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    2009Prius A Wimpy DIYer

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    The widely publicized method (within priuschat) to enter the maintenance mode (to check the 12V battery voltage, among others) involves a step to turn the headlights on and off three times. Here is one example:
    http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html

    There is another method that hasn't been mentioned much within priuschat. Instead of switching the headlights, one presses some hidden buttons on the MFD. I like this method better since it avoids unnecessary thermal shocks to the light bulbs and I think the hidden buttons are cool. ;) I saw this method on one of the "yellow top" battery web site:
    12 Volt (12v) Toyota Prius Auxilary Battery for 2004, 2005, 2006, 2007, 2008, 2009 and 2010 with installation kit

    The procedure is copied here:

    To get out of this mode you can just turn the car off or according to this post:
    http://priuschat.com/forums/gen-ii-...t-switching-headlights-off-4.html#post1446557
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    richard schumacher shortbus driver

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    Excellent! But the first step is really not right. The test should be started after the car has been OFF for a few hours, so that any misleading surface charge will have dissipated from the battery. That way any weirdness that might exist, which the car actually will be experiencing under the typical condition of trying to start it after it's been sitting for a few hours, will be present.

    As Patrick points out below the headlights will promptly remove any surface charge so the loaded measurement (the most important measurement) will not be affected.
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    ksstathead Active Member

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    The procedure title says it works with the 2010, but does it really? Only NAV package even has a MFD, and even then it does not contain the energy screens...
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    2009Prius A Wimpy DIYer

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    I just copied and pasted from the web site without any modification. I suppose we could add a step: "wait in the OFF mode for a few hours" after the first step (charging the battery by driving).

    On the other hand, how much surface charge is there? Would there be enough surface charge to run the headlight for more than a few seconds? If not then a weak battery will still fail the headlight test even with freshly replenished surface charge.
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    jdcollins5 Senior Member

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    This works with any 2010 with Nav. Follow the same instructions as below and it works just fine. The Display screen on the 2010 does not need to display the energy screens. I believe they give the audio settings instead on the 2010. Just follow the same instructions for the 1 and 2 finger dance three times.
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    2009Prius A Wimpy DIYer

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    I just copied and pasted from the web site without any modification. I don't know about gen 3 since I don't have one.

    Is it true that "Only NAV package even has a MFD"? Sounds like too much a downgrade from gen 2! :(
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    jdcollins5 Senior Member

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    No. What is called the MFD on the 2010 is located on the dash instrument display on the right hand side. This shows the Energy Monitor, Hybrid System Indicator, 5 min / 1 min mpg and last 5 tank mpg displays. This is on all cars.

    The Nav unit on the 2010 contains the GPS map plus the audio functions.

    You can only pull up the Service Menu screen on a 2010 with Nav because you need a Nav screen in order to display the Service Menu screen.
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    Patrick Wong DIY Enthusiast

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    I would say that 30 minutes at READY is not enough to charge a severely discharged 12V battery. 12+ hours is more like it.

    Use of the headlights will quickly dissipate whatever surface charge exists.
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    BAllanJ Active Member

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    But if you don't want the nav you then don't get a display for audio or back up camera? Glad I got the 2009, then.
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    2009Prius A Wimpy DIYer

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    I was thinking about the same thing. I am pretty sure for gen 3 we need to go to higher packages than gen 2 to get back up camera and smart key buttons on multiple doors. So I thought my gen 2 package 2 was a better deal, at least for me. :)
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    JimN Let the games begin!

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    Correct. I find it easier to read the voltage from a ScanGauge when I want.
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    BAllanJ Active Member

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    I don't think I'd like the car nearly as much without the backup camera.... odd since I've never had one before, but the sightlines out the back are just too limited. But adding nav on the '09 was an extra $2.5 k here and I already had a good gps in my phone (well, good for what I use it for, which isn't a lot). Not sure what the extra cost is for the '10 nav/bacup camera version...so I'm not sure I would have gone for it.
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    jdcollins5 Senior Member

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    It was $1800 more for my III with Nav. I had originally not planned to get the Nav because of the cost but when I found the color that I wanted with everything else, it had the Nav. Now that I have the Nav I am really glad that I bought it. The built-in Nav with the GPS, backup camera, audio displays is really nice.
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    w2co Member

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    Thanks for that procedure. That screen looks right out of a classic GEN1 prius.
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    burnselk New Member

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    Could someone please explain a "battery surface charge"? I've never heard of such a phrase. Thanks

    BTW, I remember my granddaughter telling me before she got the new 12 volt battery that she was only getting 30 some MPG. I haven't asked her yet if she's noticed an increase in her MPG since the new battery was installed.....but I most certainly will.

    I'll ask her because 30 some mpg is, in my opinion, very low for a Prius (even a 2001). But she could be driving totally wrong for all I know.....or have low tire pressure.......could be something as simple as that. I know nothing about the proper way to drive a Prius to get optimal MPG...........I'm asking for her in hopes of helping her save some gasoline money. She's the one who owns and drives the 2001 Prius. Apparently she's way too busy (is 18, has a to 6 month old baby, and attends beauty school) to come into this forum and look around. Basically I'm trying to learn all I can for her sake.

    Does anyone have any simple suggestions I can pass on to her about how to improve her MPG? If so, are they simple enough to be listed as "bullet" statements?
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    2009Prius A Wimpy DIYer

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    Surface charge is just a small amount of charge that would cause artificially high voltage reading but can be easily removed by turning on the headlight for a minute or two.

    If she is too busy then just drive it; check tire pressure and oil level of course.
    Short trips or cold weather leads to low mpgs. A "regular" car also suffers from such conditions.
    If she has some time then suggest her to take a look at these threads:
    http://priuschat.com/forums/gen-ii-prius-fuel-economy/14701-new-owner-want-mpg-help-read-first.html
    http://priuschat.com/forums/gen-ii-...-answer-these-questions-esp-if-youre-new.html

    Good luck!
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    Michgal007 Senior Member

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    Since my Prius is almost 5 years old and I read on PC on several occasions that it might be time to replace it, I went ahead and did the check. Car was parked overnight and I did the check without driving around. Engine was running, no lights were on. I got 14V.

    Is this normal? I saw one PChatter has gotten this number early in the morning as well (another thread).

    Thanks.
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    redrockprius redrockprius

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    When the car is in READY mode (which it must have been for the engine to be running), you are not reading the voltage produced by the battery. The voltage you are reading is the voltage derived from the main traction battery to charge the 12v battery.
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    2009Prius A Wimpy DIYer

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    Press power button once without stepping on the brake to enter IG-ACC mode. Then press the button again (without stepping on the brake) to enter IG-ON mode. Now test the battery with some load, say turning the headlights on.
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    Michgal007 Senior Member

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    Alright, I did this and got 11.5V (headlights on). I guess the battery is ok for now? I am hoping to put in a new one in August.

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