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Weird stuff happening? MPGs dropping? Test The Battery

Discussion in 'Newbie Forum' started by richard schumacher, Dec 13, 2009.

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  1. richard schumacher

    richard schumacher shortbus driver

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    Weird stuff happening? Test The Battery: Here's How

    The procedure below is for 2004-2009 models. Others must check the battery with a voltmeter or a scan gauge. If anyone has a verified procedure for use on other models please post it.

    It's best to eliminate the common, simple and (relatively) cheap causes first. Dying batteries are near the top of this list.

    12V car batteries have a lifetime of a few years. Draining a battery will usually permanently degrade it by some amount, and it will never work as well again. If the car starts having strange symptoms, or doesn't start reliably but runs fine, or if MPGs have been dropping and you've eliminated the other simple causes (cold weather, low tire pressure, lots of recent short trips), test the 12V battery. A failing battery can even reduce MPGs because the car spends more energy trying to keep it charged.

    A pass/fail battery tester, even at Toyota dealers, sometimes does not correctly test a Prius battery because the passing voltage threshold on the tester can be too low for reliable Prius operation. Low voltage is often OK for a conventional starter but computers can get confused.

    You can easily test the 12V battery yourself. With a voltmeter measure either at the jump point under the red cap inside the fuse box under the hood, or at the battery itself. With the car OFF, if the voltage is much less than 12.0V it means that the battery is dying.

    DO-IT-YOURSELF TEST PROCEDURE (thanks to jdenenberg):
    - Without brake pedal, press the Power button once and release to enter ACC mode
    - Press and hold the MFD Info button, then turn the headlights on and off three times to enter Maintenance mode; release Info button
    - Press “Menu” (on screen)
    - Press “Display Check”
    - Press “Vehicle Signal Check” - the battery voltage is shown and should be about 12.4 to 12.8 Volts (normal for an unloaded battery)
    - Again without brake pedal, press Power button and release to put a current load on the battery - the voltage should stay above 12.0V (if less than 12.0V the battery is not well, or there is a fault or unusual load somewhere)
    - Press brake pedal and press Power button once to enter "Ready" mode - the battery is now charging at about 14V (if less than 13.6V or more than 14.4 there may be a problem with the charging circuit)
    - Turn car OFF to leave Maintenance mode

    For an alternate procedure that does not require turning the headlights on-and-off see
    Entering Maintenance Mode (aka 12V Battery Check) without Switching Headlights ON/OFF | PriusChat

    If these procedures cannot begin then either the battery is dead or there is some more serious electrical problem.

    If the 12V battery is dead you're in luck because replacing it is one of the easiest and cheapest-to-fix electrical problems there is in a Prius. I have heard of one direct drop-in aftermarket replacement, the Optima YellowTop Deep Cycle Battery model #S46B24R. One mail-order source is eLearnaid (see Prius parts ). All others require modification to one or more of their terminals, mounting hardware, vent tube, or cables.

    If the Smart Key System fob no longer works well, test the fob battery. Press any button on the fob. You don't have to be near the car. A small red LED should flash in one corner. If not, dying/dead fob battery. However even if it passes this test, the battery may still have weakened enough that it does not work as well as it did originally and may need to be replaced. Also note that nearby metal or electronics such as coins or a cellphone in the same pocket or purse can interfere with fob-car communication.

    If the 12V battery is low you can try using the Prius itself to recharge it by turning the car fully ON ("Ready", as though you were going to drive it) and letting it sit with the shift in "Park" for nine hours. It can take that long for a full charge because the battery is rated at 36 Amp-hours, and the system is limited to 4A. Of course be certain that the car is physically secure, and, if indoors, has an adequate air supply.
    Colby, SnJGuin, Aaron and 97 others like this.
  2. Ichiro

    Ichiro Member

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    thanks very useful to know. my mileage has been dropping, but i chalk it up to the weather mostly.

    my radio presets have been lost a couple times in the last few weeks. might be the battery!
    I wish the ECU would automatically throw a warning light if it detected a 12V battery issue.
  3. Earthling

    Earthling New Member

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    I was lucky and got my car in to the dealer today. It turns out the inspection had run out, so there was another good reason to have it looked at.

    Diagnostics showed everything is well. I think I'll pick up one of those small jump-start units just in case.

    I checked my mileage today and I got 53.3 mpg @ 27F, on the way to the dealer (after a reset) so my mileage appears fine.

    I'll dig out my voltmeter, locate that jump-start post, and if I feel adventurous, try going into Maintenance Mode.

    Thanks to everyone for the advice.

    Harry
  4. Earthling

    Earthling New Member

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    I was told by a service person at the dealer that the red warning triangle I saw yesterday and again today when starting my Prius was a warning that the voltage on the 12-volt battery was low.
    1 person likes this.
  5. ksstathead

    ksstathead Active Member

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    Does the 2010 have the same or similar diagnostic procedure?
    1 person likes this.
  6. JimN

    JimN Let the games begin!

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    Maintenance Mode is much more convenient. Exit Maintenance Mode by shutting off the car.
  7. nagrath

    nagrath Junior Member

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    Bingo! Thanks!
  8. paprius4030

    paprius4030 My first Prius

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    Do you have to have navigation to enter the maintaince mode or can you do it with a pkg.#2?
  9. nagrath

    nagrath Junior Member

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    Nope, don't need Nav, works fine without.

    John
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  10. Rhino

    Rhino New Member

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    For all my prior cars, ICE cars. Whenever I had battery problems, I take it to a battery dealer and they check the battery, it mostly come back OK. But then my car keeps not starting and I have to tow it.

    So now I just replace the battery whenever it stops starting the car reliably.

    What I mean to say is, consider just replacing the battery. Then you don't have to carry the jump start thing around. You would save some money on the battery because you don't have to buy the jump start battery. Just my humble suggestion.
  11. usbseawolf2000

    usbseawolf2000 HSD PhD

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    This needs to be sticky.

    I would suggest the title to say "Weird stuff happening? MPGs dropping? Test the 12V battery (with instructions)"
  12. Membrain

    Membrain Junior Member

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    So i tried this technique and found that my battery after 1 year is as follows:
    Unload: 12.4V
    Load: 12.1-12.2V
    Charge/Start: 14.2 V

    I don't think my decrease in mpg is due to the battery. Has anyone else experienced a decrease in mpg, about 10mpg, and fixed the problem yet found the cause?
  13. KLear

    KLear New Member

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    FWIW, I find significant changes in MPG with the cold weather. When the weather is in the mid teens, it takes awhile for the engine to warm up, resulting in lower MPG. Yesterday, when I drove to work it was colder and overall efficiency was poor. In the ride home, it was just above freezing and mileage was excellent. You live in Anaheim, doubt you have cold weather like I have! Maybe some other environmental issue?
  14. rhncrw

    rhncrw New Member

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    My battery was shot. I tried to start the car and every light on the dashboard started blinking. I got a jump and then proceeded to the dealer. I have a 2007 Prius so the battery was under warranty. My mileage had gone from 45 mpg to 35 mpg. After a new battery was installed my mileage has not improved. Any ideas/comments?
  15. Flying White Dutchman

    Flying White Dutchman Senior Member

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    well i think this easy do rundown ( to a point of braking it ) small 12volt prius battery needs to be a recall where all 12volt battery's are replaced bt better one's
    its seems to me this is really something that happens a lot but is not a big thing just a design fault to me.
  16. KLear

    KLear New Member

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    Where do you live? Could the weather have anything to do with it? Could the dealer keep the car and go for an extended test drive to see if he can replicate the problem?



  17. rhncrw

    rhncrw New Member

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    I live in Charleston WV. I have not noticed my mileage suffering in previous winters. I will weather the cold storm and if I don't see any imporvement in the Spring I will have dealer check into it.

    Thanks
  18. Joe1987

    Joe1987 Cheyenne Joe

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    Hi,

    I read you posting regarding testing the 12V battery. I followed the instructions in the posting (thanks to jdenenberg). Fortunately, my battery passed the "test".

    I am curious though; where did you or jdenenberg find the battery test? Also, how many more tests are available in the Prius and how can I learn of them? Incidentally, I have a 2006 Prius with package 7, if that's the one with fog lights, GPS, Bluetooth, and backup camera.

    Thanks, in advance, for getting back to me.

    Joe1987
  19. richard schumacher

    richard schumacher shortbus driver

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    Sorry, I don't know where that procedure came from originally. jdenenberg may well have discovered it. I have seen screenshots in the Technical and Care and Maintenance forums of many such hidden screens; the "search" function is your friend :_>

    Others, don't forget to check tire pressures at least monthly. Cold weather flattens tires, which contributes to reduced MPGs.
  20. raholco

    raholco Ailurophile

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    This my first winter with the Gen II 2009 Touring-my MPG's has dropped from the low 40s to the high 30s. I just recently swapped the stock 195/55R16s to Nitrogen-filled Nokian WRG2 205/55R16s, which will cause a minor hit as well.

    I noticed the warmer it gets the better my MPGs are-when it gets bone cold (0 to minus 15 F), fugheaboutit.
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