The rapid growth of the hybrid-electric vehicle industry presents many new opportunities for product testing and measurement. Many of these opportunities require production-level test systems with short design times, high accuracy, and strong reliability. One opportunity involves the production testing of battery management systems (BMSs) for lithium-ion battery packs, which power plug-in hybrid electric vehicles (PHEV).
BMSs handle all of the monitoring, control, and safety circuitry of battery packs and control systems, including accurately monitoring cell charges, balancing voltages between cells to maintain a constant voltage across packs, managing charging and discharging, and protecting the system from over-voltage and over-current conditions for packs of up to 12 cells in series. In addition, BMSs monitor system temperatures, handle system power saving by entering sleep modes to reduce current draw, and communicate with external controllers to provide system feedback. While there are several types of battery management boards, including individual pack balancing and monitoring boards and system control boards, we refer to all types as BMSs in this document.
BMS Features and Requirements
Because the BMS is important to the safety, performance, and longevity of PHEV batteries, it is critical that each manufactured board perform to strict specifications. Cell voltages must be monitored to millivolt accuracy, safety faults must occur properly, and the BMS must draw current from individual cells to balance voltages across a whole pack. Functional testing of these processes requires a highly accurate, flexible, and strong test system capable of simulating packs of cells, applying system voltages, measuring cell and system-level voltages and currents, and communicating with the UUT.
System Hardware Design
By starting with the Bloomy Controls PXI-based universal test system, we produced a flexible, high-accuracy base platform consisting of a standard mass interconnect capable of testing multiple models of BMS circuit boards by using interchangeable fixtures. We centered our system around six NI PXI-4110 triple-output programmable DC power supplies, which we used to simulate a pack of up to 12 lithium-ion cells.
Figure 1. Base Platform System Layout
We also multiplexed a high-accuracy NI PXI-4071 digital multimeter (DMM) to measure voltages within the required millivolt specifications, and added an NI PXI-6221 M Series data acquisition DAQ module to provide analog outputs, TTL digital I/O, and higher-speed analog input measurements. We implemented the NI PXI-6514 industrial digital I/O module to read switches and actuate fixture relays. In addition to the PXI hardware, we used fixed power supplies and programmable high-voltage and high-current supplies to provide additional system power as required by the testing specifications.
Finally, we provided a USB connection to the fixtures to allow flexible addition of other UUT-specific communications and peripheral hardware on a per-model basis. We housed all of our hardware in a standard 19 in. rack. The test rack provided a system capable of making any measurement and supplying any source required by a BMS board.
We also used a standard fixture receiver to permit several different BMS designs to be tested using the same base hardware. Each fixture type was electronically keyed, guaranteeing that the correct test code would run for the attached fixture. By using interchangeable fixtures, we greatly reduced system cost and lead times through sharing key instrumentation hardware among UUTs. After we built the base system, we could quickly design and build new fixtures and their associated test software.
Series Cell Simulation Based on the PXI-4110
To simulate a pack of 12 lithium-ion cells, we linked the isolated ±20 V legs of the six PXI-4110 power supplies together in series; each leg simulated a single cell of the pack. During cell voltage testing, the power supplies applied individual cell voltages between 2 and 4 V for a combined pack voltage of up to 48 V. Then, the software polled the UUT for its reported voltages seen at each cell; we compared these voltages to the voltages measured by the DMM in the test system to determine UUT accuracy. For tests measuring each cell's balancing current, the 16-bit readback resolution of the PXI-4110 supplies was vital because it eliminated the need for external shunt or Hall effect current. Overall, the PXI-4110 was an excellent choice for this application because of its low ripple, fast response, high resolution, and ease of control.
System Software Design
We wrote the test software using LabVIEW and contained all test parameters in a configuration file to allow the customer to update, tighten, or loosen test specifications without making software changes. In addition, we stored all of the data acquisition channels and tasks in a separate configuration file, which allowed hardware or wiring changes to be made without affecting the underlying software. Also, because the user interface is designed for a manufacturing environment, it requires minimal operator interaction and the test technician simply opens the safety lid of the fixture, scans the barcode serial number of the unit to test, then closes the fixture for the test to start during standard operation. When testing is complete, the test result is shown, test data is logged to file, and any failed tests are highlighted for the technician.
Furthermore, we delivered all software with debugging and diagnostic modes, which provided engineers more manual control over the system. Test engineers can enable the debug mode to run smaller subsets of the main test to narrow down the possible causes of a failure. The diagnostics control screen provided access to all aspects of the system pertaining to the attached fixture. This allowed the engineer to manually read all system voltages and currents, control all power supplies, actuate relays, and communicate with the UUT.
An Accurate and Flexible Testing Solution
The NI modular instruments and LabVIEW software used in the Bloomy Controls BMS functional test system was critical in designing an accurate, easy-to-use, and flexible system. The six PXI-4110 programmable DC power supplies were ideal for simulating packs of lithium-ion cells. To date, we have delivered three base systems and nine fixtures including seven unique fixture models. We delivered two of the base systems directly to contract manufacturers, one of which is currently located in China.
Our experience with BMS testing allows for the rapid development of new test systems with low risk and short lead times. By using a modular approach and interchangeable components, the base system can accommodate testing a wide range of BMS models. This method reduces cost and new fixture design time and makes it cost-effective to test even small quantities such as R&D prototypes. In summary, the NI PXI platform coupled with the LabVIEW development environment delivered the ideal tools to quickly design and build a BMS test platform that is flexible enough to test multiple customer products, and accurate enough to meet or exceed BMS testing requirements.
For more information on this case study, contact:
Robert Cornwell, Bloomy Controls, Inc. at: firstname.lastname@example.org
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