12V Aux Keeps Dying While Off

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by ep3eddie, Nov 23, 2021.

  1. ep3eddie

    ep3eddie New Member

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    Hi everyone! First post here, lurking since I got my car back in 2018. So I have a 2008 Gen 2 Prius, I believe it is touring based upon the fact it has the touring sway bar (despite not having other touring features such as HID or the larger wheels). Great car after fixing the little odds and ends after purchasing and given proper maintenance and care was treating me well until about a year, year and a half ago. Since then, it seems to be killing 12V aux batteries like they're going out of style!

    So as far as I know, everything electrically on the car is stock (radio, speakers, etc.) I myself have a dash cam which (since the first battery death) remains unplugged unless I explicitly want to use it out of fear; I also have a bluetooth adapter set up to the charger in the center console armrest which remains plugged in 24/7 (but to my knowledge shouldn't be drawing power when off, the status light goes out when car is unpowered).

    After it killed the original battery (who knows how long it was in there), I replaced it with a Bosch Platinum as it was all that was available that day. I drove it regularly at that point, but it died a few months later after I accidentally left a cabin light on overnight (door must have been left open or something). So about a year and a half ago, I replace it with an Optima Yellow Top, I hear they are the best. It ends up killing that battery within a few months, but because of the pandemic I didn't need it, we just used my fiancees car for around a month until my school workload lightened up significantly and I had a lot of spare time. Got that warranty replacement and the cycle repeated itself. This time around I was finishing a ton of school work as it was my senior year and I had to take a heavy load to finish on time with how everything worked out after I transferred, so it sat for probably two or three months, using the other vehicle on the off chance we needed to go somewhere (home bodies through the pandemic here in NYC). I get a new battery from AutoZone this time after finding out PepBoys (last site batteries were purchased) was removed from the authorized sellers list.

    This time I *really* made it a point to drive the car at least a handful of times per week, and also to baby this battery.

    I turn off all cabin lights, I don't use my DRL it's either off or headlights, and I turned off my SKS smart key system, only entering using the fob and inserting my key to drive, worrying perhaps the proximity of my key in the house to the car was an issue.

    Well around a month later it won't open from the fob one day, won't turn on to drive, and after failing to start the first time the dash won't even do the turn on as it attempts to start. I called AutoZone today and I should be able to pick up my replacement battery tomorrow, but I had to get on here to ask for your guys' help in the matter. I'm likely going to sell the car relatively soon, hopefully the job search returns something and I can get a newer car, but in the mean time does anyone know any potential sources of the problem and fixes? I'd rather not have to replace the battery again before I wind up selling it, and I'm sure I'd get a bit more for the car if this issue wasn't persistent.

    Thank you guys in advance for all of your help
    -Ed
     
  2. dolj

    dolj Senior Member

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    Firstly, insist that the seller of the battery put the battery on charge and fully charge it before you take delivery. Take it back to them to get it fully charged and tested if it is already installed. It would be handy to have a fully automatic smart 12 V battery charger (HF for ~$30) so you can do this yourself. For future reference, always fully charge a 12 V battery ASAP if has a major drain event.

    Once you know the battery is fully charged and in good condition, make sure the connections are clean and tight. Open the battery clamps so you can push them all the way down so they sit right on the bottom of the terminals. Do the clamp nuts up with the closed end of a box wrench and be very careful not to lever the clamps up away from the bottom as you tighten the clamps. Check also that where the negative cable attaches to the car body is also clean and tight.

    You next need to rule out a parasitic drain which is easier said than done, but a good auto electrician can sort that out if you don't know how to do that. There is also so a good video on YouTube if you search for cars - how to perform a parasitic draw test by EricTheCarGuy. It's not specifically a Prius video but the principles are pretty generic.

    You are looking to confirm your car is only drawing 18 - 32 mA when fully quiesced (which takes 15 - 20 mins) of everything off and key fobs 10 ft or more away from the car.

    Good luck.
     
    #2 dolj, Nov 24, 2021
    Last edited: Nov 24, 2021
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  3. sam spade 2

    sam spade 2 Senior Member

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    This is very important.

    AND.....just because an otherwise healthy battery is mostly discharged once does NOT mean that it is done for.
    If fully re-charged they often can last years.

    BUT the onboard charging system often won't completely fully recharge it and that is important.
    An automotive battery left "half charged" for a long period almost guarantees premature death.

    SO.....even if you are not in a situation where you can use a battery tender all or most of the time,
    being sure it is fully charged every couple of weeks is almost as good.
     
  4. Tombukt2

    Tombukt2 Active Member

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    Walmart sells The Schumacher battery tender cable an eyelid on each end to a plug that can hang out the vent hole or lay up above the tray that you removed to get out the battery so you don't have to get it the battery to hook up the tender when it just takes a second to plug up you'll tend to do it

    SM-A715F ?
     
  5. pasadena_commut

    pasadena_commut Active Member

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    Obtain a Uni-T UT210 DC clamp ammeter and place it on one of the battery leads. (Note, last time I looked Harbor Freight did not sell a DC clamp ammeter, all of theirs are AC only. Fluke and others also make DC clamp ammeters, but they cost more than the UT210.) Then watch the measurement when all the doors are closed. Note, the hatch must also be closed, so you either need to be able to read it through the closed hatch or you need to crawl over the folded down rear seat. If there is no parasitic draw it will measure <35 mA once the car settles down. If you open any doors or the hatch it will be over 0.5 A.

    See this thread:

    How much driving to fully charge the 12V? | PriusChat
     
  6. Another

    Another Active Member

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    Pro model of Uni-T ammeter is on sale today Black Friday special at Amazon

     
  7. Mendel Leisk

    Mendel Leisk Sand Pounder

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    @ep3eddie what's your parking situation? Street, driveway, carport, secure garage? A smart charger might help.
     
  8. Tombukt2

    Tombukt2 Active Member

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    I get a pretty decent discount on gasoline so none of my cars ever get very much of a chance to sit I'll get bored and get in it and drive it somewhere just because so I haven't really had the sitting problem the gen 3 that we own pretty much is a drive 6 miles to the top of the street to work sit all day then start the car at the end of the day either drive the 6 miles back to the house by way of the supermarket or Walmart and that's about the daily you see there so that's maybe under 30 miles every day would seem to be not ideal car always starts always drives not too much of anything exciting exciting other than the head gasket blew it 240 250 then the people had it all apart and reassembled it with a fisher crack or something in a piston that let go 17 miles from where it was repaired when I pulled that engine out to replace the other one I'll pull the head off take pictures of the hole show why I lost compression massively at number two and that'll be that .. ZZ engines are open deck and require some strict methodology when being worked on as far as rebuilding goes so without people in the know and all that it probably generally won't go well and then trying to get warranty on work that's been done not on parts is very vague so you spend more money on labor more money on labor buying the same parts again etc etc you never get satisfaction if I get an engine from another country where they value things a little differently then I believe I personally have a better shot to keep in my car on the road without the immense United States expense which is just how it works here we don't have it down yet

    SM-A715F ?
     
  9. ChapmanF

    ChapmanF Senior Member

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    One frustrating thing typical of DC clamp ammeters is they are typically great for large currents but not nearly as useful for small ones. The UT210E, as you can see from its spec sheet, offers you a 100 amp DC current scale spec'd at ±2%+3 amps. My Fluke, while more expensive, isn't much better either, sadly. It's great for the kind of measurements you'd make on a factory floor.

    What that means in practice is that for trying to measure currents at the small fractions of an amp typical of a car when it is off, with one of these clamp meters you are more or less watching the meter's daydreams and thinking it's a real signal.

    One trick I have found with mine that can improve the results a little bit at low currents is to flip the clamp around and measure both ways, and take the average (changing the sign of the flipped reading, of course).

    But ultimately, for the kind of small currents involved in car-off load measurements, there's really no substitute for opening the circuit and putting a regular ammeter in line.
     
    #9 ChapmanF, Nov 26, 2021
    Last edited: Nov 26, 2021
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  10. mr_guy_mann

    mr_guy_mann Senior Member

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    Yeah, pretty much any clamp large enough to fit around a battery cable allows it to pick up too much "noise"- and then there is drift. My best clamp is for practical purposes good to +/- 50mA resolution. I have a couple nice low current clamps -AEMC K100 and K110, but those fit really small wires.

    Posted via the PriusChat mobile app.
     
  11. rjparker

    rjparker Senior Member

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    After the second battery failed, a series parasitic draw test is essential. A high parasitic draw kills batteries fast. By the way, a few overnight drains of a battery due to a light on does not warrant battery replacement. Especially on an agm battery such as a prius uses. It just needs a charge.

    When people say you can't count on the car's charging system to recharge a drained battery they simply mean it takes hours of driving to do so. That is why plugin 12v chargers are used for major drain episodes. I like a $25 Battery Tender Jr which also comes with quick connects. The primary purpose of a small charger is when you plan to leave the car sitting for three weeks or more. You leave it connected to the car 12v system to ensure a full battery when you return. However, a Battery Tender Jr battery maintainer will charge a discharged battery.

    I have a Uni-T UT210E meter and it is a great meter for the price but it can not accurately measure low ma currents. Don't get it for that reason. It also can not do a series parasitic draw test. However a $5 or better harbor freight meter can. Any series parasitic test requires knowledge and procedure to avoid overloading and damaging the meter. But in the hands of an experienced person it can be done in 15 to 30 minutes.

    The hard part starts if you measure an excessive parasitic draw, in this case over 35 ma. Now more expertise is required to specifically find the defective part or wire.

    An interim "anti-stranding" strategy is to carry a lithium jump pack and know how to use it under the hood. Prius v series ammeter parasitic draw setup.jpeg
     
    #11 rjparker, Nov 27, 2021
    Last edited: Nov 27, 2021
  12. Mendel Leisk

    Mendel Leisk Sand Pounder

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    @rjparker, the "jumper" is to avoid losing memories?

    And then the disconnect procedure would be to go through the steps in reverse?
     
  13. rjparker

    rjparker Senior Member

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    The jumper keeps the computers from resetting when you follow the procedure. This allows much faster parasitic draw measurements since the computers are not taking 30 minutes to go to a sleep mode.
     
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  14. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Just for a data point, when I did that procedure (albeit without jumper, but with a good wait time), I was seeing about 18~20 milliamps, with sporadic spikes to 40. The spikes didn't seem to be synced with the security system icon that blinks on the dash, though that does seem a likely suspect.
     
  15. rjparker

    rjparker Senior Member

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    I also see spikes but they are not a significant issue. A specific meter may have differences in its sampling algorithm. Plus the keyfob monitoring continues until a specific time several weeks later. Obviously long term storage of the car is best achieved with a plugin battery maintainer or by disconnecting the 12v battery.

    My memory suggests disabling the keyfob monitoring in the mfd setup menu reduces parasitic draw down to that 20 ma range. In my case, I have a Homelink mirror drawing 6 ma as a part of the ~35ma ballpark limit. 35-50ma (thousands of an amp) should keep a 12v battery functional for at least three weeks.
     
    #15 rjparker, Nov 27, 2021
    Last edited: Nov 27, 2021
  16. Mendel Leisk

    Mendel Leisk Sand Pounder

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    That's us, and yup, pretty much constantly plugged into charger. IIRC it was Thursday week before last we last used the car.
     
  17. sam spade 2

    sam spade 2 Senior Member

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    That test is not "essential" if there is another more obvious cause of the batteries going bad.

    Your statement about "overnight drains" is not necessarily true.
    If it was old and weak to start with, a charge might not accomplish much.

    The devil is in the details. :)
     
  18. rjparker

    rjparker Senior Member

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    The op states he is going on his sixth battery in less than two years, so my assumption is batteries two-six were not "old and weak". Sorry if I rocked your boat but I am staying with my advice.
     
  19. ChapmanF

    ChapmanF Senior Member

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    up to now we sailed through every storm
     
  20. sam spade 2

    sam spade 2 Senior Member

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    OK, so here's the thing:
    I have neither the desire or the time to go back and review all of the previous posts in a thread to try and "interpret" the content of a single reply.

    I think that each post should stand on it's own.
    My comments were about that one and only one post.

    I do agree that given the originally posted details, both of your statements are right........for this specific case.
     
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