AUTOMOTIVE DESIGN SECTION TITLE
Power density versus energy density in battery chemistry. Graph via
BatteryDesign.Net
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 Te 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. Tis 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. Tere 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. Tis 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. Te 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. Te 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.”
Te 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.
For further information visit
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The relationship between mechanical, electrical and thermal design is vital to cell design
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