Rechargeable Battery Testing with the NI PXI-4130 Power SMU

Überblick

The testing of rechargeable batteries is becoming increasingly important as they are relied upon in more and more designs ranging from mobile hardware to new biomedical devices. Tests on these batteries must be thorough and accurate in order to ensure proper operation. In addition, test systems must be flexible enough to accommodate new designs and new test requirements for evolving hardware. This paper discusses how to create a highly flexible and customizable battery test system using hardware and software from National instruments.

Contents

Charge Characteristics

The most common types of batteries used in new designs are Lithium Ion (Li-Ion), Nickel Cadmium (Ni-Cd) and Nickel Metal Hydride (Ni-MH), although there are many other varieties.  While these variants each have specific characteristics which are advantageous for different applications, they share many common traits that must be tested.  Most notably, the charge curve of a battery is used to show voltage as a function of time under a variety of different loading conditions.  Shown below is a typical discharge curve for a Li-Ion battery.

Figure 1. Constant Current Discharge Curve for a Li-Ion battery

 

The time taken to complete a charge or discharge cycle depends on both the capacity of the battery as well as the rate at which the battery is discharged.  Most batteries specify their charge capacity in milliamp hours (mAh), which directly translates to how long a battery can source a given current.  For instance, if a standard AA battery has a capacity of 1000 mAh, it can theoretically source 1 amp for 1 hour.  As a result of the mAh convention for specifying capacity, charge/discharge rates are generally given in terms of the current draw that will discharge the battery in 1 hour (denoted in units of "c").  Charge cycle testing is often performed at different rates (values of current) such as 1c or 0.5c.  Most batteries also specify a maximum charge/discharge rate that can be safely used without heating the battery to the point that it is damaged.

Charge cycles can reveal the characteristics of a battery such as battery life, voltage levels, the number of usable charge cycles, and other important information.  As result, charge cycle tests are commonly performed during both the research and validation of new battery designs, as well as during the design of devices that employ batteries.

 

Charge Cycle Testing

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.

Hardware

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.  

 

Software

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

 

Rechargeable Source Simulation

In addition to performing charge cycle testing on batteries, the NI PXI-4130 Power SMU can also be used to source power to other devices in order to simulate a battery or stack of batteries.  In doing so, it also has the ability to characterize the total power drawn by a device under test or measuring leakage currents with down to 1nA resolution.  More rigorous tests of other devices can be achieved by sweeping the maximum voltage and current supplied by the SMU in order to check functionality at different power levels.  Alternatively, the 4-quadrant supply on the NI PXI-4130 can be useful for testing battery chargers which must source power to an active load.  

Note: The PXI-4130 has a specified rise time of 200us on its SMU channel (channel 1), so if an application requires faster bursts of power, another source may be needed.  For more details visit the NI PXI-4130 Specifications.

 

Conclusion

The NI PXI-4130 Power SMU can be used to perform charge cycle tests on a wide variety of batteries due to its high-power, 4-quadrant output.  Additionally it can serve as a source simulator to test the devices that a battery will be used with, thereby providing a means of validating power characteristics on all elements of a battery powered design.  For more information on specifications and pricing visit:

NI PXI-4130 Power SMU

Or, for more information on other NI Precision Sources visit

Programmable Power Supplies and Precision Sources