Electric vehicles (EVs) 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 continue to deliver us safely from here to there. However, EV design and manufacturing is a clearly a paradigm shift for the auto industry – new drive systems, new technologies… and new test plans. EVs are bringing new test and validation challenges to the automotive industry 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 battery packs in a manufacturing environment.
Motivation for EV Battery Testing
The battery packs used as the rechargeable electrical storage system (RESS) in 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, abnormally high discharge rates, 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 system (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 which perform the basic required functions of the pack.
We will start with the actual battery cells, which 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 total voltage and current requirements of the pack. In fact, battery packs for all electric drive EVs can contain several hundred individual cells.
To assist in manufacturing and assembly, the large stack of cells is typically grouped into smaller stacks called modules. Several of these modules will be placed into a single battery pack. Within each module the cells are welded together 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.
Somewhere in the middle, or at the ends, of the battery cell stack is a main fuse which limits the current of the pack under a short circuit condition. Also located somewhere within the electrical path of the battery stack is a “service plug” or “service disconnect” 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 no high potential electrical danger to service technicians.
The battery pack also contains relays, or contactors, which control the distribution of the battery pack’s electrical power to the output terminals. In most cases there will be a minimum of two main relays which connect the battery cell stack to the main positive and negative output terminals of the pack, 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 which will also have their own associated control relays. For obvious safety reasons these relays are all normally open.
The battery pack also contains a variety of temperature, voltage, and current sensors. There will be at least one main current sensor which measures 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 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 BMS or its battery monitoring unit (BMU). 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
Almost all electronic functions of the EV battery pack are controlled by the BMS, 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, making logical decisions to control pack performance and safety, and reporting input status and operating state through a variety of analog, digital, and communication outputs.
Modern BMS systems for PHEV 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 2 categories. The monitoring of the temperature and voltage of individual cells is done by a BMS ‘sub–module’ or ‘slave’ circuit 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’ or ‘master.’
The sub-modules and main module communicate on an internal data bus such as a 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 the 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 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.
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 PHEV applications the number of cycles can be very high due to the use of regenerative braking mechanisms.
Assume degraded cells with a diminished capacity existed within the battery stack. During the charging cycle, there is a danger that once it has reached its full charge it will be subject to overcharging until 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. During discharging, the weakest cell will have the greatest depth of discharge 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 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.
In order to determine when active cell balancing should be triggered, and on 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
A battery’s state of health (SoH) is a measure of its 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 the battery 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.
In automotive applications, CAN is the most common communications bus, although RS232/RS485 serial, TCPIP or other networks could be used. CAN networks come in a variety of implementations and can include a range of higher level protocols.
Aside from a digital bus, separate analog and/or digital inputs and outputs could be considered as BMS communication. Discrete inputs and outputs can be used for redundancy 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 critical points in the development and manufacturing process will reduce the size of the problem. Key points for most pack manufacturers are BMS development, pack development, module assembly, and pack assembly. However, the tests that are performed at each step is a different matter altogether, and depend 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. Testing such as hardware-in-the-loop (HIL) is often performed at this stage. HIL testing involves simulating physical inputs and external digital connections to the pack while monitoring its outputs and behavior relative to design requirements.
It is not easy to accurately simulate all of the real-world conditions a BMS will be subjected to. But what does it cost you to skip testing over every condition? 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 Assembly 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 Assembly 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 the service disconnect, 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 in order to set the initial charge state of each cell to a nominal condition, or to 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 is unique and testing requirements are primarily left up to the end user and manufacturer to agree upon, 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 in specific, are standardized. As such, pre-packaged, off–the–shelf hardware and software solutions exist for these particular test steps. These systems make use of the only elements common to every battery pack, the positive and negative output terminals. 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 guy needed?
- Are you using everything you’re going to pay for, or are you paying for things you won’t use?
- Is the system flexible enough to accommodate your future needs, but not so flexible that it becomes burdensome to use?
Arguments for a Customized, Modular Test System Approach
Building a functional test system customized to your battery pack and your specific testing needs often sounds like a more costly and time consuming approach. However, the route you take to achieve that end goal makes a world of difference in the outcome.
Choosing a modular hardware and software testing platform which can be customized 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, a testing approach utilizing a modular platform offers several benefits:
- It can dramatically lower cost of the test system, both in initial capital expenditure, and in 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 redesign 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.
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.
Brent Hoerman is an Engineering Manager at Eaton in Galesburg, MI, and a subject matter expert in high voltage lithium-ion battery pack modeling, design, and testing. Brent holds a PhD in Materials Science and Engineering from Northwestern University and a BS from the University of Wisconsin – Stevens Point. Prior to joining Eaton, Brent worked at DMC in Chicago, IL and specialized in development of battery test systems for several organizations.