Developing a PXI solution for testing automotive battery management systems
As the developed world attempts to move to a more “green” lifestyle, electric vehicles are clearly becoming a growing part of the automotive scene. They promise low or no emissions, conceivably low cost of fuel from the Power grid, yet they will deliver us safely to and from work and shopping.
But their design is a paradigm shift for the Auto Industry – new drive systems, technologies… and test plans. These vehicles are bringing new test and validation challenges to the industry as the electronics content of the vehicles grows.
The auto industry has embraced the use of Lithium Ion Batteries for most future Hybrids and Plug-in Hybrids. This battery design requires a carefully designed charging system to provide long life and safety; which means that one of the major challenges to be tackled in electric vehicles concerns the effective testing of the Battery Management Systems (BMS) – the electronics that manage the state of the battery that stores the high levels of energy required to propel the vehicle.
To aid in the testing of the BMS, DMC Engineering & Software Services and Pickering Interfaces have collaborated to help
provide a solution to BMS testing for a major manufacturer, based on PXI modules that emulate the battery systems. In
this article, we will discuss some of the tests that must be performed and why. We’ll also show how PXI was utilized
and why it was a perfect solution to a complex problem.
Building and Testing a BMSManufacturing processes for Lithium Ion battery cells have a high degree of inherent variation, requiring a more advanced and robust BMS. The BMS must compensate for any under performing cells in a module, or “stack”, by actively monitoring and balancing each cell’s state of charge (SOC). A battery stack design can have infinite combinations of good and bad cells, and will be subject to a huge range of environmental conditions. These variations and usage scenarios necessitate Battery Pack simulation for development and qualification of effective Battery Management Systems. It can also influence the type of testing utilized in a production environment as well.
The BMS is therefore a critical component of Hybrid-Electric Vehicle (HEV), Electric Vehicle (EV), and Plug-In Electric Vehicle (PHEV) electric drive systems. A typical BMS controls all functions of the Energy Storage System (ESS), including battery pack voltage and current monitoring, individual cell voltage measurements, cell balancing routines, pack state of charge calculations, cell temperature and health monitoring, as well as ensuring overall pack safety and optimal performance.
The BMS modules and related sub-modules must read voltages from the cell stack and inputs from associated temperature, current and voltage sensors. From there, the BMS must process the inputs, making logical decisions to control pack performance and safely, and reporting input status and operating state through a variety of analog, digital, and communication outputs.
Effectively testing a BMS system involves two primary functions, (1) accurately simulating the required sensors and battery
cell stack inputs to the BMS, and (2) measuring, collecting, and processing the digital and analog outputs produced by
the BMS system as a result of those inputs.
Why the BMS challengeThere are two main reasons for separately validating the performance of a battery stack’s BMS, including Safety and Longevity.
Everyone has heard horror stories about lithium-ion batteries exploding in laptops and cell phones. The good news is that Lithium-ion batteries have six times the energy density of lead-acid chemistries and three times that of nickel-metal hydride – in addition, more charge/discharge cycles are possible with a properly designed and maintained battery stack. But, any time you pack more energy into a smaller space, with increased usability, your safety concerns likely increase.
Controlled release of the battery’s energy provides electrical power in the form of current and voltage. Uncontrolled release of this energy can result in release of toxic materials (i.e. smoke), fire, high pressure events (i.e. explosions), or any combination thereof. All lithium-ion systems use an electrolyte that is flammable and has a tendency to undergo "thermal runaway". When you heat this material up, it will reach an onset temperature at which it begins to self-heat and progresses into fire and explosion.
Uncontrolled energy releases can be caused by severe physical abuse, such as crushing, puncturing or burning, which can be mitigated by mechanical safety systems and proper physical design. However, they can also be caused by shorted cells, abnormally high discharge rate, excessive heat buildup, overcharging, or constant recharging, which can weaken the battery. These causes are best prevented by a properly designed and validated electronic safety and monitoring system, better known as a battery management system.
The BMS is also responsible for tracking a battery pack’s exact state of charge, which is critical to maintaining its usable lifetime. Usable battery life can be dramatically reduced by simply charging the pack too much, or discharging it too deeply. Accordingly, the BMS must include a very accurate charge estimator. Since you can’t directly measure a battery’s charge, the state of charge percentage has to be calculated from measured characteristics like voltage, temperature, current and other proprietary (depending on the manufacturer) parameters. The BMS is the system responsible for these measurements and calculations. Validating the accuracy of the BMS state of charge calculation is critical to pack performance and longevity.
Battery Stack Emulation
Lithium-ion batteries with a lithium iron phosphate cathode and graphite anode have a nominal open-circuit voltage of 3.2 V and a typical charging voltage of 3.6 V. Lithium nickel manganese cobalt (NMC) oxide cathode with graphite anodes have a 3.7 V nominal voltage with a 4.2 V max charge. A typical battery stack for automotive use has 96 cells in series capable of generating voltages to over 350V. High voltages allows energy to be transferred to the drive system with thinner wires and lower losses than would be the case with lower voltage systems, but high voltages need careful management if damage to electronic systems is to be avoided. Testing and validating new BMS systems on a real battery stack is not a practical solution because the consequences of an error are likely to result in significant damage to either, and could hurt the test operator as well. The test cannot be repeatedly run. Only when confidence is high can the BMS be connected to a real battery stack. The second problem is that faults and the characteristics of a real battery stack cells cannot be varied to simulate the conditions the BMS is designed to handle. Injecting faults, especially in development and NPI can be crucial to successfully validating hardware and firmware designs.
DMC approached Pickering Interfaces with a request for a battery simulator that could be used to simulate a low power battery stack for the purpose of validating the BMS design. The battery stack had to be fully programmable for output voltage for each cell, and the stack had to be capable of both sourcing current and sinking current (the charging state).
Pickering Interfaces already had experience in designing single cell PXI based battery simulators for mobile applications.
So following the enquiry, a design feasibility study was undertaken. To keep the systems compact and low cost, it was
determined that we needed to pack as many channels on a single module as possible. This made for a compact footprint
at a price that could be justified to the auto industry.
Not to be negative, but PXI is not an ideal platform for designing high density, multi cell, battery simulators. Not impossible, but the modular format did limit some of the specifications. However, the availability of all of the other modules in PXI to address this test as well as the wide acceptance of PXI indicated that we needed to focus on a PXI solution.
Clearly, the end product had to be reliable, compact and safe to use. A PXI chassis can typically support up to 18 PXI peripheral slots, so the requirement to make the design compact required each single-slot wide module to simulate 6 off cells of a battery stack – requiring 16 modules in a chassis to simulate a 96 cell stack. This density places some space restrictions on the design of each cell. The requirement was on an urgent schedule to meet the end user project times, so for the most part components had to be available off the shelf.
Each cell had to be capable of providing up to 300mA and generate voltages in excess of 4.2V for each cell, which was a significant challenge for the chassis backplane to provide enough power for each slot and for the system as a whole – keep in mind we also needed fast transient response to simulate a battery.
The PXI backplane can supply up 6A to each module on the 5V supply – by far the most capable of the PXI backplane supplies. However, DC conversion from 5V is notoriously inefficient, so significant losses had to be expected in the design, those losses contributing to the thermal load on the chassis. The solution to this problem was to take the power for the cells predominantly from the +5V supply, but to supplement it with power from the +12V and -12V supplies.
Each cell of the battery simulator uses a fixed isolated DC to DC converter to provide an isolated supply which is then regulated by a fast acting linear regulator. The fast linear regulator is required to allow the battery to be emulated at a point remote from the PXI supply without introducing poor voltage regulation on the output, and without the use of an excessively large output decoupling capacitor.
The linear regulator was required to shed significant amounts of power under the worst case load conditions, but clearly each cell on the module was very constrained on space which in turn limited the amount of heat sinking available to cool the regulator. The solution to this problem was to use a regulator designed specifically for automotive use that could withstand high temperatures, had built in thermal protection and used the PCB copper surface for cooling. The efficient cooling system in a PXI chassis ensured these copper areas were well ventilated – an aspect of the PXI standard that works very well for this type of application particularly when the active device is placed low down on the PXI module.
Safety and isolation also presented significant design challenges. In a battery stack the 96 cells are connected in series, if each cell is set to its nominal 4.2V output voltage the result is a potentially lethal output voltage. The isolation barrier had to be designed to withstand up to more than double of the common voltage on each cell and yet still have programmatic control of the battery cell. The proposed design used digital isolators to provide the control interface to the PXI backplane, and a safety interlock systems was defined that allows the user to connect the modules in such a way that if that user disconnected the cable assembly from the front of the PXI module either that module or all the modules in the system would close down.
The BMS also had to simulate a cell being charged. Simple power supplies are unidirectional—they either source or sink current, not both. The BMS had to be capable of doing both, but the sink current required could be much less than the source current. This problem was solved by integrating a programmable current load in the design that would pre-load the power supply, when the BMS was being “charged” the current load would ensure the power supply was still sourcing current.
All these challenges and solutions were assessed and a proposal put together for a new 6 channel battery simulator to be designed and supplied by Pickering Interfaces to DMC. After the exchange of ideas on modifications to the design and questions to clarify the expected performance, an order was placed for a complete battery stack emulator.
Pickering Interfaces rapidly progressed implementing the design and constructing the first modules, which became the model 41-752 PXI Battery Simulator Module. The challenges were not confined simply to the hardware design. To test and use the 41-752 required the support of a software support team working in parallel with the hardware design team to produce firstly software capable of testing the module hardware and secondly software, including manual soft front panels, that the user could use to control each cell in each module.
Once this module was shown to have satisfactory performance the remaining modules were constructed and shipped to DMC. DMC integrated the solution into their test system for onward shipment to the end user. After successful trials and some very minor adjustments of the design the user accepted delivery of the module and the 41-752 was commercially launched.
Although the use of PXI as platform generated a number of difficult challenges for this application the flexibility and speed of design possible with PXI was demonstrated by how quickly the design progressed. The backplane power supplies, the PCI control bus, chassis ventilation system and PC based software all allowed the design to proceed at a high rate of progress. In the case of the 41-752 from the placement of the order to the dispatch of the first system took just eight weeks, a feat that would not have been possible on other hardware platforms.
The 41-752 now commercially available, it offers 6 channels of battery emulation for voltages up to 7V and current up to 300mA. The high isolation barrier, rated at 750V, permits many cells to be placed in series on D Type user connectors. A flexible safety interlock systems allows systems to be connected that are intrinsically safe to use. The Pickering 40-923 PXI chassis used to support the high power modules was shown to be capable of providing the power and the current needed to meet the task of emulating up to a 108 cell battery stack.
Functional diagram of PXI battery simulator module (model 41-752)
16 off battery simulator model supported by PXI chassis (model 40-923)
In addition to the Battery Simulator, PXI modules supported the simulation of the temperature sensors, Analog and Digital I/O, high voltage switching of the battery stack output, as well as communicating with the BMS via a CAN port. The system consisted of two PXI chassis and some external circuitry. The entire system was a compact unit that fitted into a 1.5 meter deep rack and met all the customer expectations.