By on December 6, 2016

 

2010 Toyota Prius

Peter writes:

I drive a 2010 Toyota Prius. From top to bottom, I’m thoroughly impressed by the technology in this car. And yet this engineering marvel is so easily disabled by its inferior owner leaving a dome light on overnight and draining the 12V battery.

Is there any technical reason cars allow the 12V battery to be drained down beyond the point where the car will start? Who needs that extra 6 hours of dome lighting?

Sajeev answers:

And here I thought every modern vehicle implemented a battery saver relay/switch/doohickey, eliminating human error! The Best and Brightest better find us a reason not to isolate such “human-error only” items, as hybrids aren’t exempt in my book.

Since when did a map/dome/cargo light play such an influential role in an electrical system’s design?

Let the people have an always-on USB port/cigarette lighter for an emergency phone charge (they stop charging at 100%) or for a hit of that sweet, sweet nicotine monster. Everything else open to human errors should shut off after 10-30 minutes of inactivity from a human or proximity key.

I’d recommend one of these:

Easy: change the offending (reading/dome/cargo) interior lights to LED bulbs. Not all are created equal in terms of light quality, but that’s a bigger concern for exterior lighting. LED’s lower power consumption could be the difference between a start or a no-start on that tiny 12V battery.

Hard: Install a battery saver relay intended for another application, a battery saver system, or a trigger module: all require extensive knowledge of the Prius’ chassis wiring to make sure the 12V system won’t go berserk over such a change.

[Image: By Mariordo Mario Roberto Duran Ortiz (Own work) [CC BY-SA 3.0], via Wikimedia Commons]

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry … but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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51 Comments on “Piston Slap: Foiled by a Dome Light?...”


  • avatar
    zoomzoomfan

    That’s really odd that it doesn’t shut the light off after a pre-determined time. Even my ’99 Pontiac Grand Am did that. I think it was 30 minutes or so and the light would turn off. My current car, I can adjust it to time out after 10 minutes, 30 minutes, or 60 minutes.

    Is it possible this is a setting buried deep within the Prius’ infotainment menus? My friend has a 2009 Prius and I had to use YouTube to figure out how to disable it’s weird backup alarm thing.

    • 0 avatar
      psarhjinian

      This seems to be a Toyota thing, or at least it was. My old Sienna did the same thing: there didn’t seem to be much in the way of body-computer subroutines for “if this…then that” control of lights and such based on other parameters. Off meant off, on meant on.

      • 0 avatar
        dukeisduke

        Yeah, Toyota is way behind the curve on this. Our ’08 Sienna and ’13 Tacoma don’t have a battery saver feature, but GM has been doing this for years. Come on Toyota!

        • 0 avatar
          psarhjinian

          I’m not sure if this is really a serious issue, and in fact might be a good thing: it adds a fair bit of complexity and it’s a real challenge to figure out why something can gone wrong.

          Case in point, I spent quite some time trying to figure out why our Montana couldn’t turn off the radio when the engine was off and the key out of the ignition: sometimes it would turn it off when the door opened, sometimes not. Turned out that it couldn’t reliably tell when the door was open due to a dirty sensor, which was causing other issues.

          My old Toyota (a Sienna) and Honda (a Fit) didn’t bother to try: if you took the key out of ACC, the radio was staying off: no BCM guesswork required. Ditto the lights: switch on = lights on. The BCM didn’t get involved.

          • 0 avatar
            WheelMcCoy

            To avoid the complexity of “smart” bulbs that turn themselves off, auto makers could put interior lights on a different circuit powered by household batteries (e.g.: AA or 9 volt) in the glove box. This will surely annoy some, but it would prevent killing your car battery. And the batteries would be fairly easy to replace (maybe once a year) with LED bulbs extending that period.

            The other option is to carry around a portable usb / cigarette lighter battery jump starter. They go for about $200 though.

            BTW, I recognize the location in the photo! It’s downtown NYC, opposite the Federal Reserve Bank.

          • 0 avatar
            Erikstrawn

            If manufacturers put a separate battery in the glove box to power the interior light, it would be a 9v in proprietary packaging, it would cost $50, and it would only be available at the dealership.

  • avatar
    heavy handle

    My guess is that your 12v battery is done, and/or you have some other power drain.

    Have your battery tested. Ideally, you should do both a load test (using a tester that’s the size of a breadbox) and a new-style no-load test (using a tester that’s the size of a hoagie).

    You may also have a phantom power drain, caused by a component that is not shutting-down. Those can happen in almost random places, I’ve seen a power window regulator take-down an Acura, a radio drain a Mazda, and so on. It’s a time-intensive diagnosis. You need to go through your fuse boxes, see what circuits are hot, and then unplug each and every component on that circuit, one at a time, until the drain stops. Some components will wake-up overnight (Mini had a famous issue with that), so not finding a power drain doesn’t rule-out having one.

    • 0 avatar
      pdieten

      This is the right answer. The car is a 2010. OEM 12v battery is only good for about six or seven years, and yours is worn out. My wife’s 2010 crossover needed a new battery because it failed a load test, and my ’09 car needed one last year for the same reason. A dome light isn’t enough draw to kill a healthy battery, but it could kill a tired one.

      • 0 avatar
        DenverMike

        A dome light will easily kill a healthy battery overnight. If the car has other drain/electrical problems or an already weak battery, that’s besides the point. It still should have an auto-off timer. What you leave the car alone for more than a overnight with the stupid dome light on? There’s no excuse for this.

      • 0 avatar
        Higheriq

        My experience with many new cars over the years is that OEM batteries last ~2 years. That is one area that OEMs cut costs to save a few bucks on every vehicle – owners neither notice nor care. I always replace original batteries at the two year mark.

        • 0 avatar
          LeMansteve

          Two years? That is an absurdly short replacement interval.

          • 0 avatar
            dukeisduke

            Agreed. That’s one thing that Toyota owners crow about on the forums – how good the factory-installed batteries are. The one in my Tacoma was made by Johnson Controls, and some owners get as much as eight years out of them. The ones sold in the Toyota dealer parts department don’t have the same rep as the factory batteries.

            One kind of battery I’m hearing good things about is AGM (absorbent glass mat) batteries. They’re supposed to last up to two times longer:

            https://www.pepboys.com/car_care_corner/car_care_basics/maintenance/advantages_of_agm/

          • 0 avatar
            whitworth

            It depends where you live.

            If you live in a really hot climate, it’s ever 2 years. And basically every battery these days comes from 2 manufacturers with just different stickers and warranties on them.

          • 0 avatar
            Wheeljack

            The batteries in my ’02 Wrangler and ’06 Wrangler are both original and work fine. I keep a trickle charger on them when the vehicles aren’t in use, so that helps, no doubt. I even drove the ’06 across country earlier this year towing a trailer and it did just fine. I’m sure a lot of people would think it’s insane to take a 1,500 mile trip on a 10 year old battery, but I wasn’t worried.

            Incidentally, my Dad got 14 years out of the OEM Motorcraft battery in his Explorer, and he didn’t give it the kid gloves treatment with a trickle charger like I’ve done with mine. Color me impressed with Motorcraft batteries.

          • 0 avatar
            dal20402

            The factory batteries in the Lexus LS460, although made by a perfectly reputable supplier (Panasonic), are notorious for having a short replacement interval. My car had an OEM battery installed in early 2014, before I bought it, which started to die in late 2015 after one overlong session of listening to the Mark Levinson stereo at high volume with the engine off. The replacement is Johnson Controls (via AutoZone) and so far is OK after a bit more than a year with the car driven only a couple of times a week.

          • 0 avatar
            rpn453

            The Panasonic in my ’04 Mazda3 is now over 12 years old and still going strong. I’ve only topped up the electrolyte level once.

    • 0 avatar
      Lack Thereof

      What kind of weak-ass car battery is killed by leaving a dome light on overnight?

      I’ve left the dome-light on in my Cavalier for days with no problem.

      The letter-writer’s battery is completely shot.

    • 0 avatar
      TheEyeballKid

      The Prius 12v Auxiliary battery is for starting the computer, unlocking the doors, and the dome light (as well as other auxiliary functions) – it does not start the engine (that’s done by the big hybrid traction battery), so it does not need to handle a large load (and load testing would probably indicate it needed replacement years before it really did). My OEM Prius battery lasted 11 years – if the car gives weird errors in which it refuses to “boot up” because it is parked on an incline (or intermittently does not boot up), it is time to change the battery.

    • 0 avatar
      Scoutdude

      Note a Prius has different 12v battery demands than a conventional car. It just has to have enough power to wake up the PCM and energize the HV battery relay, once that relay is closed the HV battery takes over and charges the 12v through the inverter. It doesn’t have to crank an engine. So it has to be really low to not start the vehicle. Because of that and for weight savings purposes they use a really low capacity battery. That means that a light left on will drain the battery pretty quickly.

      Personally I find it inexcusable that in this day and age they don’t have a battery saver system other mfgs have been doing it since the 90’s and it doesn’t cost that much.

    • 0 avatar
      brandloyalty

      That’s the answer. Because full hybrids do not use the 12v battery to crank the engine, the 12v battery can be in very bad shape but there are no symptoms. As long as the 12v battery has enough juice to close the relays that connect the hybrid battery (and this is why parking on an incline may have an effect), the hybrid will start. Started, the 12v systems will seem normal due to the charging system.

      If the 12v battery gets low enough, the first signs of trouble will be strange electronic issues that don’t seem to be relared to anything. Hybrid owner forums regularly feature such cases.

      So far as I know, the Escape Hybrid has a regular capacity 12v battery. It also has battery saver systems. That, in my opinion, cut power to interior lights etc. too soon. Like if you want interior lights on to work on something or read.

      One more thing for the auto writers to learn about hybrids.

  • avatar

    If it’s anything like the Insight, it uses a lightweight battery, since it is just needed for the control system sand not used for the starter. When I went to test drive one years ago, ever tester on the lot had a dead battery.

    The part I didn’t get is that the thing should be able to jump itself with a simple transformer from the high voltage system. :-/

    Oh, fix your site, TTAC. I had to try to type this 6 times because the page kept reloading.

    • 0 avatar
      sirwired

      The car won’t jump itself because down-volting DC for that amount of current would require expensive and/or large and heavy conversion electronics, which don’t make sense for something that happens so rarely.

      • 0 avatar
        Scoutdude

        That stuff is already there, the 12v battery is charged from an inverter by the HV battery pack. The problem is that you need 12v to wake up the computer and have it activate the inverter. In theory there should be a simple way to have a button you could push that mechanically activates the relay. Of course it would need to be on the inverter itself which is not necessarily in an accessible place.

    • 0 avatar
      Click REPLY to reload page

      My username is a gentle protest and reminder of that bug, and will remain until it gets fixed.

  • avatar
    DenverMike

    It’s stupid is all. My 11 year old Super Duty and 13 yo F-150, both crank-window XL beasts, kill the dome/spot/map lights, 20 minutes after you walk away.

  • avatar
    Vulpine

    Clearly it’s a combination of issues, of which the battery itself is one likely factor. Sajeev’s suggestion of an LED replacement is a valid one and offers the potential to be brighter (old tungsten elements tend to dim over time) as well as more efficient. You can find LEDs with a warmer color like the tungsten bulbs, though they might be fractionally more expensive.

    • 0 avatar
      dukeisduke

      Yeah, an LED is a good suggestion, as it wouldn’t use as much power. It’s not a cheap solution for something like our Sienna, that has two map/dome lights up front, four map/dome lights for the second and third row seats, and two courtesy lights (one at each sliding door), plus one in the rear compartment, which we normally keep switched off.

    • 0 avatar
      whitworth

      The power consumption though really isn’t enough of a game changer that you could not worry about leaving it on for over night.

      Yes, LED is quite a bit more efficient, but usually the LED replacements are quite a bit brighter than what the original filament bulbs put out, so a lot of those gains go out the window in the quest for brightness (unless you go with a really “dim” led replacement.)

      Most car led users though want as bright as possible, that is the main selling point, not the energy savings. So the LED will end up consuming about the same as the original halogen, but be orders of magnitude brighter.

  • avatar
    SilverBullett

    I have a 2007 Prius which I have left the interior light on for 8 hours, whiile at work. The car started again just fine. This is still the original 12v that it came with 150k miles later. In the case of the 2010 Prius, maybe another battery drain elsewhere?

  • avatar
    W.Minter

    Problem: Prii use their 12 V battery mainly only to boot the car, therefore you have a simple user experience. It works or it works not, and there are no signs of a weak battery before.
    Leaving the dome light on overnight is no big deal with a healthy battery. Source: own stupidity, thankfully without having to learn it the hard way.

    Solution: buy a CTEK Multi US 4.3, recondition your 12 V AGM battery and everything should work. Unsure? If you know how a wrench looks like, you can replace the battery on your own. Many folks go for a Optima Yellow Top (DS46B24R).

  • avatar
    DenverMike

    The Prius is just a crappy, overpriced throwaway. I’m puzzled as to why it’s vehemently defended by they BB.

    • 0 avatar
      King of Eldorado

      To the extent that the B&B defend the Prius, it’s because it’s NOT a “crappy … throwaway”; it’s repeatedly been found one of the most reliable cars out there. From an economic standpoint, it might be considered “overpriced” only because at current low gas prices it will take longer to make up the hybrid premium in gas savings.

      • 0 avatar
        DenverMike

        I didn’t say it was unreliable. So go ahead and make an excuse for an absent feature found standard on Kias and other junk for decades. Probably standard on Daihatsus too.

        • 0 avatar
          psarhjinian

          This is one of those features that Toyota doesn’t seem to go in for. Frankly, when you’re topping the reliability charts for more than three decades, I’ll tend to favour your judgement on what you consider a frivolous feature.

          • 0 avatar
            heavy handle

            Toyota has a problem with lights. They are the only manufacturer that hasn’t figured-out automatic headlights either.

            Driving home last night, it’s dark. I notice at one point that the only cars around me that don’t have lights on are Toyota, including one late-model Lexus ES.

            I understand that Corolla owners respond positively to low-spec cars (even if a feature wouldn’t cost a penny), but a Lexus? It’s just insane that they don’t have automatic lights.

          • 0 avatar
            Scoutdude

            @ heavy handle, that is because many of the Toyota instrument clusters light up with the car is on. So the owner doesn’t get the clue of not being able to see the instruments as a reminder that the headlights are not on. Throw in DRLs and yeah they need to go the autolamp route, yet another feature that has been implemented with out issue for several decades by other automakers.

    • 0 avatar
      dukeisduke

      They’re definitely not throwaways; 200,000 miles or more is a chip shot for a Prius, and 300,000 isn’t uncommon. Some friends of ours bought two used 2nd-gen cars. He’s a retired USAF Lt. Col., and commutes 50 miles each way every day (he’s a test pilot). He’s already over 200k, and the only real problems he’s had are generic car stuff, like struts wearing out, and the optional HID headlights (a known problem on them).

  • avatar
    whitworth

    I never have understood with all the incredible technology on cars these days, why an “automatic off” on something like a dome light hasn’t been a standard feature on cars for like the last 20-30 years.

    Has anyone ever sat hours on end using a dome light?

    I have a Lexus LS, their flagship, and if my kids open up like a vanity light and forget to close it, it can leave me stranded.

    • 0 avatar
      rentonben

      It is a standard feature on just about anything other than Toyota. The feature is so old, it’s reached drinking age.

    • 0 avatar
      Scoutdude

      Yea it has been standard on cars from other mfgs for many years. I can’t count the times that back when my kids were young that they left the reading lights on in the back seat. Since the dome light stay on for a little while after the door is closed it is easy to walk away w/o noticing that a reading light is left on. I do have to admit it is annoying when you get in the car and start it up only to find that light on in the back, with a switch that is not an easy reach from the driver’s seat and no kids back there to turn it off, but it certainly beats the car not starting.

    • 0 avatar
      Steve65

      Every car I’ve ever owned had an “automatic off” dome light. When you close the door, the light goes off.

  • avatar
    MWolf

    Every car I’ve owned has automatically turned off lights left on more than 45 minutes. Why doesn’t that Prius have that feature? Not sure. You’d think it’d be a given.

    On the other side of the same coin, I suppose with some things, some manufacturers assume if you don’t know enough to pay attention to such things, one mistake will teach you not to forget. It’s never wise to assume something is going to turn off itself unless otherwise noted or you’ve observed it.

  • avatar
    Kyree S. Williams

    My car came from the factory with LED overhead lights everywhere except the rear cargohold, and that light only comes on if the liftgate is open. I’m not sure whether or not my car has a battery-saver feature that will cut lights off before they drain the battery, but based on other people’s battery experiences with newer VWs, I would guess not.

  • avatar
    Von

    If installing this doodad or that thingamajig is too complicated for you, how about just look at your car before walking off?

  • avatar
    scottcom36

    I’m pretty sure my ’15 Civic has a battery saver for the interior lights, but accidentally pop the trunk and the light will flatten the battery.

    • 0 avatar
      sgeffe

      Interesting — my 2013 Accord in the avatar will kill the trunk light. IIRC, it doesn’t kill the map lights, but I don’t recall accidentally leaving one on.

      Last oil change, the original battery tested ABOVE rated CCAs, so I have a decent one in my Touring V6. Unfortunately, the 4-cylinder batteries are routinely knocked for short lifespan.

  • avatar
    Master Baiter

    This would only happen with an incandescent bulb. Even a 2W LED would make a bright dome light, and only draw 170mA from the battery. A quick web search says the Prius’ battery is 45Ah, which could power this bulb for 132 hours on half its capacity, not including any other battery draws when the car is “off.”

  • avatar
    el scotto

    During some times in my misspent youth; we’d deliberately take the dome light out. So “the cops couldn’t see us”; yeah, like they didn’t know. Why doesn’t the OP just take the dome light out? Three minutes of effort, problem solved.

  • avatar
    manu06

    Instead of an on/off switch, why not a mechanical timer you twist ? Maybe15 minutes max ?

  • avatar
    LTDwedge

    Call me aware ! Any ORANGE colored high voltage wiring – read “LETHAL” needs to be left alone. Just having safety gloves and a wiring diagram may not prevent DEATH. Just don’t want to read about someones unplanned demise. Insufficient knowledge will kill.

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