inaccurate picture of the condition of the battery.
Charge acceptance is the other side of battery testing. It is not sufficient that a battery passes capacity; it must also demonstrate that all cells can properly transform the charge current into charge stored in the plates.
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