Hybrid Energy Storage Key to Electric Auto Features

The automotive industry increasingly is looking toward electrification as a solution to meet changing emissions standards across the globe and as regulations become stricter, they are taking these initiatives even further.

While automakers traditionally have approached vehicle electrification in micro-hybrid vehicles through start-stop technology, which shuts off the engine when the car comes to a stop, the industry is evaluating new electrification strategies to meet future carbon-dioxide emissions standards while increasing performance and features.

Several of these new technologies will be adopted in mild-hybrid vehicles where an electric motor assists the combustion engine during short accelerations or braking and by allowing it to turn off for longer periods of time while the car is coasting. As a consequence, the combustion engine potentially can be downsized to significantly reduce emissions without compromising performance.

To push fuel economy even further, mild-hybrid vehicles often combine the efficiency advantage of an electric turbocharger with an electric braking system for recuperation.

In addition, premium vehicles will continue offering a wide variety of new, innovative electric features that provide comfort for the driver and the passengers, such as electric active-roll control, electric power steering and electro-turbocharging. Collectively, these features have demanding power requirements and each requires frequent, quick, high-power charge and discharge events, which is pushing energy-storage engineers to revolutionize the board-net strategy for the platform.

The Limits of Lead-acid and Lithium-ion Batteries

Traditionally, lead-acid batteries have been used in automotive applications due to low cost and simple monitoring. But, when exposed to high discharge rates or rapid cycling applications, those batteries experience accelerated aging, which automakers traditionally have overcome by oversizing the battery. However, using larger lead-acid batteries cannot reasonably compensate for peak power demands that can be up to 10 times higher than traditional architectures, while still meeting strict weight and size objectives.

In recent years, lithium-ion batteries have been increasingly adopted by the automotive industry because of their improved power and energy density performance compared to lead-acid batteries. Although the technology has made significant performance improvements, their low temperature profile and sophisticated battery-management system with heating and cooling remain a constraint to achieve optimal performance and lifetime.

Solving Batteries’ Shortcomings With Ultracapacitors

Because of batteries’ high-power performance limitations, ultracapacitors are gaining traction as an alternative energy-storage technology. There are two possible ways to integrate ultracapacitors to the energy-distribution network: as exclusive energy storage in a sub-network, also called an island solution; and in parallel with batteries, creating a hybrid energy-storage system.

While the first topology has been designed multiple times to assist high peak-power loads of a single function, hybrid energy storage is gaining traction in combining the high-energy density of the battery with the high-power density of the ultracapacitor.

By having ultracapacitors and batteries work in tandem, the key strengths of each device can be better utilized: Ultracapacitors can absorb high-power discharge requirements and transient events, saving the battery from high discharge rates and potentially increasing the life of the battery, while the batteries can provide long, constant energy discharge during periods of lower power requirements.

Taking a hybrid energy-storage approach offers the possibility to downsize the battery which, in the case of a lead-acid battery, can result in potential weight savings of nearly 40%.

Such hybrid energy-storage systems demonstrate good power and energy performance in temperatures ranging from as low as -40˚F (-40°C) to as high as 149˚F (65°C). Thus, regardless of external ambient-temperature conditions, ultracapacitors enable demanding high-power applications to be utilized over a significantly wider temperature range than battery-only systems. Unlike batteries, ultracapacitor energy-storage systems have a long operational life and can be designed to survive for the life of the vehicle with minimal maintenance requirements.

To meet the demands of power-hungry electrification applications, the automotive industry is continually seeking new, high-performing and reliable energy storage solutions. Marrying traditional battery energy storage with ultracapacitors not only can improve efficiency and reduce emissions but also save time and reduce maintenance costs.

Bertrand Renaud is director of product management for Maxwell Technologies. His responsibilities include new-product strategy, roadmap definition, analysis of market trends and product P&L.