Stop-Start Idle Elimination – Slashing Fuel Consumption By Up To 17%


John Petersen

I’ve written several articles over the last year that explain why idle elimination is a crucial first step in the global effort to increase fuel efficiency and curb CO2 emissions. For readers who are new to my blog, or confused by a torrent of news stories and analysts reports that wax poetic on the expected benefits, costs and challenges of gee-whiz vehicles that are “coming soon to a showroom near you,” describes stop-start systems, or micro-hybrids, as follows:

“These are conventional vehicles powered either by gasoline or diesel engines in which the 12-volt starter motor has been eliminated and a specially designed, belt-driven integrated starter/generator, or ISG, has been installed in place of the conventional alternator. While the ISG of a micro hybrid cannot help to propel the vehicle, it can provide two important hybrid features. First of all, a micro hybrid will feature idle stop. Engine control circuitry is included in a micro hybrid which will shut down the internal combustion engine when the vehicle is at rest. This feature alone can improve fuel economy by 10% to 15% in city/urban driving environments. The electronic control system in a micro hybrid can also control the charge cycle of the alternator so that it produces electricity to recharge the vehicle battery primarily during deceleration and braking. This provides a mild amount of regenerative braking and an additional gain in efficiency.”

I usually talk about an 8% improvement in fuel economy for an incremental cost of $400 when I write about stop-start systems. Since I know that blog entries from guys like me who have an economic dog in the fight are often viewed as less credible than articles from writers who merely have a philosophical or political axe to grind, I also spend a good deal of time searching for concrete supporting data from reliable collateral sources.

I recently found a fascinating and somewhat disturbing slide in a presentation that General Motors R&D made at the 2010 Annual Meeting of the Minerals, Metals & Materials Society titled, “Challenges and Opportunities Relative to Increased Usage of Aluminum Within the Automotive Industry.” The following schematic from page 13 of the presentation tells me that the 8% estimate I’ve been using is too pessimistic by half and the real fuel economy target for stop-start systems is closer to 17%.
5.28.10 GM Efficiency.png
Stop-start is not a complete solution to the fuel efficiency challenge, but it is the lowest and juiciest fruit on the conservation tree. Is it any wonder that industry analysts are predicting that stop-start systems will be built into 20 million cars a year by 2015?

The most common question on articles that discuss stop-start systems is, “if stop-start is so important, where are the automakers’ press releases touting the technology?” The answer is simple. Stop-start will not normally be offered as a stand-alone option and will usually be bundled in packages like the EfficientDynamics system from BMW that has begun to attract praise from the mainstream media. More importantly, stop-start may be optional equipment for a couple years, but it is almost certain to become standard equipment because there is no compelling reason to waste fuel while waiting at a stop-light.

Automakers in Europe and North America are under tremendous pressure to meet new fuel efficiency and CO2 emission standards or pay huge penalties for failure. The following table summarizes the CO2 emission standards adopted by the European Union in April 2009.

Calendar Year Percent of Fleet CO2 Emission Standard MPG Gasoline MPG Diesel
2012 65.00% 130 g/km ~42 ~48.2
2013 75.00% 130 g/km ~42 ~48.2
2014 80.00% 130 g/km ~42 ~48.2
2015 100.00% 130 g/km ~42 ~48.2

In April of this year, the NHTSA and EPA created comparable standards for the U.S. when they adopted a joint final rule establishing the following fuel economy standards for light duty vehicles including cars, pickups, SUVs and vans.

Model Year Passenger Cars Light Trucks Combined Fleet
2010 (1) 27.5 mpg 23.5 mpg
2011 (1) 30.2 mpg 24.1 mpg
2012 (2) 33.3 mpg 25.4 mpg 29.7 mpg
2013 (2) 34.2 mpg 26.0 mpg 30.5 mpg
2014 (2) 34.9 mpg 26.6 mpg 31.3 mpg
2015 (2) 36.2 mpg 27.5 mpg 32.6 mpg
2016(2) 37.8 mpg 28.8 mpg 34.1 mpg

(1)  Source: Wikipedia Corporate Average Fuel Economy
(2)  Source: NHTSA CAFE-GHG Fact Sheet

The bottom line business dynamic is that every Prius, Volt or Leaf the automakers sell will simplify the task of regulatory compliance, but the lion’s share of the progress will come from building simpler efficiency technologies into cars that will be sold to consumers who think the green in their wallets is more important than the green in their conversation.

The second most common question is, “why do you think the widespread adoption of stop-start technology will be a boon to developers of advanced lead-carbon batteries and other systems that combine supercapacitors with conventional starter batteries?” My response has always been that current starter batteries are not robust enough to start an engine several times in a daily commute and systems based on exotic chemistries like NiMH and lithium-ion batteries are too expensive. Until recently, data to prove my point has been limited, which led to some skepticism. Now that hard data is beginning to make its way into the public domain, the task gets easier.

The big problem with stop-start systems is that starting an engine several times in a daily commute is very hard on starter batteries and the constant punishment gives rise to two related problems:

  • First, the dynamic charge acceptance rate falls off rapidly, meaning that charge cycles that take 30 seconds with a new battery can take 2 minutes or more after a few months of use;
  • Second, charging efficiency falls off rapidly, meaning that more energy is needed to bring the battery back to a full state of charge.

Both of these factors limit the frequency of stop-start events because control electronics won’t turn the engine off unless the battery is fully recharged and ready for another start cycle. As the frequency of stop-start events declines, so does the fuel economy.

Last week a reader referred me to a Journal of Power Sources article (Volume 194, Issue 4, Pages 1241-1245) that compared the stop-start cycle-life performance of a conventional starter battery, an advanced lead-acid battery with carbon additives, and a lead-carbon battery-supercapacitor hybrid from Australia’s Commonwealth Scientific and Industrial Research Organization called the Ultrabattery. The following graph shows the relative performance of all three devices in simplified cycle life testing that slightly under-charged the batteries to show the differences in dynamic charge acceptance rates.

5.28.10 Ultrabattery 1.png

A graph of their cycle-life testing using a normal charging protocol follows.

5.28.10 Ultrabattery 2.png

Axion Power International (AXPW.OB) reported comparable results in its May 19th presentation at the Advanced Automotive Battery Conference 2010.

5.28.10 Axion.png

The bottom line take-away points for investors are:

  • In response to government mandates, stop-start systems will ramp from a few hundred thousand vehicles in 2010 to 20 million vehicles a year by 2015;
  • Initial implementation of stop-start systems is planned the 2012 model year, which will require OEMs to reach design specification decisions by the third or fourth quarter of 2010;
  • Roughly half of the $400 incremental cost of a stop-start system will be spent on better energy storage devices and the balance will be spent on control electronics and electro-mechanical components;
  • While some automakers may choose higher quality conventional lead-acid batteries for stop-start systems, OEMs that want to maximize vehicle efficiency and avoid service problems will prefer technologies that combine the performance characteristics of supercapacitors and batteries; and
  • Incremental revenue for manufacturers of storage devices for stop-start systems will run to several billion dollars a year by 2015.

Five public companies are actively developing specialized materials, components and energy storage devices for stop-start systems and will enjoy a substantial first-mover advantage over the next few years, including:

  • MeadWestvaco (MWV), a packaging material and container manufacturing company that is developing carbon additives for the lead pastes used in ISS batteries;
  • Maxwell Technologies (MXWL), which has teamed-up with Continental AG to develop storage systems for stop-start applications that use supercapacitors in tandem with conventional lead-acid batteries;
  • Furukawa Battery Company (Frankfurt – FBB.F), which licensed the Ultrabattery from CSIRO and then sublicensed North American manufacturing rights to privately held East Penn Manufacturing Company, the recipient of a $32.5 million ARRA battery manufacturing grant award in August 2009;
  • Axion Power International (AXPW.OB) a manufacturer of lead-acid batteries that has built a formidable patent position in lead-carbon technology and teamed-up with Exide for the commercialization of its PbC® battery-supercapacitor hybrid; and
  • Exide Technologies, Inc. (XIDE), a leading global manufacturer of lead-acid batteries that has teamed up with Axion and was awarded a $34.3 million ARRA battery manufacturing grant in August 2009.

While each of these companies is working feverishly to complete OEM testing, build manufacturing facilities and negotiate their first contracts, none of them is truly ready for the anticipated surge in demand. As a result, I believe every company that brings a product to market this year will have more business than it can handle by the middle of next year. When the first design wins are announced later this year, the market response should be impressive, especially in the case of Exide and Axion which are rumored to be trading at depressed prices because of liquidations by troubled funds. Other battery manufacturers will undoubtedly enter the fray, but they’ll all be playing catch-up ball for a long, long time.

Disclosure: Author is a former director of Axion Power International (AXPW.OB) and has a substantial long position in its stock.


  1. Working from the Energy Efficiency of vehicles diagram, I actually get a *potential* of more than 18%, and a best-guess reduction of 12%.
    While idling energy (1.36) is 17% of the total energy (7.94) in, the reduction in alternator losses should be the power used by non-belt driven (e.g. air conditioning) accessories, such as headlights. This probably is enough to increase your estimate by another 1%.
    On the other hand, my experience driving a Prius for over 8 years now makes me think that stop-start will only eliminate about 2/3 of idling losses, since the vehicle continues to idle whenever the engine is not completely warm or the battery needs charging.
    Hence my back-of-the-envelope estimate for anti-idle would be a 12% reduction in fuel consumption.


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