In recent years, mild hybrid systems that utilise batteries and motors to assist gasoline engines have been gaining traction as a technology that can lessen fuel consumption. Hitachi Automotive Systems Ltd has developed a high-output 48V Li-ion battery pack for such vehicles. The company claims that the output density and energy density of its battery pack is greatly improved compared to existing 48V Li-ion battery packs.

Increasing the output density was accomplished by making a thinner membrane on the cell electrode, thus reducing resistance. However, in return for the increased output density, the amount of energy that could be stored was reduced. For the developed Li-ion battery, resistance was reduced and output density was increased without making it thinner by modifying the cell electrode on a microscopic level to provide a structure that facilitates the flow of lithium ions.

Furthermore, energy density was increased by improving the material composition of the positive and negative electrodes and increasing the amount of lithium that can be stored per unit of weight. This made it possible to achieve an output density of 1.25x the existing level and an energy density of 1.5x the existing level.

In addition to a lower internal resistance within the battery cells and reduced heat emissions, a thermally-conductive and exoergic metal is used in the body of the Li-ion battery pack that eliminates the need for a cooling fan. This enables the production of thinner batteries, provides greater freedom in installation, and reduces noise.

Improving output density makes it possible to increase the motor’s torque performance for assisting acceleration, providing maximum torque upwards of 12kW and enabling powerful driving when accelerating from a standstill. It realises a maximum input of 15kW or more, enabling the recovery of the substantial recycled energy that is instantly produced when decelerating, and reducing energy loss. In addition to such improvements in input and output characteristics, fuel consumption is improved due to the increased energy density.

20170607_Hitachi_48V specs Figure 1: Specifications of the 48V Li-ion battery pack for mild hybrid vehicles.

Production of mild hybrid vehicles Hitachi is targeting is expected to increase from 450,000 units in 2016 to over 12.8 million units in 2023. Likewise, mild hybrid systems using 48V Li-ion batteries are expected to spread rapidly, especially in China and Europe.

Hitachi Automotive Systems is developing the battery pack, which will be manufactured by Hitachi Vehicle Energy Ltd. Samples will be provided to automobile manufacturers, while mass production is scheduled to commence in Hitachi's fiscal 2019.