The Shocking Truth About Start-Stop Systems

Edward Niedermeyer
by Edward Niedermeyer
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TTAC has long seen stop-start systems (which turn off the engine at idle) as one of the many common-sense technologies that will continue to improve internal combustion engine efficiency at a relatively low cost. Outside of these digital pages, though, the systems have taken longer to gain awareness in the United States, resulting in the lagging adoption rate pictured in the chart above. Up to this point, we’ve assumed that this can largely be blamed on the EPA test’s unwillingness to acknowledge the urban-driving advantages of stop-start systems, pointing to Mazda’s protests on the matter as evidence that government intransigence was keeping the technology out of the market. But recently Mazda has announced that all of its vehicles will get stop-start as standard by 2015, and Ford has said that it will begin offering the technology on “some” four-cylinder models for the North American 2012 model-year… and the rest of Detroit isn’t far behind. So what’s the deal? The EPA hasn’t changed its test… why are stop-start systems finally starting to trickle over?

Thanks to new research obtained by TTAC from the cleantech investment fund Pacific Crest, we now have a better understanding of stop-start technology, and why we’re actually glad it’s taking so long for the systems to get here.

Not having looked into the technology in any appreciable depth, Pacific Crest’s research was eye-opening. It turns out that the first generation of systems actually used variations of lead-acid battery technology known as “ absorbed gas mat” (AGM) and “enhanced flooded batteries” rather than the newer NiMh or Li-ion chemistries. Though these technologies clearly offer advantages over standard lead-acid batteries (see comparison from Johnson Controls, above), they are still far from perfect. Pacific Crest’s research notes:

Gen1 AGM and enhanced flood batteries perform poorly, leaving future market share in doubt. The start-stop battery cranks the engine 10x more than a traditional battery, and the lead-acid chemistry is simply unsuited for this workload. Current AGM and EFBs degrade rapidly, with AGM batteries losing half of the charge acceptance within two weeks after first use (i.e., it loses half of its fuel-efficiency gains). They are not good at holding steady voltage during a stopping event (e.g., car stereos/windshield wipers may not work when the car engine turns off).

Yes, you just read that right: the current generation of stop-start systems lose half of their benefits after two weeks. Which means they’re great for juicing up scores on Europe’s urban-efficiency test, but they are as good as useless for the vast majority of the life of the vehicle (we already knew they were less-effective in cold weather). In an industry that typically validates equipment for hundreds of thousands of miles of useable life, this is nothing short of shocking. Especially when you jump ahead and find out the OEM response to these concerns

Ultracapacitor prices need to decline by approximately 50% to gain significant Gen2 share. Overwhelmingly, OEMs are interested in satisfying the European legislation at the lowest cost. With few exceptions, cost is the first, second and only consideration. The poor performance of the AGM battery is something these OEMs are willing to live with as they assume consumers (and regulators) will not notice or care about the fuel-efficiency losses soon after purchase. Representatives from Ford, BMW and Porsche all dismissed ultracapacitors solely on cost, even while recognizing the greater performance. The good news for ultracapacitors is that a few OEMs are interested in offering consumers the better performance. But in order to gain real market share, the module price needs to fall significantly.

That’s right, the OEMs can “live with it.” Because they literally don’t have to live with it: consumers do. How this issue has not erupted into a semi-serious scandal in Europe is beyond us. After all, if this is accurate and verifiable, it means manufacturers are building cars that may not actually comply with emissions standards in the real world for most of their lives. Whether consumers will “live with this” is one question… whether governments will is a very different proposition. But, as the paragraph above implies, manufacturers aren’t likely to abandon advanced lead-acid designs for some time. Luckily, though, the next generation of AGM batteries should be much improved… and they had better be!

By 2013, start-stop batteries need to get much better. The next-generation battery, expected to be introduced initially in 2013 models, is expected to do much more than current models and stretch lead-acid battery technology further. OEMs are demanding significant technology advancements to meet customer demands and higher fuel efficiency standards. The Gen2 battery not only will do start-stops, but also basic regenerative braking, start-stop during deceleration and electric boost during acceleration. The battery will, therefore, need to be able to handle more cranking events, and more charge. Also, the battery will need to operate a partial state of charge (i.e., normally hold 70% to 80% charge) in order to absorb energy from braking events.

Generally speaking, batteries need a 4x improvement in charge acceptance and a 3x increase in cycle life to meet OEM demands for 2013. Lead-acid batteries, ultrabatteries and ultracapacitors are all vying for share in the Gen2 start-stop market. No OEM is committed to a single future technology, although most of the OEM testing is focused on improving AGM batteries. Most OEMs (e.g., BMW) are waiting to see the final specs on Gen2 AGM batteries before turning their attention to alternatives such as ultracapacitors. If lead-acid manufacturers can produce an improved version at the current price point, AGM will continue to dominate the start-stop market, in our view.

Pacific Crest goes on to argue that ultracapacitors (which are good for over a million cycles and don’t lose capacity like lead-acid) need to see something like a 50% price reduction to compete for attention from price-sensitive OEMs, and points out that a lack of Chinese players in the ultracapacitor space is part of the problem. Though ultracapacitors sound sexy, if the next-generation of AGM batteries improve to the point where stop-start systems are offering full benefits for longer than two freaking weeks, then we say “bring ’em on.” Meanwhile, every auto media outlet with stop-start-equipped long-term loaners (and possibly government emissions-control agencies) should be running regular tests to verify or disprove these troubling claims. A two-week lifespan for full-function on any automotive system is nothing short of misleading.

Edward Niedermeyer
Edward Niedermeyer

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4 of 66 comments
  • Johnswift Johnswift on Jun 30, 2011

    A very good article answering questions regarding AGM and start stop can be found here:

    • See 1 previous
    • Grover432 Grover432 on Jun 16, 2014

      @BigOldChryslers But my BMW shuts down most accessories and degrades performance on the others until the engine restarts. Is it still drawing 50 amps? In the winter, every couple of weeks I plug in the factory trickle charger to top up the battery as most of my trips are short. In cold climates the auto start system on BMWs don't function below 0C, so fuel savings are not as great in the winter months (northern climates).

  • Grover432 Grover432 on Jun 16, 2014

    "After all, if this is accurate and verifiable, it means manufacturers are building cars that may not actually comply with emissions standards in the real world for most of their lives" Are you kidding? You have come to a conclusion without verifying the claims of Pacific Crest who are providing a research paper with the aim of selling stock in a competing technology ("Maxwell Energy"). Their research piece doesn't provide sources for their conclusions and nor do you. I have had a car with ASS for 7 months now and it continues to operate the same as it did when I got it. If the car continues to shut off when you come to a stop then I don't see how it is failing to provide the advertised fuel mileage. Of course, if battery capacity drops and the car fails to shut off at red lights and stay off until you take your foot off the brake, that is another matter. The manufacturers don't seem to say how long a car should be able to sit in traffic on a 100F day with the AC running on battery before the engine needs to re start. Maybe you can do some research before you come to a conclusion? You may be right, but without research, you are just a blog regurgitating some else's (possibly) poor and biased writings.

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  • ToolGuy "Images © 2023 Tim Healey/TTAC; Mercedes-Benz"• I bet I can tell you which is which.