Testing and quality control in automotive engineering: Taking a look at the battery test landscape.
When designing a battery, engineers must think at cell, module, battery management system (BMS) and pack level. However, they must also consider vital electrical, thermal, mechanical, control and safety elements. Nigel Taylor, Founder of BatteryDesign.Net – an educational website providing knowledge, information and ideas on battery design – shared his key considerations for battery design, testing and quality control at Advanced Engineering in Birmingham.
KNOW YOUR PRODUCT REQUIREMENTS
There are many considerations to take into account when embarking upon the design of a new battery – how many cells are needed, how will they be connected together, what energy and power density will be required, and so on. When starting a project, it is vital to understand exactly what outcomes you are trying to achieve.
“The same as any system you design, you need to know exactly what your product requirements are,” says Taylor. “When developing a battery pack, you need to start with some format requirements, and if you can’t write them down at the beginning you’re really going to struggle to design anything. Look backwards through the actual design chain – start at the end and work out what you can pull to the front of the system design process in terms of generating data and validation.”
The main chemistry currently used in batteries is lithium-ion, however there are many variations on this. The cathode is a lithium transition metal oxide, such as manganese, cobalt, or a combination of transitional metals. The anode is normally a graphite-based material which can intercalate or release lithium. The design and engineering of a cell, meanwhile, is a complex systems approach that requires numerous specialists. As a battery pack designer, it is important to understand the cell in detail so that you can optimally interface with it in terms of mechanical, electrical and thermal design.
“When we put battery packs together, we have to place a large number of cells in parallel in order to get to a certain voltage and energy density that we need,” Taylor explains. “A battery pack is as strong as its weakest cell, and so you need to take into account charging and discharging cycles, charging times, and durability. If one cell is underperforming, all the cells in that string or series will be limited by that, and eventually the whole pack. Cells aren’t perfect, they have an internal resistance which correlates to the voltage and therefore determines the heat generated within the cell. By playing around with these different elements, you can figure out what the actual maximum power of that cell is and, when arranged in parallel with one another, you can increase the power capacity while keeping the voltage the same throughout the series of cells.”
Benchmarking is also an important step in the cell and battery pack design process, allowing engineers to learn and develop a future roadmap for their products. For battery pack design in particular, there are several key metrics to use, such as pack gravimetric energy density and cell-to-pack mass ratio.
BUILDING A BMS
The hardware and software control unit of the battery pack, the BMS is a critical component that measures cell voltages, temperatures and current. It also detects isolation faults and controls the contactors and thermal management system in order to protect the operators of the battery-powered system and the battery pack itself against overcharge, over-discharge, overcurrent, cell short circuits and extreme temperatures.
“Remember, a cell changes capacity with temperature, age, what it has done previously, how it has been discharged, how hard it is to charge, and so on,” says Taylor. “To ensure that all cells within your pack are operating optimally, you need to think about how you connect them all equally. Each of these cells needs to behave and discharge in the same way and at the same time, otherwise individual cells will reach their voltage limits faster than others. This makes the design of the BMS absolutely vital so that it can measure cell voltage, current and temperature in order to estimate state-of-charge. There is a lot of estimation going on to work out what is happening to each cell, and thus the battery pack as a whole.”
TEST, TEST AND TEST AGAIN
Testing is an integral part of the battery pack design process, however it is also a costly one. This is why it is important to start testing your designs and components right from the very beginning, in order to collect the data you need in order to model and predict cell behaviour in a wide variety of scenarios. The testing process should cover everything from a single cell through to the complete battery pack.
“Specifications for cells are getting more difficult to locate and are containing less data, so you need to be able to generate your own data in order to inform your design process going forwards,” Taylor explains. “Prior to having an actual cell the chemistry will need to be tested as a half cell against a reference electrode in order to give directional data for the electrochemists. The battery cells will then mature over the course of several stages, and at each stage these cells will be performance-tested, undergo ageing cycle testing, parameterisation for models, and stages of legislative testing.”
The final sign off will be at complete pack level, and should be against test data, BMS control response, legislative test results, and against the model of the pack.