In start-stop systems, the internal combustion engine is shut off when the driver stops and the engine is seamlessly restarted when the driver accelerates, which lowers emissions and improves fuel economy. Battery-based start-stop systems augmented with an ultracapacitor-based voltage stabilization system implementation provide burst power needed to restart the engine, thus reducing high currents and repeated cycling that can shorten battery life. The voltage stabilization electronic control results in a smooth start, reduced engine vibration and a superior driving experience.

Maxwell’s ultracapacitors, in Continental’s VSS design, also serve as an additional power source for stabilizing the vehicle’s electrical system during periods of high power demand.

Unlike batteries, which produce and store energy by means of a chemical reaction, ultracapacitors store energy in an electric field. This electrostatic energy storage mechanism enables ultracapacitors to charge and discharge in as little as fractions of a second, perform normally over a broad temperature range (-40°C to +65°C), operate reliably through 1 million or more charge/discharge cycles and resist shock and vibration.

The VSS system consists of two main parts, a control unit (PEU - Power Electric Unit), and the power storage unit (ESS - Energy Storage System). The latter has two 1200F ultracapacitors linked in series. The excess energy required for restarting the engine is stored in this unit, which is recharged by the vehicle’s battery. The PEU controls the charging of the ESS, and the utilization of the accumulated energy.

The ultracapacitors provide all the power for the engine cranking event; this also takes that load off the 12V battery, potentially reducing the requirements for a higher-cost lead-acid battery in a start-stop application. There is no regen in this basic application.

Unlike in conventional cars, where the negative terminal of the battery is connected directly to the metal body of the car, in cars equipped with this Start-Stop system, the chassis connection is made by the control unit, ensuring that the power storage unit is chargeable and that the car can retrieve the energy from there.

The system to be applied in the Cadillacs is very similar to those which have gone into service in Europe in PSA Peugeot Citroën vehicles over the past five years. The power electronics between the two are slightly different, noted Mills. The ultracapacitor cells are the same.

Maxwell is working with GM and other OEMs on other uses of ultracapacitors in transient load applications such as active suspension, and also as backup power for telematics or infotainment. Such systems would likely be distributed for each function, Mills suggested, depending upon the duty cycle and load profiles. While it could be a larger single ultracapacitor system, economically it might make more sense to localize, he noted.