34 The BMS String Control Unit provides safety monitoring, diagnostics and control of the system. Its primary function is to maintain the lithium-ion cells within their safe operating range, regardless of the demands placed on them. This gives the BMS the ability to accurately predict operating situations with the potential for creating over- or under-voltage conditions, as well as adjusting performance for high or low temperatures, and reacting quickly and safely to prevent unsafe combinations from occurring. With hundreds or even thousands of lithium-ion cells in a heavy-duty vehicle, mitigating the risk of thermal runaway is key. This can be achieved with hightemperature barrier materials, software algorithms to find defects and early gas detection. Containment measures ensure any potential thermal incidents are localised within the pack, preventing damage to the rest of the system. Cell characterisation From a performance standpoint, it is not the form factor of the cell but the quality that matters. Detailed characterisation is needed to ensure the quality and consistency of cells, making sure one poor-quality cell does not pull down the performance of the others in the module, or even the overall pack. This has led to the creation of an architecture, rather than a single module, which uses three modules with varying serial and parallel configurations to provide different voltage and energy combinations, but these can all be built on the same lines within a 3 ft envelope that gives flexibility for production. One advantage of this architecture is that another type of battery cell can be used in the module for a higher-power version in the same 2170 format; for example, for the 4680 or 4695 form factor in future. The architecture with NMC 2170 cells has two bricks of cells with a cooling plate in between. Water glycol runs through the cooling plate to remove heat, and every module has its own plate. LFP cells tend to be prismatic and there are different formats, from the narrow and long Euro MEB form factor to a tall version for China, and the Blade form factor used by some Chinese car makers. Going to a prismatic cell changes the module design, and allows a battery pack to fit between the frame rails. For Class 6 trucks, it is 650 mm, and the LFP prismatic cells allow a thinner pack to fit between the rails as it can use a simpler cooling system in the 850 mm rails of Class 3 trucks. An LFP battery system could have a bottom cooling plate integrated into the structure of the pack to optimise cost and reduce size. This would use an aluminium base, and composite or sheet moulding compound (SMC) or fibreglass cover. One of the big differences for heavy-duty battery packs is that all the mechanical structure is in the pack rather than being part of the chassis. While swappable batteries are an interesting technology gaining traction in mining applications, many heavyduty vehicle designs put the batteries in two, three or four locations, partially to spread the weight but also to fit into the frame-based chassis originally developed for diesel engines. This limits the ability to remove a battery pack easily for recharging. Focus | Battery tech for heavy-duty apps July/August 2024 | E-Mobility Engineering Characterisation of battery cells is key for heavy-duty applications (Image courtesy of American Battery Solutions) Schematic of lifetime modelling workflow (Image courtesy of Freudenberg)
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