E-Mobility Engineering 017 l ECE Doosan electric excavators dossier l In Conversation: Matt Faulks l Battery testing focus l Battery Show North America 2022 report l Ariel Hipercar digest l Cathode materials insight l Thermal management focus

sandwich, which protects both battery and underbody from impacts and punctures. Although the battery pack is solidly mounted to the chassis, it is not fully stressed. Optimal battery management Cosworth is also responsible for the BMS, which is the focus of considerable engineering effort. “The main thing is that we populate the BMS with the knowledge that we have of the cell,” Dowson says. “So we characterised the cell and are able to set the limits, understand where the optimum performance is and ensure that the system operates within that window. “Because it’s a prototype, we’ve got a lot of voltage and temperature sensing within the pack, just so we can see how far we can push it. As we go into volume production we will take some of that sensing out, because we will have more experience with the system.” The conduit cooling system allows the battery to be pushed further than would otherwise be safe, but Dowson emphasises that the overall task is one of optimisation. “Everything we do is about optimising the system, downsizing and pursuing weight efficiency and performance targets in terms of power and energy,” he says. That involves using the BMS to calculate the battery’s state of health and to work out its likely service life. Dowson says that is a challenge, because it is not clear how much time each individual car will spend on track, so Cosworth carries out extensive testing, typically at module level. “We put them through a harsh duty cycle to generate state of health information over time,” he says. “Obviously this is important for Ariel when considering warranties and so on. We have a safety factor in there, so I think we are happy with where we are.” Simulation, FEA and physical testing are also crucial to other aspects of battery safety, such as the risk of pack penetration and thermal runaway. “There are regulatory criteria in terms of possible ingress,” Dowson explains. “When you run over a stone at high speed for example and it fires into the baseplate, it is not allowed to penetrate, so we do a lot of FEA-based simulation work on that. “In terms of thermal propagation we perform physical testing. We will encourage a cell to go into thermal runaway at our test facility, then manage that runaway within the module construction and the pack. “The first line of defence is to sense that you have an unhappy cell or group of cells, and you start looking at how to shut the system down. If that thermal runaway continues, we have to look at what happens then, whether we can limit it to one cell and work out what damage that cell might do to the rest of the system.” While Cosworth is happy with the results of that work, Dowson points out that such testing is an ongoing process, as the car is likely to go into production with new 21700 cells with a different chemistry that offers a better power/energy balance. “That will require a lot more characterisation testing. The temperature window the new cell operates in can vary, and what happens in thermal runaway changes,” he says. Catalytic range extender Although it is an option, the 35 kW gas turbine range extender had a profound effect on the Hipercar’s powertrain because it allowed the battery chemistry to be weighted more towards power density than energy density. It also allowed the pack to be smaller and lighter without sacrificing too much range, although its contribution to the car’s performance on track is limited. “At the beginning of the project we ran lots of simulations, ranging from the WLTP cycle through to track use,” Saunders explains. “One of them we called – and this is the polite version – ‘driving stupidly on the road’. The range extender will keep up with that all day, but take it onto the track and it will not keep up with the power demands of the battery. “We can probably do 20 minutes on track before the car needs charging. Then you could leave the range extender running for 45 minutes and it will have the battery back to a useful state of charge again, but it is not designed to be driven around a track all day. We decided it needed a track capability to measure its performance against IC-engined cars.” The Cat Gen range extender puts out 35 kW and weighs 50 kg, spinning at a constant 110,000 rpm (Courtesy of Cosworth) 54 January/February 2023 | E-Mobility Engineering Digest | Ariel Hipercar