By on June 29, 2011

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.

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66 Comments on “The Shocking Truth About Start-Stop Systems...”

  • avatar

    How does losing half of its charge acceptance translate into losing half of the fuel mileage efficiency gains? Won’t the engine still shut-off and stop using fuel? Won’t the problem be with running electrical systems with the engine off and then re-starting the engine (since the battery isn’t accepting enough charge)?

    I just don’t clearly understand why it loses half of the efficiency gain.

    • 0 avatar

      I agree-
      If the car turns itself off and back on, isn’t the fuel saving accomplished?

      Maybe this loss of efficiency applies to vehicles with regeneration capabilities (ie. mild hyprids).

    • 0 avatar

      maybe because it takes more energy from the engine to recharge, once it’s restarted… if it has the capacity to restart it.

      • 0 avatar

        As Mikemann points out, batteries don’t generate the energy, they just store it. I assume that this degradation manifests in the battery drawing more engine power rather than the system failing completely, otherwise we’d definitely have heard of this by now.

        To be clear though, there’s no definitive test cited in the research… and as usual, we welcome any hard data that either supports or refutes this claim.

    • 0 avatar

      The other posters make good points. Also, if the system determines that the power available is insufficient to perform a stop/start cycle, then the stop/start is canceled and the engine continues to run until enough power is available.

      This is exacerbated by cold temperatures where battery efficiency is lower and electrical loads from defrosters, wipers, headlights and other electrically demanding systems is highest. Its is conceivable that in the winter, a driver might see zero advantage from a stop/start system with a degraded battery.

      • 0 avatar

        redliner, you hit the nail on the head. This is how it was explained in previous discussions. Say you take a trip across town when the system is brand new and the engine stops 10 times at ten stop lights. Two weeks later the same trip the engine stops only 5 times and runs the entire time you sat at the other 5 stoplights. Is that not losing half of your fuel mileage savings?

    • 0 avatar

      I’m having trouble understanding the loss of fuel efficiency as well. I can see that a lead-acid battery might have trouble with the highly variable charge/discharge cycling presented by start-stop, but the battery is either capable of restarting the car or not. The fuel efficiency stems from the start-stop. Is the confusion stemming from this line?
      “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).”
      Is this really referring to a loss of battery capacity?

      • 0 avatar
        SVX pearlie

        I think it’s a loss of Charging Amps – kind of like how batteries lose discharging Amps “Cold Cranking Amps” in the winter / over time, these batteries lose their ability to “fast charged” via “stop”.

        If you look at a battery, you can think of it as having a finite number of combined “starts” and “stops”. Start/stop technology increases the number of “starts” per trip 5-fold or 10-fold, which rapidly eats into the number of starts / stops before degradation becomes noticeable.

        In the BMW case, it’s always the driver’s fault, so you’ll just have to drop another $500 on a new battery and “registration” with the car.

    • 0 avatar

      It doesn’t loose half in charge acceptance. It loose half the improvement in charge acceptance compared with a standard lead-acid battery. So half the improvement is still there.

      Pacific Crest is an investment fund which if i read between the lines has invested money in competing battery technology.

    • 0 avatar

      Unless the battery is at an adequate state of charge, the engine will not shut off.

      So if lose half your charge acceptance and it takes twice as long to charge the battery then you get half the engine stopping events and therefore less fuel savings from your start/stop.

      Disclosure: Long AXPW

  • avatar

    Not all start-stop system are the same.

    My understanding of Mazda’s system, that is cited in this article, doesn’t actually require a battery or the alternator to restart the engine as it uses combustion to restart the engine through use of its direct-injection system.

    • 0 avatar
      bumpy ii

      Right. Computer controls shut the engine off at a certain point in the Otto cycle that allows a variation in injector and ignition timing to kickstart the engine.

      It occurs to me that Mazda has been doing a lot of innovation in ICE operations recently, generally flying under the radar.

    • 0 avatar

      Not all start-stop system are the same.
      You’re right. It sounds like great technology and doesn’t seem to have the issues that the other systems mentioned in article have. Probably pretty cheap too. I wonder if the normal start for the car uses the direct injection method? Eliminating the starter would save some weight and would be one less potential failure point in the vehicle.

      • 0 avatar
        Brian P

        The kick-start method that Mazda is using for their start/stop system will not work to start the engine from stone cold. Think of how much effort it takes to crank an engine over with the oil in the sump at -30 C. Still need the normal starting motor for that … but at least Mazda’s system minimizes extra wear on the system.

      • 0 avatar

        The Mazda system (i-Stop) doesn’t operate unless the engine is up to temperature or if the electric demand in the car exceeds what the computer figures the battery can handle. It is also extremely sensitive to driver input (changing pressure on the brake pedal), so the engine doesn’t stay off as much as calculations would predict.

        Also, on the current i-Stop systems, the combustion restart does not fully start the engine; they still need a starter, but it doesn’t get strained as badly as in other manufacturers’ systems.

        All-in-all, the Mazda system looks the best, but I expect it will continue to evolve & improve for the next few years. I am eagerly awaiting its arrival in the US.

  • avatar

    I had a car with start stop ‘technology’. it was annoying as he**, and didn’t save me any gas. (it was a 1987 Ford Sierra, and the engine would shut down as soon as you pushed the clutch and brakes at the same time, possibly a vacum related problem)

  • avatar

    Nice catch Ed – its another example where meeting mandated standards is no guarantee of real world performance.

    • 0 avatar
      Greg Locock

      Fraid not. The main sentence of interest “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).” makes no sense, so attempting to analyse any conclusions from it are not revealing much.

  • avatar

    I thought that the redesigned 2012 Kia Rio will be the first car in the US equipped with a start/stop system.

  • avatar

    OMG this is bad.
    But car makers probably take that into account in their design; i.e. the star-stop system using less than 50% of the battery capacity.

  • avatar

    I’ve often thought that with stick shift cars the mechanism could be simple: when the car is stopped and in neutral and the clutch is let out the car should turn off the engine; as soon as the user pushes the clutch in the engine should start. By triggering from what the driver is doing you wouldn’t get a huge number of starts and stops, but instead only triggering for the long duration stops that make the biggest difference; I don’t go into neutral if I think the stop light will soon turn or I’m just coming to a stop at a stop sign. With a quick restart, the engine should be ready to go by the time I shift the transmission into gear.

    • 0 avatar
      Brian P

      I believe that’s the logic they’re already using; expecting the driver to place it in neutral in order to activate the auto-stop. If that’s what is done, and if I drove per my usual habits, it would seldom activate. I always have it in gear at a stop, ready to go.

      • 0 avatar

        I haven’t heard about any system that requires being placed in N. Instead, they use a computer that interprets events to predict if you will be stopped for a while, and it shuts off the engine. When you let off the brake, it automatically restarts the engine; this system is more seamless and offers faster response so you are less a nuisance to those behind you.

    • 0 avatar

      I currently have a BMW 116d here in Germany with this technology and this is exactly how it works. The way that I drive, it also rarely kicks in, although it is rather rough when it does and has to restart. I friend has the same technology in his Golf BlueMotion and it is much smoother and quieter.

  • avatar

    Is this different for hybrids? Since Cadillac started using the technology in the ’80’s there’s been millions of miles driven with it. Shouldn’t the problems have cropped before now?

  • avatar
    Hildy Johnson

    Nothing of this makes any sense. As long as the battery is strong enough to crank the engine, it will work. Should the battery fail, the car would simply not start up again after stopping, which would hardly go unnoticed. Alternatively, if the car detects the battery failure and does no longer shut off the engine, that would not go unnoticed either.

    A reduction in battery capacity would only matter to regenerative breaking. But that’s real hybrid territory, and I suppose those systems use more durable batteries.

    • 0 avatar

      The reduction in battery capacity causes less start-stop cycles(skips the cycle and the engine continues to run while you sit at the light) so less fuel savings.

  • avatar

    Furthermore, the capacity of batteries is rated in cold cranking amps, to indicate starting a cold engine in cold temperatures. Neither of which should be an issue with a warmed-up engine & engine compartment.

    Don’t all batteries lose some of their theoretical “factory-rated” capacity shortly after leaving the factory? Are AGMs worse then others? I know when I buy a battery for the bike I won’t take anything else than an AGM, as they are much more tolerant to periods of sitting unused than a conventional battery, even a conventional battery plugged into a tender.

    • 0 avatar

      Battery capacity is rated in Amp-hours. Once you start using up those amps for restarts, you gotta recharge regardless of temperature. CCA is just the measure of a battery’s max amp output at a specific cold temp.

  • avatar

    Just one idea on why a degraded battery may reduce fuel savings; a low battery may require even a warm engine to turn over 2 or 3 cycles before it fires up. While this is happening raw fuel is just spewing out through the exhaust.

  • avatar
    Beta Blocker

    If Mazda is doing it, but no one else is doing it — or if very few other car makers are doing it — that’s the end of Mazda in the American car market.

    Too bad, as I am very pleased with my 2009 Mazda 6.

    • 0 avatar
      Brian P

      What are you talking about? Why would this specifically affect Mazda and no one else? I don’t get the point of your post.

      In any case, the start-stop system that Mazda uses doesn’t rely on the battery for restarting the engine. They use trickery with the direct-injection system to kick-start the engine – and it has already been discussed in this thread – so Mazdas with start-stop would not have any of the alleged (and I suspect, overstated) adverse effect on the battery.

  • avatar

    Can anyone address why a nickel metal hydride battery wouldn’t work in this application? They’re not much more expensive then lead-acid these days. I realize Exxon owns GM’s NiMH patents, but I doubt they would apply to this use of the technology.

  • avatar
    Tommy Boy

    These systems will greatly increase vehicle operation costs to the consumer:

    1) Frequent start-stops, with the accompanying drop / loss of oil pressure, has to significantly increase wear and shorten the life of the engine; and

    2) Batteries will cost more, and have to be replaced much more frequently; and

    3) Particularly for people in cold climates, older vehicles that don’t instantly restart will require repair / maintenance to keep them “drivable” in traffic, lest there be a safety hazard at every stop light, and

    4) For those who don’t dump money into their car to keep it “instantly restartable” (or can’t afford to), we’ll likely see in increase in rear end collisions at stop lights.

    • 0 avatar

      1. Use a few check valves to keep a supply of oil in the upper reaches of the engine. There’s still a film of oil running along the insides of the engine, and you’re not leaving the car stopped long enough to drain all the oil with the gearbox still in Drive. The auto start/stop function is disabled when the engine is cold, so wear isn’t that much of a problem.

      3,4. Have the computer disable start/stop if the battery isn’t up to snuff. Easily fixed.

      • 0 avatar
        Tommy Boy


        While a check valve of some sort (if it can be done) might help, I don’t think that it’d totally resolve the issue. I’ve been in stop and go traffic (e.g., Boston) in which you may be sitting still for five minutes or more (ditto some stop lights), so I’m skeptical that by then most oil wouldn’t still drain into the pan. Indeed, in Boston-like traffic you can be stop for several minutes, then move may 30-50 feet, and be stopped again for several minutes … this cycle repeating for a half-hour or more. Thirty or forty start-stop cycle every morning for a commuter just has to be hell on the engine.

        As for disabling stop start just on battery capacity, I’m also concerned about aging spark plugs making starting take longer, the electrical issues of the various sensors and relays required for start-stop, etc. (Not to mention if they get older and funky and start “stopping” the engine at the wrong time, or even just too early (e.g., in stop and go and the engine cuts out before the vehicle is at full stop and an individual’s power brakes / steering cut out).

        All such systems do is add (at best) incremental increases in fuel economy at a significant increase in complexity (and possibly cost) … and certainly higher maintenance costs in batteries, starter motors and God knows what else over the (reduced?) life of the car, likely wiping out (and more) and fuel savings.

    • 0 avatar

      Engines suffer very little damage during restarting when already warm. If you were cold-starting your car at every light, then your points would be correct.

      Note that hybrids have been doing this for over a decade, and where is their increased service costs?

  • avatar

    I am wondering about the little icon “advertisement” pop up for the dodge ram that I got over the graph… what’s with that?

  • avatar

    I agree with TommyBoy. I do not care how much fuel these gimmics save if it causes premature wear of the engine/transmission due to constant start stop cycles. More wear occurs to the engine during these cycles than any other time of operation. Mfgs don’t care if maintenance costs increase. They just see it as additional revenue as long as it gets through warranty and they can claim they meet the EPA standards for that year.

    • 0 avatar

      Full hybrids, such as the Prius, have been doing start/stop for years without a problem.

      • 0 avatar

        Prius situation is very different – no lead acid battery and no conventional starter involved in ICE start/stop. “Engine start: To start the engine, power is applied to MG1 to act as a starter. Because of the size of the motor generators, starting the engine requires relatively little power from MG1 and the conventional starter motor sound is not heard. Engine start can occur while stopped or moving.” Not only does engine stop/start when the car stops but it cycles on and off all the time as acceleration, hills, etc call for more power or less.

      • 0 avatar

        Yes, but they were trying to say that by simply starting/stopping the gas engine over and over again you’re putting excessive wear on the engine, which hasn’t been the case for the Prius, which is doing stop/start constantly.

  • avatar

    The 2012 Buick LaCrosse eAssist system uses a lithium-ion battery. It is like a souped-up stop-start system.

  • avatar

    I’d like to see a flywheel energy storage system be used instead, but the weight/space penalty might be too great.

    • 0 avatar

      Flywheels also cause significant handling problems because of gyroscopic effects. If the flywheel is spinning and the axis of the car changes (off-camber road, change of incline/decline), the flywheel will cause the car to want to turn.

  • avatar
    John Horner

    ” … cleantech investment fund Pacific Crest ”

    Hmmm, does Pacific Crest have a dog in this race, perhaps an ultra capacitor company or three? Also, is the Pacific Crest in question this company: ? If so, they are an investment bank, not an investment fund. Those are two different things (though both are known for producing plenty of spin).

    Also, this reporting seems to conflate start-stop technology with mild hybrids. All mild hybrids that I know of also use start-stop, but they are not the same thing. All hybrids also tend to come with low rolling resistance )LRR) tires, but that doesn’t preclude non-hybrid vehicles from getting fuel economy gains out of LRR tires.

    Finally, I think there is a misunderstanding in this bit: “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).”

    If indeed AGM batteries loose some of their ability to recharge after two weeks of use in a start-stop equipped vehicle, that does NOT mean that there is necessarily any significant loss of fuel economy. Start-stop does not equal mild hybrid, and I don’t know of any mild hybrid vehicle which use a lead-acid battery as the energy storage medium. If indeed the AGM batteries are loosing some of their capacity early in life it simply means that there is not as much reserve cranking capacity available. Modern cars rarely call on anything approaching the full capacity of a decent sized battery, so this may not be much of an issue. Now, if said batteries become useless and have to be replaced in only a few month, then you have a big problem. But, start-stop is in use on millions of vehicles in many markets and I’ve not heard any reports of premature battery death issues.

    All in all, this story doesn’t pass the smell test. My guess is that either Pacific Crest was misunderstood, or that they are selling something :).

    • 0 avatar


      Are AGM batteries even used in start-stop systems?

      Does sound like they are pushing ultra-capaicators….

      Bigger issue: the macro claim is driver behavior means they get different fuel mileage than the EPA or european test? Is that a surprise to anyone?

  • avatar

    If a driver ends up NOT using the start-stop system, maybe then the fuel efficincy gains are parly lost.
    Also, if a battery is not kept fully charged because of this system, the fuel efficiency may also be affected.

  • avatar
    M 1

    I have a hard time swallowing some of those claims about AGM batteries.

    They’re VERY common with motorcycles, I’ve been running AGMs for… maybe 10 years? Something like that. In fact, I have one motorcycle still going strong on an AGM that I bought in 2003. Certainly bikes don’t take as much juice to get started, but the batteries are often no larger than two paperback books, so I’d think a 50% power loss would rapidly become apparent.

  • avatar

    Not to sweat the small stuff too much, but isn’t AGM absorbed glass mat, like an Optima battery?

  • avatar

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

    • 0 avatar

      That is a much more informative article than the one cited above. I have read it and have two comments:

      1. The article states that they have modelled a stop-start cycle as an accessory load of 50 amps for 60 seconds, followed by starter load of 300 amps for 1 second. The accessories in new vehicles consume 50 amps of current?!?!

      2. In the comments, the author states that applying stop/start to manual transmission vehicles is easy, but manufacturers have had a difficult time applying it to vehicles with automatics. My guess is that goes a long way towards explaining the lack of stop/start availability in the US til recently.

      • 0 avatar

        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).

  • avatar

    “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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