Test Equipment Requirements
From the various tests described so far, we can see some common requirements for test equipment. All the tests require a discharge cycle using a constant current. A constant discharge current cannot be attained with a simple resistor because the battery voltage changes as current is drawn from it. An active device is required, such as an electronic load with a constant current mode of operation. Also note that, because many levels of constant current are used from test to test, you should be able to control the electronic load dynamically as the test demands.
The ability to control the load with a computer is important because dis- charge is typically over a long period of time and, if the test were not auto- mated, constant attendance would be an unproductive use of an operator’s time. Long term tests also bring about another requirement: reliability. The electronic load must be very reliable because, if it should fail, the test would take a long time to repeat.
In battery or single cell testing the electronic load only has to function down to the EODV, not zero volts. See Figure 5. If the minimum load operat- ing voltage is above the EODV for the battery being tested, two alternatives are available: stack more than one battery in series until the required voltage is reached (Figure 6) or place a DC power supply (of sufficient volt- age and current) in series with the battery (Figure 7). A power supply applied in this way is sometimes called an “offset supply.”
Figure 5. Single Battery Test Configuration
The first alternative (Figure 6) requires a method of scanning the voltage of each battery in the stack so that when any one battery reaches its EODV, either the test can be halted or the battery switched out of the circuit and replaced by a short circuit. Even as each battery is switched out of the circuit, the discharge current will remain the same if the load has a constant current mode of operation.
Figure 6. Batteries in Series
The second alternative (Figure 7) shows that using a power supply may be more desirable because timed fast charge,
Figure 7. Using an Offset DC Power Supply
Voltage and current must be moni- tored throughout all the tests because actual battery voltage varies with the battery chemistry as well as the dis- charge rate involved. Therefore, a voltmeter and ammeter are required. They should be computer controlled so that the various tests can be halted when the EODV is reached. If an ammeter is unavailable, a current shunt can be used in conjunction with either a second voltmeter or a scanner.
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