A123: Increasing 48V battery power up to ~25 kW enables advanced mild hybrid capabilities with greater fuel savings
In a paper presented at SAE WCX 2017 this week in Detroit, engineers at Li-ion battery maker A123 Systems conclude that 48V battery packs with strong power (up to ~25 kW) and HEV-level energy capabilities (<200Wh for most scenarios) are needed to enable advanced mild hybird (MHEV) capabilities, with optimal power-to-energy ratios between 30 and 160.
Improving battery power to such as level would further enable hybridization to near-HEV levels as well as engine downsizing, thereby enabling fuel economy improvements beyond the current 10-15% MHEV limit. Additionally, new electrified features could be added, such as electric turbo/supercharging, electric traction, electric power steering, electric suspension and electric air conditioning. To address these needs, the A123 team presented a high-power, lithium-iron-phosphate chemistry with excellent rate capabilities.
Current MHEV systems typically consist of an engine coupled to a belt-integrated starter generator (BISG). The BISG is connected to a 48 V (nominal battery) and also typically to a 12V powernet via a DC/DC convertor. Such an MHEV system is expected to provide approximately a 10-15% improvement in fuel economy compared to a standard vehicle with start-stop.
A typical MHEV can provide from 8kW up to 16kW. The power output of a conventional MHEV system is fundamentally limited in terms of the ability of the BISG to provide torque to the ICE; excess torque can create belt slip. The specific torque at which slip occurs can be mitigated through the use of belt tensioners and other techniques, but is nonetheless fundamentally limited. In summary, while the MHEV battery may provide high power in short discharge bursts to the electrical network, it is limited in its ability to generate power at the wheels, thus limiting fuel economy opportunities.
While typical BISG architecture limits MHEV power, demand for vehicle electrification only continues to increase, extending a decades-long trend. Accessories that are typically powered via hydraulic power or the serpentine belt may be converted to electrical power. Additionally, advanced features such as electric low-speed drive, park-assist, electric-turbo, and others may utilize the 48V system. Finally, partial hybridization using the 48V powertrain may be integrated to improve fuel economy.
Our work builds on previously established analyses that define requirements focused on optimization of fuel economy in typical MHEV and Hybrid Electric Vehicles (HEV). Here, we consider new architectures and applications expected to emerge in MHEV-based vehicles over the next ten years. These new architectures will likely stretch the fundamental limit on MHEV performance to approximately 25kW or more. Within that context, we investigate the power and energy requirements associated with high-power MHEV systems focused on different objectives, including fuel economy but also electrification of high-power accessories for consumer benefit. We also outline the tradeoffs between fuel economy benefits and high-power accessory usage.—Abdellahi et al.
|Left: Conventional MHEV architecture. Right: Generalized MHEV topology with advanced high-power 48V battery packs. The extra power can be directed toward either fuel economy or extra electric accessories. Abdellahi et al. Click to enlarge.|
For their study, the A123 team used two cycles—FTP-72 for the US and NEDC for Europe. They considered two batteries—both based on lithium-iron-phosphate (LFP) cathode chemistry paired with a conventional graphite anode. Both are based on formulations developed to provide high levels of power over a wide temperature range, leveraging small cathode particles (<100nm) to allow rapid transfer of ions and electrons in/out of the cathode particle matrix. One featured much thicker cathodes and anodes than the other (2.35 and 2.21 times, respectively). The basic cathode material was common for both, with minor differences on the anode to accommodate differences in the layer structure.
By using a flexible scaling approach, batteries can be designed based on either Technology A or Technology B that vary in energy and power capabilities while providing a consistent power to energy ratio (P/E), since both power and energy scale with added layers. This flexibility allows us to choose the basic technology based on the nature of the application, while we generally scale the number of layers to accommodate changes in scale (i.e., a larger motor generator).—Abdellahi et al.
Abdellahi, A., Khaleghi Rahimian, S., Blizanac, B., and Sisk, B. (2017) “Exploring the Opportunity Space For High-Power Li-Ion Batteries in Next-Generation 48V Mild Hybrid Electric Vehicles,” SAE Technical Paper 2017-01-1197 doi: 10.4271/2017- 01-1197