Ultracapacitors for Start-Stop Systems in Micro-Hybrids

The C4 uses PSA Peugeot Citroën’s diesel auto platform for stop-start and idle-elimination.

As more states offer incentives for the purchase of electric vehicles (EVs), governments are overlooking the benefits that alternative eco-friendly and energy efficient vehicles offer. By only providing credits for pure EVs, states are missing out on an opportunity to ignite adoption of other cars, like micro-hybrids, which also offer environmental advantages compared to traditional combustion engines.

Micro-hybrids aim to combine the best characteristics of modern combustion engines with energy storage systems to reduce emissions and improve fuel efficiency—the same benefits that appeal to EV drivers.

Start-Stop Systems and Vehicle Electrification

In a micro-hybrid vehicle, start-stop technology allows the engine to shut off when the car comes to a stop at a red light or while sitting in a traffic jam. The system is based on an intelligent combination of engine, brake and energy management. As soon as the driver removes pressure from the clutch or brake pedal, the system restarts the engine.

As automakers look to comply with stricter fuel-economy and emissions regulations, more are choosing to implement start-stop systems in micro-hybrid vehicles. This helps ensure fuel is not wasted during periods when the car is idling, thus reducing emissions.

Batteries have high energy density (storage capacity) expressed as watt-hours per kilogram while ultracapacitors have high power density (charge/discharge rate) expressed as watts per kilogram. (Image courtesy of Maxwell Technologies.)

As start-stop vehicles evolve into the standard platform in future cars, the ongoing electrification of other high-power vehicle components increases the requirements of energy sources. That’s why micro-hybrids are designed with a secondary energy source that complements primary sources like batteries.

While batteries suffer when repeatedly providing quick bursts of power, a secondary energy source can be implemented to overcome this limitation. When a secondary energy source is used in a start-stop vehicle, it can also power high-power features like electric power steering or active suspension. Traditional battery energy storage systems have problems supporting such high-power features, especially at lower temperatures.

The Role of Ultracapacitors

Maxwell ultracapacitors and supercapacitors. (Image courtesy of Maxwell Technologies.)

As automakers try to comply with stricter emission standards, there has been an increase in the electrification of mechanical subsystems.

For smaller vehicles with 12-volt (12V) architectures, which are ubiquitous across the globe, developers are transitioning energy storage components from traditional lead-acid batteries to ultracapacitors. These offer nearly instantaneous power bursts during periods of peak power demand.

Ultracapacitors can store and discharge energy with high power very quickly and effectively compared to batteries, which are better suited to store large amounts of energy, but should be charged and discharged at low power levels to avoid premature aging.

More manufacturers are choosing ultracapacitors to power start-stop applications, especially when fast restarts are required, and for maintaining the car’s supply voltage. The voltage can be reduced significantly because frequent restarts tend to drain the battery. With their high power density, ultracapacitors can ensure energy is delivered quickly, allowing for cars to restart without a pause in vehicle operation.

Compared to battery-powered start-stop systems, the restart powered by ultracapacitors typically is faster, smoother and creates less vibration. Therefore, more power and torque is delivered to the starter resulting in less time required to crank the engine.

Aside from the environmental benefits and improved fuel efficiency, drivers also experience firsthand benefits from micro-hybrids that employ ultracapacitors. Ultracapacitors are virtually maintenance-free and typically last the whole vehicle life. A hybrid energy storage system that leverages both batteries and ultracapacitors reduces stress on the batteries, meaning cars are less likely to need costly replacements.

Ultracapacitor/Battery Hybrid for Stop-Start Applications

Ultracapacitor/battery hybrid. (Image courtesy of Maxwell Technologies.)

Maxwell Technologies has been exploring the use ultracapacitor/battery hybrid for stop-start applications. The prototype configuration uses a direct parallel connection between six 2000F ultracapacitors and a 30 Ah motorcycle battery.

UC peak-shaving of battery load demonstrated in actual vehicle engine cranks. In-rush (peak) currents are as follows: Battery (10mΩ): 204A; Ultracapacitor (2.5 mΩ): 716A; Hybrid: 907A. (Image courtesy of Maxwell Technologies.)

Testing showed that the ultracapacitors could be used for peak-shaving of the battery load in vehicle engine cranks, where load sharing is a function of the relative resistance of the two devices.

Repeated cycles of Charge (1-sec, 100 A/CC to 15V/CV) and Discharge (1-sec, 100A/CC) for the hybrid device and a 55Ah absorbed glass matt (AGM) battery in good condition. (Image courtesy of Maxwell Technologies.)

In addition, a rapid cycling test showed that the hybrid storage device exhibited a lower charge as well as a higher discharge voltage compared to a conventional battery.

Repeated Cycles of Charge (7-sec, 25A/CC to 14.4V/CV), Discharge (2-sec, 30A/CC) and Crank (2-sec). Test samples were a 50Ah AGM battery, a 55Ah flooded lead acid (FLA) battery and the hybrid device based on a 38Ah AGM. (Image courtesy of Maxwell Technologies.)

Most importantly, the hybrid device was able to maintain a higher minimum voltage during high frequency stop-start cycling.

The Future of Micro-Hybrid Platforms

Future micro-hybrids will implement new functions like coasting or sailing. These functions shut the engine off during times when the vehicle is driving at higher speeds, not only during periods of standstill. This results in a significantly higher number of engine restarts compared to current start-stop vehicles.

In combination with other functions like electric power steering or active suspension, the requirements for the energy storage will increase, which needs to provide very high reliability and power availability over a wide temperature range.

By 2025, it is expected that global sales of light-duty start-stop vehicles will exceed 61 million, accounting for 59 percent of all light duty vehicle sales. Micro-hybrids are growing in popularity because they can achieve more fuel savings than a traditional internal combustion engine model, with a portion of the savings coming directly from start-stop systems.

Micro-hybrids relying on batteries alone face potential disinterest from drivers due to the limitations of lithium-ion batteries. In cold temperatures, the batteries become compromised and drivers or the vehicle’s energy management are likely to disable the start-stop system altogether.

Automakers can overcome the problems of battery-only systems by adding ultracapacitors into the start-stop application. This will ensure reliable engine cranking regardless of external temperatures, and that the driver’s operation of the vehicle is never stalled. States looking to get drivers interested in more fuel-efficient and environmentally-friendly transportation should leverage this growth and offer incentives for a range of vehicles that are not purely electric, but still offer benefits.

Stefan Werkstetter is applications engineering manager at Maxwell Technologies.