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Ways to lower 12v power draw when car is sitting?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Tnx, Feb 14, 2018.

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  1. Tnx

    Tnx Junior Member

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    So I had thought my battery is dying, went and tested it at Toyota dealership and Pep Boys, both said the battery is still good. The receipt they gave me said 12.6v on it, I thought these batteries are supposed to read 13+ when fully charged?

    Anyways, since my car sometimes sits idle for long, I've been trying to read up ways to lower its power draw while sitting parked.

    1. Faraday Cage for key FOB: Does this actually work? Seems like this would only save the little CR2032 battery inside the FOB right?

    2. Disable SKS system: This seems like a good solution... Except the gen 3 doesn't have that button where you can turn it off with ease, so no go here I guess.

    3. Solar Charger: Seeing a few posts talk about a solar charger, but also seeing people say that you can't just connect it to the cigarette port. What do you have to do to get this to work? Some people also report this working well with just a 1.5-2.5 watt solar charger, but that seems way too low?
    ALSO: Alot of brands say "trickle charger", I thought these aren't to be used with an AGM battery?

    4. Battery Tender: I have a Deltran JR, but no garage so it's a huge hassle to route a cord out to the front to charge the battery all the time.

    Any other recommendations? Those of you using Solar Chargers, faraday cage for FOB, what are your thoughts on this? Sorry if this post is hitting too many points at once.
     
    #1 Tnx, Feb 14, 2018
    Last edited: Feb 14, 2018
  2. bisco

    bisco cookie crumbler

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    welcome!

    a new battery will read 13+, is yours new? what made you think it was dying?

    just keep the key fob away from the car, no need to do anything crazy.

    how long does your car usually sit idle?

    there may be a way to shut off sks in the menu, i'm not sure.
     
    #2 bisco, Feb 14, 2018
  3. Tnx

    Tnx Junior Member

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    Hello. The battery in my car now is the Truestart 84 month warranty one from the dealership. It was installed in the car about 3~ years ago, but the car was rarely driven then (aunt owned it then and was going through chemo). I assume it had lots of full discharges then, and it had another one with me while sitting in my driveway. I took it out and recharged it using a Deltran Jr, and the multimeter reading of it read 12.55 (my understanding is it should be much higher). I had it fully charged today and took the prius out for errands, logged about 25 miles total with 4 cold starts, the battery then read 12.32 while connected to the car.

    So with all the full discharges + the low voltage after driving, doesn't that show that it's dying? The car is used for lots of short trips, and on weekends long trips. Also, my allotted parking spot is about 5 feet from my room here, so it's hard to keep the key fob far away from the car, which is why I asked about the faraday cage.
     
    #3 Tnx, Feb 14, 2018
  4. bisco

    bisco cookie crumbler

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    gotcha.

    1) all batteries are dying from the day they are made, just like us. it sounds a bit low, but i'm not sure you're checking it correctly.
    pop the hood in the evening, or if that is not possible, go through the passenger door in the morning and pop the hood. check the voltage at the jump point in the fuse box before you do anything else with the car, and see what it reads.

    2) my 2012 oem is still reading 12.5, but the heat where you are may shorten life. also, running it down as has been done is also bad for it, i'm surprised it is still working for you.

    3) short trips are no big deal. if the car isn't sitting idle for days at a time, the 12v will be fine.

    4) can't you place the fob on the other side of the room?
     
    #4 bisco, Feb 14, 2018
  5. Tnx

    Tnx Junior Member

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    I'm testing it by removing the back trunk cover and testing right at the battery contacts- is that not the best way to do it?
     
    #5 Tnx, Feb 14, 2018
  6. bisco

    bisco cookie crumbler

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    yes, that is fine. are you testing after the car has been off for 8 hours or more?
     
    #6 bisco, Feb 14, 2018
  7. Tnx

    Tnx Junior Member

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    Oops, the 12.32 reading I took was 1 hour after the running the car. That's still low though no? I should be waiting 8 hours so it doesn't get a skewed high reading right?
     
    #7 Tnx, Feb 14, 2018
  8. bisco

    bisco cookie crumbler

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    correct. there is a surface charge that needs to wear off. that's why i always recommend testing first thing in the morning.
     
    #8 bisco, Feb 14, 2018
  9. Tnx

    Tnx Junior Member

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    So if I'm getting a reading of 12.32 after 1hr of the car being driven, it's technically even lower right? I don't get how this battery tested to be still good then.
     
    #9 Tnx, Feb 14, 2018
  10. bisco

    bisco cookie crumbler

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    battery testing is a funny thing. most dealers don't know how to do it, but pep boys should put a tester on it, and provide you with a printout you can post here.
     
    #10 bisco, Feb 14, 2018
  11. Tnx

    Tnx Junior Member

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    Here's the picture of the receipt from pep boys:
     
    #11 Tnx, Feb 14, 2018
  12. bisco

    bisco cookie crumbler

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    you're going to have to try and figure out how they are getting 12.6 volts?
     
    #12 bisco, Feb 14, 2018
  13. Tnx

    Tnx Junior Member

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    This test was this morning- I had charged the battery fully the night before, put it in my car and drove straight to pep boys. The guy there said doing this would not skew the results of the battery (was he wrong?). He also mentioned that the machine they use was from Toyota, it looked like a skinny arcade machine. It was after the test that I went and did my short errands before returning home, tested after 1hr and got 12.32.
     
    #13 Tnx, Feb 14, 2018
  14. bisco

    bisco cookie crumbler

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    wrong. even driving to pep boys skews the results. don't charge it next time.
     
    #14 bisco, Feb 14, 2018
  15. Tnx

    Tnx Junior Member

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    Yeah I thought that was BS but I didn't know enough to question it- so what should I do now? Go back tomorrow without charging battery and test again?

    EDIT: Btw I want to thank you for talking me through on this. Would it have been easier if we did the "start a conversation" thing under our names?
     
    #15 Tnx, Feb 14, 2018
  16. bisco

    bisco cookie crumbler

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    yes, go back.

    no, an open thread is better, hopefully, others will add their opinions as they see fit. i could be giving you bad advice.:eek:
     
    #16 bisco, Feb 14, 2018
  17. Maarten28

    Maarten28 Active Member

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    According to the manual: 12.6-12.8V: fully charged, 12.2-12.4 partly charged, 11.5-11.9: discharged. Measurment to be taken 20 minutes after powering off the car and consumers.

    In rest, batteries are not 13V. They will be a short time after you switch off the car (with 14.8V charging temperature), but not in rest.
     
    #17 Maarten28, Feb 15, 2018
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  18. David Beale

    David Beale Senior Member

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    If you test the battery in the car by accessing it through the hatch, you've opened the hatch. That puts around a 1A draw on the battery (lights come on - some that you can't disable). This is actually a "good thing" (tm) because testing a battery with no load is useless. All it tells you is it's a capacitor of some sort. ;)

    The SKS in the 3rd gen turns off on its' own. You don't need a switch. RTFM.

    If your FOB is within about 2-4 metres of the car store it in a metal box. Otherwise don't worry about it. It's out of range.

    If you install a solar panel all it needs to overcome is the less than 50 mA parasitic drain of the car and the internal loss in the battery. Most of the reasonably sized ones for car maintaining put out at least 100 mA (in full sun). You will need to connect it to the battery or to the "boost point" or other "always on" power terminal. The accessory power socket in the car (cig. lighter socket) turns off then the car is turned off. A 100 - 200 mA solar panel won't overcharge the battery. It's not powerful enough. So a regulator isn't needed. If you select a larger panel a regulator is a good idea.
    Of course getting "full sun" on the panel for a reasonable amount of time each day will be the challenge.

    The best and easiest solution is the one most of us who leave the car unused for extended periods of time use. A battery maintainer of some kind. Perhaps you could "dig in" an extension cord (lift lawn turf and lay cord under it) so only the end 2-3 ft is out of the ground right at the car parking spot? Or maybe overhead with the cord end hanging down? I see both those options used here for block heaters.

    There is one other option. Disconnect the battery when not in use. The one disadvantage to this is you will need to connect 12V to the boost point to open the hatch to reconnect the battery. A small "alarm battery" or portable boost device could be used for this. Oh, and you will probably loose all the set stations in the radio memory as well as the "auto up" window settings. ;) Both are easy to reset if you want them.
     
    #18 David Beale, Feb 15, 2018
  19. lumpy95

    lumpy95 Member

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    I started a thread awhile back on keeping the 12 v charged because I drive very little anymore. My solution was a battery tender.
    The thread has a lot of good info in it and is located here: 12 volt Aux. battery Question | PriusChat
     
    #19 lumpy95, Feb 15, 2018
  20. Grit

    Grit Senior Member

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    Wrap some box like a shoe box with aluminum foil and keep the key there. Layer up if needed.
     
    #20 Grit, Feb 15, 2018
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