Performing these tests requires instrumentation hardware which can both charge and discharge these batteries while accurately monitoring and controlling the voltage and current. For performing charge cycle testing, an SMU (source measure unit) is commonly the instrument of choice, as it provides the ability to accurately source to (charge) and sink from (discharge) batteries while measuring both voltage and current. In this manner, a single instrument can be used to gather a variety of important information.
The NI PXI-4130 Power SMU is a flexible, high-power source measure unit that is based on the PXI Platform. It offers a 4-quadrant supply that can source up to 40W (20V, 2A) and sink up to 10W, and measure current with down to 1nA sensitivity on its lowest range. This SMU channel also features remote (4-wire) sense capability for accurately controlling and measuring the voltage directly at the terminals of the battery. Remote sense capability is especially important when testing batteries that have small internal resistances and high current outputs, as those factors can make the effects of lead resistance very detrimental to accurate measurements. By offering a second pair of terminals with high-impedance measurement capability, the NI PXI-4130 can more accurately characterize voltage on a wide variety of cells. The NI PXI-4130 also incorporates an additional single-quadrant utility channel that acts a programmable power supply with both source and readback capability. Shown below is a connection diagram for testing a battery using the SMU channel on the NI PXI-4130 Power SMU with remote sensing enabled.
Figure 2. Connection diagram for performing charge cycle testing with the NI PXI-4130 Power SMU
To perform a charge cycle, the NI PXI-4130 should be configured to source the desired fully-charged voltage of the battery being testing (such as 4V), and the current should be limited to the desired rate of charge (such as 1c). Since an uncharged battery will have a lower voltage than the voltage setpoint on the SMU, the current limit will immediately be enforced and the SMU voltage will drop to equal the voltage of the battery. In this instance, the SMU is sourcing current to the battery at the current limit, and as a result the battery will charge and the voltage on both the battery and the SMU will rise until the desired fully-charged voltage level is reached. In order to display the charge curve, voltage measurements on the SMU can be performed at regular intervals and plotted on a chart.
To perform a discharge cycle, the NI PXI-4130 should be configured to source the desired un-charged voltage of the battery being testing (such as 3V), and the current should be limited to the desired rate of discharge (such as 1c). Since an charged battery will have a higher voltage than the voltage setpoint on the SMU, the current limit will immediately be enforced and the SMU voltage will increase to equal the voltage of the battery. In this instance, the SMU is sinking current at the current limit, and as a result the battery will discharge and the voltage on both the battery and the SMU will decrease until the desired un-charged voltage level is reached. Finally, in order to display the discharge curve, voltage measurements on the SMU can be performed at regular intervals and plotted on a chart.
Note: When the NI PXI-4130 is sinking current, the maximum power that can be continuously dissipated is 10W. Refer to the NI PXI-4130 Specifications for more information.
In addition to the requirement for instrumentation hardware as discussed above, a key consideration for charge cycle testing is flexible software that can be used to control the hardware and retrieve measurements as well as analyze and present the data. The NI PXI-4130 can be programmatically accessed in a variety of languages such as C++ and Visual Basic or National Instruments LabVIEW. Shown below is the user interface of a simple LabVIEW program which performs charge and discharge cycles as described above, and displays the battery voltage on a tank indicator as well as the most recent charge cycle on a waveform chart.
Figure 3. A simple program for performing charge-cycle tests written using the National Instruments LabVIEW Graphical Programming Environment