New 48 V battery technology reportedly boosts mild hybrid performance
The new 48 V battery design launched by MAHLE Powertrain at the Future Powertrain Conference in February 2019 reportedly boosts the performance of mild hybrid road vehicles. The engineering services company, part of the MAHLE group, has designed and developed a working prototype battery to showcase the potential for the technology.
“The draw of a 48 V system is that it offers similar fuel economy benefits to high-voltage systems, but at a much lower cost. However, we quickly saw that the architecture is inherently limited by battery module compromises necessitated by cost and packaging constraints,” explained Mike Bassett, MAHLE Powertrain’s Chief Engineer for Research and Advanced Engineering. “We have carried out extensive studies into the requirements of a 48 V system to establish an optimal balance of the powertrain components, and our analysis has identified a pathway to develop an MHEV that is able to provide CO2 benefits of between 12 and 15%.”
Mild hybrids need to recover energy efficiently and at a high rate during deceleration. However, packaging and cost factors encourage the use of more compact batteries with reduced storage capacity. What’s needed is a small battery pack that is capable of high-power charge and discharge cycles, and MAHLE identified this requirement as providing the greatest potential for advancing mild hybrid powertrain technology. According to MAHLE, it decided to design and build a suitable battery because one meeting those needs couldn’t be found.
“We revisited the very question, 'What is needed from a 48 V vehicle?' to minimize any limitations and to demonstrate the attainability of an enhanced powertrain solution,” explained Bassett. “We based our analysis on our existing C-segment 48 V demonstrator." This downsized vehicle already developed by MAHLE is fitted with a 1.2-L, turbocharged 3-cylinder engine equipped with a belt-integrated starter generator (BSG) and 48 V electric supercharger. The vehicle reportedly provides performance akin to the production 2.0 TGDi equivalent, with CO2 figures 12% lower over the new WLTP test procedure through downsizing. This is expected to improve further when fitted with MAHLE Powertrain’s new 48 V high-power battery and 48 V e-axle.
“Setting the right high-level performance targets was crucial, as it enabled the appropriate selection of cells for our battery pack development," said Bassett. “Balance is key: matching the total mass of cells needed with packaging requirements and pack performance targets enables the development of a pack with a high power capability and a relatively low energy storage capacity. It is a case of selecting the correct tool for the job, not making do.”
Inadequate battery cooling can have a significant impact on charge/discharge performance and battery life. Bassett said the difficulty was the narrow difference between battery and ambient temperatures: “We wanted to replace traditional air-cooling with a liquid coolant, which could then potentially be linked to the vehicle air-conditioning system in hot climates, but of course we needed to overcome the challenge of keeping coolant electrically separate from the cells.” Settling on a cooling strategy, MAHLE Powertrain says its simulations show cooling performance to be on target.
MAHLE says its engineers also paid a great deal of detail attention to circuit and bus bar design and materials to minimize the chances of short-circuit. Basset explained: “Even though 48 V is technically a safe system when compared to much larger 350-400 V battery packs, short circuits can still release considerable energy, and we wanted to maximize the reliability of the system as well as its performance.”
The new design will reportedly enable better energy recovery and a faster charge, which boosts efficiency. Bassett emphasized that a higher power capable battery pack creates tangible efficiency benefits on the road, offering a 12% fuel saving over the new WLTP standards, assuming ICE efficiency remains constant.
“However,” he continued, “we can further improve this figure by how we use electric motive power. For example, boosting the supplement from electric drive during inefficient ICE scenarios can increase WLTP-achieved fuel economy by a further 3%. Careful calibration of a refined system continues to add incremental benefits, and the flexibility afforded by a 48 V system makes it ideally suited to real-world use in a variety of environments.”