MUR Blog - Cell Selection: Things to Consider

Cell selection is one of the most important tasks for an electric FSAE car as the battery pack essentially fuels the car. This decision, made very early in the design stage affects not only the tractive system but also the performance of the whole car. This selection process is not an easy one to make as cells come in many different chemistries and various packaging styles; each with their own advantages and disadvantages. On top of that, there is a wide range of manufacturers such as A123, Kokam, EIG, Melasta etc. Ideally, you’d want to build a tractive system with excellent safety, high specific energy and specific power, good temperature characteristics, long cycle life and at low cost. [1]

Figure 1: Comparison of different types of Lithium Ion Batteries [1]

Let’s now delve into how we carried out our selection process. With reference to the table above, Lithium manganate and Lithium iron phosphate are relatively safe chemistries. Considering that this will be MUR’s first ever electric car, safety is extremely important as inexperience and unsafe practices are a recipe for disaster. Indeed, safety takes precedence over the performance of the car because whatever you do, you do not want a fire or an explosion that could damage and hurt personnel or property. Thus, our battery pack of choice will be A123s AMP20M1HD-A which is a lithium iron phosphate pouch cell that offers high usable energy and is very abuse tolerant (perfect for noobs like us!). These cells are expected to have a nominal voltage of 3.3V and an operating voltage range between 2.0V and 3.6V. This is lower than other chemistries such as lithium manganate (4.2V) or lithium polymer (3.7V, 4.2V). The lower voltage of Lithium iron phosphate means that more cells are needed in series to achieve a given system voltage, and the watt-hour content is correspondingly lower for a given amp-hour capacity. [4] If we had gone for a lithium polymer or a lithium manganate cell, our battery pack would have been smaller in size and hence be easier to fit in the car from a mechanical point of view, but we decided that safety took top priority.

Figure 2: Li-ion cell formats - Cylindrical, Prismatic and Pouch [2]

A123 also offers cylindrical cells which do not require as much design effort but pouch cells are packaged more efficiently [5] and their lack of metal enclosure around each cell reduces weight of the overall pack. They do, however, require more design time as the cells need support and must have room to expand in the battery enclosure. Both cell faces must also be subjected to evenly distributed pressure, while allowing for cell expansion when fully charged; this will allow the cells to operate at their peak performance and achieve optimum cycle life. [3]

Selecting AMP20M1HD-A also allowed us to configure the cell in series only (i.e. No cells in parallel) because these cells can supply the required current for our car while on track. This greatly simplifies the design of the accumulator pack and reduces fire risk from electrical shorts. However, there are challenges in using a series configuration too such as cell matching especially when replacing cells in an old battery pack. This is an issue because old cells generally have less capacity than the new ones and the overall capacity of a pack made by cells in series is determined by the cell with the lowest capacity. Cell balancing[i] while charging and discharging can also be an issue; hence why a Battery Management System is needed to maximise the capacity of the pack and to ensure the cells are not over-charged/over-discharged.

While safety will always be paramount, it will be interesting to see what cells MUR decides to use in the future to maximise performance, reduce weight and enable easy cell replacement in a battery pack.

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About the Author:

Foad Munir
Senior Accumulator Engineer, 2017

[i] Individual cells can have different capacities due to different internal resistance and some other factors such as manufacturing variances or cells from different production runs being mixed together, which results in different levels of State of Charge (SOC). This means that some cells might reach 100% SOC before the others resulting in charging being stopped and therefore some cells are still below their maximum capacity; reducing the capacity of the whole pack. So, to be able to use all the pack capacity, these cells must be brought to the same level of SOC as the other cells in the pack. This is done using various techniques and is referred to as Cell balancing.