An Overview of Battery Pack Design and Testing Considerations
Electric vehicles are clearly a rapidly growing part of the automotive scene. They promise low or no emissions, conceivably low cost of energy from the power grid, yet they will continue to deliver us safely from here to there. However, electric vehicle design and manufacturing is clearly a paradigm shift for the automotive industry – new drive systems, technologies, and test plans.
Electric vehicles are bringing new test and validation challenges as the electronic and software content of the vehicles grow. In this article, we will discuss the basics of electric vehicle battery pack designs and some of the tests that should be performed on them in a manufacturing environment. We’ll also discuss a conceptual solution to this complex testing challenge.
The Motivation for EV Battery Testing
The battery packs used as the rechargeable electrical storage system (RESS) in electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) are large and complex. Controlled release of the battery’s energy provides useful electrical power in the form of current and voltage. Uncontrolled release of this energy can result in dangerous situations such as release of toxic materials (i.e. smoke), fire, high pressure events (i.e. explosions), or any combination thereof.
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, an 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 (BMS).
One of the major validation and safety challenges to be tackled in modern EVs, HEVs, and PHEVs concerns the effective testing of the Battery Pack itself and the Battery Management Systems (BMS) – the complex electronic system that manages the performance and safety of the battery pack and the high levels of electrical energy stored within. In the sections below, we will describe both the battery pack and the BMS in greater detail.
Inside an EV Battery Pack
Battery pack designs for EVs are complex and vary widely by manufacturer and specific application. However, they all incorporate combinations of several simple mechanical and electrical component systems that perform the pack’s basic required functions.
Cells and Modules
Battery cells can have different chemistries, physical shapes, and sizes as preferred by various pack manufacturers. However, the battery pack will always incorporate many discrete cells connected in series and parallel to achieve the pack’s total voltage and current requirements. In fact, battery packs for all electric drive EVs can contain several hundred individual cells.
The large stack of cells is typically grouped into smaller stacks called modules to assist in manufacturing and assembly. Several of these modules will be placed into a single battery pack. The cells are welded together within each module to complete the electrical path for current flow. Modules can also incorporate cooling mechanisms, temperature monitors, and other devices. In most cases, modules also allow for monitoring the voltage produced by each battery cell in the stack by the BMS.
Safety Components and Contractors
Somewhere in the middle, or at the ends, of the battery cell stack is a main fuse that limits the pack’s current under a short circuit condition. There is also commonly a service plug or service disconnect located somewhere within the battery stack’s electrical path, which can be removed to split the battery stack into two electrically isolated halves. With the service plug removed, the exposed main terminals of the battery present reduced electrical danger to service technicians. A high voltage interlock circuit will often run throughout key elements and connection points of the pack to establish hard-wired safety functions.
The battery pack also contains relays, or contactors, which control the battery pack’s electrical power distribution to the output terminals. In most cases, there will be a minimum of two main relays that connect the battery cell stack to the pack’s main positive and negative output terminals, those supplying high current to the electrical drive motor. Some pack designs will include alternate current paths for pre-charging the drive system through a pre-charge resistor or for powering auxiliary busses with their associated control contactors. For obvious safety reasons these contactors are all normally open.
Temperature, Voltage, and Current Sensors
The battery pack also contains a variety of temperature, voltage, and current sensors. At least one main current sensor will measure the current being supplied by, or sourced to, the pack. The current from this sensor can be integrated to track the actual state of charge (SoC) of the battery pack. The state of charge is the pack capacity expressed as a percentage and can be thought of as the pack’s fuel gauge indicator. The battery pack will also have a main voltage sensor, for monitoring the voltage of the entire stack and a series of temperature sensors, such as thermistors, located at key measurement points inside the pack.
Collection of data from the pack sensors and activation of the pack relays are accomplished by the pack’s battery management system (BMS). The BMS is also responsible for communications with the world outside the battery pack and performing other key functions, as described in the following section.
Inside an EV Battery Management System (BMS)
The BMS controls almost all electronic functions of the EV battery pack, including battery pack voltage and current monitoring, individual cell voltage measurements, cell balancing routines, pack state of charge calculations, cell temperature and health monitoring, ensuring overall pack safety and optimal performance, and communicating with the vehicle engine control unit (ECU).
In a nutshell, the BMS system must read voltages and temperatures from the cell stack and inputs from associated temperature, current, and voltage sensors. From there, the BMS must process the inputs, make logical decisions to control pack performance and safely, and report input status and operating state through a variety of analog, digital, and communication outputs.
Modern BMS systems for EV applications are typically distributed electronic systems. In a standard distributed topology, routing of wires to individual cells is minimized by breaking the BMS functions up into at least two categories. The monitoring of the temperature and voltage of individual cells is done by a BMS sub-module board, which is mounted directly on each battery module stack. Higher level functions such as computing state of charge, activating contactors, etc. along with aggregating the data from the sub modules and communicating with the ECU are done by the BMS main module.
The sub-modules and main module communicate on an internal data bus such as controller area network (CAN). Power for the BMS can be supplied by the battery stack itself or from an external primary battery such as a standard 12V lead acid battery. In some cases, the main module is powered externally, while the sub modules are powered parasitically from the battery modules to which they are attached.
BMS State of Charge Calculation
The BMS is responsible for tracking a battery pack’s exact state of charge (SoC). This may simply be for providing the driver with an indication of the capacity left in the battery (fuel gauging), or it could be used for more advanced control features.
For example, SoC information is critical to estimating and maintaining the pack’s usable lifetime. Usable battery life can be dramatically reduced by simply charging the pack too much or discharging it too deeply. The BMS must maintain the cells within safe operating limits. The SoC indication is also used to determine the end of the charging and discharging cycles.
To measure SoC the BMS must include a very accurate charge estimator. Since you can’t directly measure a battery’s charge, the SoC must 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.
BMS Cell Balancing Functions
The BMS must compensate for any underperforming cells in a module, or stack, by actively monitoring and balancing each cell’s SoC. In multi-cell battery chains, small differences between cells (as a result of production tolerances, uneven temperature distribution, intrinsic impedance, and/or aging characteristics) tend to be magnified with each charge and discharge cycle. In EV applications the number of cycles can be very high due to the use of regenerative braking mechanisms.
When degraded cells with a diminished capacity exist within the battery stack, the performance of the pack as a whole is degraded. During the charging cycle, there is a danger that degraded cells would be subject to overcharging before the rest of the cells in the chain reach their full charge. As a result, temperature and pressure may build up and possibly damage that cell. The weakest cell will have the greatest depth of discharge during discharging and will tend to fail before the others. The voltage on the weaker cells could even become reversed as they become fully discharged before the rest of the cells resulting in the early failure of the cell.
Cell balancing is an active way of compensating for weaker cells by equalizing the charge on all the cells in the chain and thus extending the battery pack’s usable life. During cell balancing, circuits are enabled which can transfer charge selectively from neighboring cells, or the entire pack, to any undercharged cells detected in the stack.
To determine when active cell balancing should be triggered and which target cells, the BMS must be able to measure the voltage of each individual cell. Moreover, each cell must be equipped with an active balancing circuit.
State of Health and Diagnostics
The state of health (SoH) is a measure of a battery’s capability to safely deliver its specified output. This metric is vital for assessing the readiness of the automobile and as an indicator of required maintenance.
SoH metrics can be as simple as monitoring and storing the battery’s history using parameters such as number of cycles, maximum and minimum voltages and temperatures, and maximum charging and discharging currents, which can be used for subsequent evaluation. This recorded history can be used to determine whether it has been subject to abuse, which can be an important tool in assessing warranty claims.
More advanced measures of battery SoH can include features such as automated measurement of the pack’s isolation resistance. In this case, specialized circuits inside the battery pack can measure the electrical isolation of the high current path from the battery pack ground planes. Such a safety system could preemptively alert the operator or maintenance technicians to potential exposure to high voltage.
Most BMS systems incorporate some form of communication with the world outside the battery pack, including the ECU, the charger controller, and/or your test equipment. Communications interfaces are also used to modify the BMS control parameters and for diagnostic information retrieval.
CAN (controller area network) is the most common communications bus in automotive applications, although automotive ethernet, RS232 / RS485 serial, SPI, TCP/IP, or other networks could be used. CAN networks come in various implementations and can include a range of higher level “application layer” protocols like unified diagnostic services, OBD II, J1939, etc.
Aside from a digital bus, separate analog and/or digital inputs and outputs should be considered as supplemental parts of the BMS interface and communication. Discrete inputs and outputs can be used for redundancy and for operations requiring a separate interface such as activating an external contactor, fan, or dashboard lamp.
Testing an EV Battery Pack
Developing a test strategy for an assembly as large, complex, and powerful as an EV battery pack can be a daunting task. Like most complex problems, breaking the process down into manageable pieces is the key to finding a solution. Accordingly, testing only at carefully selected points in the development and manufacturing process will reduce the effort required. These key points for many pack manufacturers include BMS development, pack development, module production, and pack production. What tests are performed at each step is a different matter altogether and depends on the specifics of the process and the device.
BMS Development Testing
During BMS Development, engineers need a way to reliably test the BMS under real-world conditions to complete their verification and validation plans. Test strategies such as hardware-in-the-loop (HIL) testing are often performed at this stage. HIL testing involves simulating physical inputs and external connections to the pack while monitoring its outputs and behavior relative to design requirements.
Accurately simulating all the conditions to which a BMS may be subjected during real world operation is not easy. However, one must consider the long-term cost of skipping testing over a full range of conditions, remembering that any given condition could lead to a critical failure in the field. In the end, simulating nearly every combination of cell voltages, temperatures, and currents you expect your BMS to encounter is really the only way to verify that your BMS reacts as you intended in order to keep your pack safe and reliable.
Pack Development Testing
At the pack development stage, engineers are typically concerned about testing the entire assembly through various types of environmental stress testing as part of design validation or product validation plans. Environmental stress could include exposure to temperature extremes, thermal shock cycling, vibration, humidity, on-off cycling, charge discharge cycling, or any combination of these. The testing requirements here typically include performing a full batch of performance tests on a pack both before and after application of the stress. Live monitoring of the pack throughout the environmental stress period may also be required.
Module Production Testing
Requirements for module level testing vary widely depending on the actual design of the system. The main testing to be done at this point involves simple charge/discharge testing to ensure that connections between cells are robust and can handle the intended current loads without failing or shedding excessive heat. Further testing could involve ensuring the cell voltages are reported correctly, that the cells are balanced, and/or that the cooling and temperature monitoring sensors are working properly.
Pack Production Testing
Pack level testing is done after the pack has completed, or is at least very close to, the point of final assembly, or end of line (EOL). At this stage, the pack must complete a full batch of tests to ensure proper functioning of every major pack subsystem (functional testing). These tests include simple pinout and continuity checks, confirming proper relay operation, testing functionality of safety devices such as high voltage interlocks, carefully measuring the isolation resistance under high potential (hi-pot testing), and testing proper communications and operation of the BMS.
After EOL functional testing is completed, packs may also be subjected to charge/discharge cycling and drive profile cycling, which will simulate the typical conditions the pack will see when integrated into the EV drivetrain. Packs can also be run through active cell balancing routines to set each cell’s initial charge state to a nominal condition or set the Pack SoC to a level appropriate for shipping and storage.
EV Battery Pack Testing Solutions
Once you have decided where you are testing and what you are testing, you need to determine how you will be testing. Since every battery pack design has unique elements, and since testing requirements vary accordingly based on agreements between the manufacturer and end user, in reality, there is no one-size-fits-all solution for everyone’s battery pack testing needs.
Off-the-Shelf Testing Solutions
That being said, some portions of the testing, such as charge/discharge/drive cycle evaluation, are standardized. As such, pre-packaged, off-the-shelf hardware and software solutions exist for these particular test steps. These systems typically use only the positive and negative output terminals, as these are the only elements common to every battery pack. These turn-key systems may even allow you to add in options required to test components and functions specific to your battery pack, such as CAN communications, external relay activation, etc.
When considering off-the-shelf systems for use in your test plan, make sure to ask yourself these three basic questions:
- Are you getting everything you need just the way you want it, or are you settling for what the other person needs?
- Are you using everything you will pay for, or are you paying for things you won’t use?
- Is it flexible enough to accommodate your future needs but not so flexible that it becomes cumbersome to use?
Arguments for a Customized, Modular Test System Approach
Building a functional test system tailored to your battery pack and your specific testing needs often sounds like a more costly and time-consuming approach, and it can be. However, the route you take to achieve that end goal makes a world of difference in the outcome and in long-term ROI.
Choosing a modular hardware and software testing platform tailored to meet your requirements can be used to jump start this approach, making it a very viable option. This is especially true if the platform you choose leverages proven commercial technologies and open industry standards.
In the end, this modular platform-based testing approach can have several benefits:
- It can dramatically lower the cost of the test system, both in initial capital expenditure and overall cost of ownership, through the use of commercial technologies and standards.
- It can increase your test throughput with fast measurement hardware and software capable of managing multiple test routines in parallel.
- The time required to adapt such test systems for new products will decrease through the use of flexible, modular software and hardware.
- You can get exactly what you need, the way you want it. You can get everything you paid for and your test station will be flexible, without being cumbersome to use.
- The system is tailored to your product and workflows, resulting in a simplified user experience, shorter learning curve, and corresponding personnel time savings.
A Platform Approach
The preceding sections describe the challenging problem statement of thoroughly testing a complex, high power system like an EV battery packs and BMS.
It is highly desirable to achieve standardization, cohesion, and efficiency of testing throughout the EV component product cycle and during inevitable future product evolution. It is best to take a platform-based approach to address this testing challenge to achieve this. This means establishing a unified suite of test equipment built on common reusable building blocks (both hardware and software) and utilizing various configurations of this platform to cover testing of battery cells, modules, packs across various testing regimes (R&D, validation, HIL, production, and lifecycle tests).
This requires incorporating reliable software and hardware architectures and flexible and reliable subsystem components, which can be customized to specific use cases and changing requirements. Utilizing high-quality COTS (commercial off-the-shelf) hardware assembled from best-in-class instrumentation vendors typically improves system performance, reliability, and maintainability while significantly reducing the engineering effort involved in deploying the system.
Battery packs used in today’s EVs are complex systems designed to provide safe and efficient electrical power. As such, a comprehensive testing strategy to evaluate possible safety and performance considerations is essential to the battery pack manufacturing process. However, testing requirements often vary from manufacturer to manufacturer and from one battery design to another, further complicating the testing process. Using a customized modular test system can be an efficient, cost-effective approach to conducting necessary battery pack testing in a manufacturing environment.