New 48 volt battery technology boosts mild hybrid performance

“The draw of a 48V 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 48V 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. Since no suitable battery existed, MAHLE decided to design and build one.

“We revisited the very question, 'What is needed from a 48V vehicle?' to minimise any limitations and to demonstrate the attainability of an enhanced powertrain solution,” explained Bassett. "We based our analysis on our existing C-segment 48V demonstrator."

This heavily down-sized vehicle already developed by MAHLE is fitted with a 1.2-litre, turbocharged 3-cylinder engine equipped with a belt-integrated starter generator (BSG) and 48V electric supercharger.

The vehicle already provides performance akin to the production 2.0 TGDi equivalent, with CO₂ figures 12% lower over the new WLTP test procedure - purely achieved through downsizing. This will obviously improve further when fitted with MAHLE Powertrain’s new 48V high power battery and an equally capable 48V e-axle.

Optimising the battery pack
“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 robust cooling strategy, MAHLE Powertrain’s simulations show cooling performance to be exactly on target.

MAHLE engineers also paid a great deal of detail attention to circuit and bus bar design and materials to minimise the chances of short circuit. Basset explained: “Even though 48V is technically a safe system when compared to much larger 350-400V battery packs, short circuits can still release considerable energy, and we wanted to maximise the reliability of the system as well as its performance.”

On the road
The new design will enable better energy recovery and a faster charge, which boosts efficiency. Bassett emphasises 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 48V system makes it ideally suited to real-world use in a variety of environments.”

MAHLE Powertrain set out to demonstrate the potential of 48V architecture and the benefits from working with a dedicated engineering consultancy.

It has taken the company’s specialists only nine months to evaluate, design, analyse and prototype a battery that could improve efficiency by up to 15% on an already efficient powertrain technology.

Having been central to the development of some of the most important high performance combustion engines over the past two decades, MAHLE Powertrain is now playing just as crucial a role in the development of the next generation of powertrains.