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Maxwell and Valeo to Collaborate on Ultracaps for Next-Generation StARS+X System

Maxwell Technologies signed a memorandum of understanding (MOU) with Valeo covering a development collaboration to incorporate Maxwell’s BOOSTCAP ultracapacitors in Valeo’s next generation StARS+X—a stop/start and regenerative braking system. (Earlier post.)

The new StARS 14+X (starter alternator reversible system) consists of a reversible starter-alternator, a multi-cell ultracapacitor energy storage module and other power and control electronics in a 14-volt architecture. It enables standard gasoline and diesel engines to shut down during idle phases and restart instantly, eliminating fuel consumption and emissions anytime the vehicle is stopped.

Valeo estimates that the system can reduce fuel consumption and associated emissions by about 12% in normal operation and more than 20% in stop-and-go urban traffic.

In its earlier efforts with StARS+X, Valeo was working with multiple ultracapacitor technologies with capacitance values of 200 farads with nominal voltage between 20V and 30V, depending upon the technology. The company is using different size powerpacks—12-, 10- and 6-cell—to support a range of powertrains and internal space constraints.

The powerpacks must last for the lifetime of the vehicle, deliver reliability in the harsh thermal environment created by being close to the engine, and be safe and environmentally friendly. Valeo is specifying 600,000 stop-starts, 1 million regeneration events, and 200,000 boost events for the system.

The first generation StARS with start-stop capability, which did not include a dedicated energy storage component, initially was offered in 2004. The new 14+X architecture incorporates enhanced electronics and an ultracapacitor energy storage module that allows it to recuperate and store energy from braking, which otherwise would be dissipated as friction and heat in a conventional braking system.

The recovered energy is then available to power peak electrical loads such as de-icing and rapid cabin and seat heating and cooling, avoiding increased fuel consumption for such functions.

Because its energy storage employs high-efficiency, low-cost, ultracapacitor technology rather than costly nickel metal hydride or lithium-ion batteries, StARS 14+X can deliver 80 percent of the benefit of a mild hybrid system at 20 percent of the cost. Tax incentives and free access to high occupancy vehicle lanes have helped to stimulate demand for current premium-priced hybrid cars as niche products, but we believe that mass adoption of low-emission vehicles and much greater benefits in reduced CO2 and other greenhouse gas emissions will be driven by the availability of more cost-effective hybrid architectures.

—Daniel Richard, Director, R&D Valeo Electrical Systems

Richard also noted that the flexible 14+X system’s ability to be adapted for integration with a wide variety of existing motor platforms and to be applied to any new fuel technology such as flex fuel will make time-to-market for new models incorporating it much shorter than more radical hybrid approaches.

The MOU also covers terms of a proposed multi-year development and supply agreement through which Valeo will source ultracapacitors from Maxwell.



Combine this with engine-off accessory function, and this could be a huge win.


Is there anyone who knows the price of Maxwell Boostcap(3000F)? That will be great help.

Rafael Seidl

I think microhybrids are a great idea if they actually make it into a sizeable fraction of all new cars built world-wide. The small fry would really add up in terms of CO2 mitigation/energy security. Note that idle stop phases typically last no longer than 40 seconds. Such brief interruption of power to the AC can be overcome with a system that incorporates a small
latent cold store.

Adding a few ultracaps is a sensible extension even in a 14V system. German transmission specialists LuK used this concept in their
integrated package based on their dry-plate DCT. Note that it would have been ok to have the M/G on the engine side as well, as there is typically an excess of kinetic energy relative to storage capacity.

The other clou is the ancient art of Chi Ting: a second or two of substantial boost power is all it should take to increase the fun-to-drive factor of an otherwise modestly powered vehicle.

For example, hydraulic systems for flight simulators and thrill rides overcome their inherent stroke limitations by exploiting a sleight of hand. When confronted with conflicting information from two senses, e.g. vision and the vestibular accelerometers, the brain tends to trust the dominant sense and fills in the blank. Even if only the visual clues suggest continued acceleration, that is what the brain perceives for a few additional seconds.

Similarly, a well-tuned microhybrid system equipped with just a few ultracap cells should be able to sustain the illusion of boost torque/power for a few seconds after the ultracaps run dry - at least if there are no contervailing visual clues or, a bona fide hill to climb.


Are capacitors a technology that makes one choose between them or batteries?

Or is it something where most of the expensive batteries' "work" could be replaced with caps and thus 100% (or more) of the batteries' benefits can be had in conjunction with caps for less money?

Could a long range (300 mile) plug in Li-ion BEV/PHEV be made for a lower price tag by incorporating capacitors into the design?

John Schreiber

Excellent post Rafael,
I had not seen the "cold storage" thing before, but had been wrestling with the idea.

Ultracaps are going to be everywhere, as they provide the best way to supply inrush current to motors.

Rafael Seidl

@ Elliot -

a number of researchers have proposed combining ultracaps and batteries into a hybrid electrical energy story. Options include the integration of separate systems and the use of pseudocapacitors, in which one electrode acts electrostatically and the other electrochemically.

The upside is that you can optimize the batteries for capacity and longevity at the expense of peak power, which is taken care of by the ultracaps (at least for short bursts).

The downside is that you add yet another level of complexity to the power converter design, state of charge management as well as the manufacturing supply chain. Overall, both volumetric and gravimetric power and capacity densities should be in-between those of pure ultracaps and pure batteries.

@ John Schreiber -

the main reason ultracaps have been out of favor is the way California skewed its ZEV legislation in favor of high all-electric range rather than high fuel economy.


BMW and Citroen can already do this with lead-acid / AGM lead-acid. I can't imagine that ultracaps could possibly be cheaper than these.

Moreover, to contain the same amount of energy, an ultracapacitor bank would have to be 6 times the weight of the equivalent cheapo lead-acid battery.

Rafael Seidl

Clett -

I think you may be missing the point. The StARS+X system would use both ultracaps and a 14V battery, much like LuK's ESG concept does. The ultracaps will not need to have a lot of capacity. Rather, their job is to buffer power peaks in either direction.

The combo energy store lets you fit a larger M/G than would be possible with a battery-only system, while increasing battery life and recuperation efficiency. The result is robust idle-stop technology for its most demanding applications: diesels, large gasoline engines, cold weather conditions etc.

Increasing the electrical grid's burst power rating also means you can also switch a number of key peripherals to electricity, e.g. power steering & water pump. This leads directly to a little additional fuel economy.

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