Governments want cars to produce fewer tailpipe emissions, whether it’s to improve local air quality through less particulate matter or NOx being pumped out or a reduction in carbon dioxide gas with its wider global ramifications.
It’s meant that just about every major vehicle manufacturer is investing in scaleable electric powertrains that can propel a diverse range of vehicles from small city cars to large SUVs.
As more R&D money is invested, huge improvements are being made to vehicle performance, whether that’s measured by traditional metrics such as acceleration and top speed or more contemporary issues such as vehicle range on a single charge.
But there’s another figure that also needs to be improved: recharge times.
Cutting recharge times
Plug an electric vehicle into a domestic socket and it’ll take 17 hours for the battery to be replenished (based on recharging a 40kWh battery from zero to 100%). That’s not good if you’re in a hurry, which is why fast charging is becoming an important target for many firms. The ability to recharge batteries in minutes rather than hours is another critical component in determining whether electric vehicles will be widely adopted. It’s happening – public 150kW charging stations allow you to drive up, plug in and see battery capacity rise from 20% to 80% in just 30 minutes.
But the practice of plugging in at higher charge rates causes challenges, and could in some cases speed up battery degradation. The difference between charging an EV at home at a rate of less than 3kW and at a fast-charging site running at 150kW will be huge on the system. Which is why companies are researching ways to improve battery life even under regular fast-charge conditions.
One project, and there are many looking at this area, brought engineering firm D2H Advanced Technologies and chemical company Croda together to improve thermal management in production EVs. Managing the thermal flow as charge is taken onboard is extremely important.
“Battery performance is a critical aspect of inspiring further take-up of EVs,” says Chris Hebert, D2H engineering director. “All areas of battery performance must be considered, especially their behaviour during high C-rate charging and discharging. Providing enhanced thermal management of the battery has the potential to offer further accelerated fast-charge times in the future.”
Stage one involved exploring the cause of excess noise during a battery fast charge, identified as being caused by the high pump power required with existing cooling systems, needed to dissipate heat.
D2H built a 32-cell battery test rig to evaluate the performance of immersive versus contemporary cold-plate cooling methods before computational fluid dynamics (CFD) modelling was used to accelerate comparative studies examining the cooling characteristics of various fluids.
CFD modelling then suggested clear benefits of using a dielectric fluid, which was validated in physical tests against alternative fluids. The second stage investigated the difference in performance between immersive and cold-plate cooling systems, which utilised the same simulation correlation process.
Fewer hot spots
Correlation of simulated tests demonstrated that the dielectric fluids promoted more efficient heat transfer, with fewer hot spots and more stable characteristics, which are less likely to negatively impact battery performance.
The work conducted by D2H and Croda, and others in similar spheres, will be incredibly important. If battery thermal management systems can be optimised to maintain a battery’s stability over time even under fast-charging conditions, it could lead to higher recharge rates beyond even the 150kW we see today. The implications are obvious: if you could refuel an EV in minutes rather than hours, it would make them comparable to combustion-engine vehicles and there would then be one less reason to stick with fossil fuels.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.