There are many things that need careful consideration in the design and management of a battery pack for hybrid or electric vehicles
To the uneducated, what could be simpler than connecting some cells together and putting them in a box with a + and – marked on the outside? If only it was that easy.
Designing a battery starts with understanding the fundamental performance requirements of the vehicle. The key vehicle targets to start with are acceleration, top speed and range. Acceleration and top speed will give an indication of the power and torque requirements of the electric motor and factoring in any losses (efficiency etc) will give the levels of power and current the battery has to supply. Analysing the vehicle range will give what capacity the battery has to have – usually measured in kilowatt hours (kWh).
Fundamentally, cells can be divided into two kinds – primary cells (non-rechargeable) and secondary cells (rechargeable). For the purposes of this article, only secondary cells are considered. The current cell chemistry of choice will usually be something from the Lithium family of chemistries and identifying exactly which one can be a difficult task as there are so many variations in the cell chemistries, characteristics and technology maturity.
There are three main architectures for lithium cells: cylindrical, prismatic and pouch. Cylindrical is a familiar cell format – for many years we have been used to primary and more recently secondary cells in sizes such as AAA, AA, C and D for home electronics and toys. A prismatic cell is a ‘flat’ format like that of a book but is enclosed in a hard casing. A pouch cell is another ‘flat’ format cell that has a foil enclosure rather than a hard casing and while pouch cells may offer packaging benefits, careful mechanical design to hold or mount the cells will be needed.
A cell can be optimised for power density or energy density by altering the thickness of the active chemical layers on the electrodes. As hybrid vehicles typically have a reduced electric range, power density is more suited where the pack can be optimised in size and weight to produce power. For Pure EVs, capacity is the key for the vehicle driving range, therefore the pack can be optimised in size and weight to store energy.
A process of iteration and refinement is necessary, where managing trade-offs becomes a constant task to identify the most appropriate cells for the application from the large and growing supply base. The cell data sheets provided by the manufacturers are used to build a mathematical model of the cell so that the characteristics of the full battery pack can be understood. The choice of battery management system (BMS) is an important one as, firstly, some are better than others. Which BMS is chosen may lead to defining how many cells make up a module and how many modules make up a pack. A future proActive article will look at Battery Management Systems in more detail.
The electrical definition of the pack can be determined with respect to how many cells are connected in series and parallel to give the overall battery pack specification for voltage, current and capacity. Simulations that include a basic control strategy will predict range and performance figures for the whole vehicle. Included in the ‘performance’ is battery lifetime and this is a large consideration from a customer point of view due to the cost of ownership and from an OEM point of view regarding warranty. Cell data produced by the cell manufacturers will include lifetime figures for the number of cycles a cell can be subject to before the cell performance starts to degrade. This is typically derived from empirical data from the supplier or a third party. There are other factors that affect lifetime such as temperature and storage. Testing has shown that temperature is the factor that has the largest effect on lifetime. This is true for both the operating temperature and the temperature cells may be stored at.
Cycle life can also be affected by the state of charge (SOC) the cells have when they are stored. If they’re stored with a high SOC, it is shown that there will be a decrease in cycle life. It must be understood that when a cell reaches the end of its cycle lifetime, it doesn’t cease to operate but it does start to degrade. The way the cell degrades is generally a capacity loss and the auto industry has defined the end of useful life as when the whole battery pack capacity has dropped to 80% of the capacity when new. What this means practically is that the vehicle range will decrease over lifetime – the vehicle is still usable but the range will never be able to be restored.
This degradation however can be mitigated in a number of ways. One method is to set a narrower band of usable voltage compared with the one stated in the cell data sheets. The result is that the cell can be taken through more cycles before the degradation occurs but the downside is that the pack will need more cells for the desired voltage, current and capacity. It’s one of the many trade-offs that needs managing when designing battery packs. Choice of cell chemistry can also mitigate lifetime issues but the trade-off here may be specific energy or power density.
Taking the above into account will start to identify the basic complete pack specification. The detailed design then follows and there are a number of considerations here too. Once the electrical architecture has been defined (what cells; how many cells connected in series or in parallel; how many cells per module etc), the mechanical design of the pack can be conducted which needs to take account of the vehicle dynamic forces. The structural integrity of the pack and the pack installation into the vehicle can be defined with the assistance of structural analysis tools. The pack and installation will need to be robust enough to withstand the forces subjected by normal road use as well as crash pulses set down in legislation and corporate test requirements. The vehicle environment is a very dynamic thing where loading and vibrations are present and changing all the time. Understanding this environment should lead to a pack that is fit for purpose.
An example of a production battery pack is the one that’s installed in the Tesla Roadster. This uses ‘commodity cells’ typically found in household electronics such as laptop computers. They are cylindrical cells,
designated ‘18650’ and are similar in size to a normal AA size cell. What makes the pack stand out is the large number of cells – there are 6,831 of them making the mechanical design and electrical connectivity a challenge. What’s becoming more commonplace is the production of larger format cells which may be pouch or prismatic in their architecture. The benefits are that there should be fewer cells to assemble and connect – a pack equivalent to the Tesla pack may only need around 100 large format cells.
Another aspect of the mechanical design is design for safe assembly. As the cells are supplied with typically 50% state of charge, it doesn’t take many electrical connections before a ‘high voltage’ is present. The health and safety aspect for assembly must be accommodated in the mechanical design stage. A service disconnect must also be included so that the terminal voltage can be isolated by means of a mechanical, removable disconnect. This has advantages in the vehicle assembly process but also improves the safety of vehicle occupants and first responders to an incident such as a vehicle crash
The thermal management of the battery needs to be consistent with the cell characteristics with respect to operational temperature. This reiterates the earlier comment on temperature and lifetime. Again, choice of cell chemistry will define the requirements for a thermal management system, which in some cases may be relatively straightforward but in other cases could be a sophisticated system with significant costs. The vehicle operational temperature is usually defined in OEM corporate standards and might be (for example) between -35 degrees C to +45 degrees C. However, what this doesn’t account for is heat-soak where, for example, the rear storage compartment temperature may reach 80 degrees celsius or an engine bay temperature which may be higher still. It’s not all about cooling though. Typically, damage will be caused if lithium cell chemistries are operated in temperatures approaching freezing so a method of warming might need to be considered. If the cell temperatures are not controlled in some way, it will cause the cells to operate outside of their limits. At a minimum, this may have a detrimental effect on performance and lifetime but in serious cases it may result in thermal runaway with possible explosion or fire consequences.
To sum up, the battery pack will probably be the single largest, most costly system that is installed in the vehicle and to design the most appropriate pack that meets all requirements for performance, durability and crashworthiness is a complex task. The use of analysis techniques is an important factor in battery design to make sure the end result is robust and fit for purpose.
Author: Phil Barker