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    donee New Member

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    Hi All,

    As you may know a weak 12 V battery has been traced to poor mileage in the 2nd Gen Prius. Recently, I read how the 3 rd Gen Navi diagnostics are accessed, and found the 12 V battery volt meter. My car is less than a year old, and it read 11.7 V. Hmmm...

    Maybe some others with the Navi can read their battery voltages and post on here?

    Sit in the car, do not press the brake pedal. Press the power button once. Wait for the screen to come on. Then press and hold the INFO button on the right side of the Navigation screen, and while holding it cycle the head lights on and off three times. Then find the soft key on the screen which takes you to the screen which shows the voltmeter display.

    Let me know what you get.

    Then press the gas pedal and hit the button again. My reading rose to 13.5 V as the car became ready, as it should.
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    kbeck Active Member

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    Basic deal: A fully charged 12V lead-acid car battery usually clocks in a bit north of 13 V.

    Second: There's mostly six cells in a car battery, each a bit more than 2 V when the battery is charged. If you've got a battery that, just sitting there, is clocking in around 11 V a nickel bet says that one of your cells is shorted. This can happen due to wear, age, or deep discharging the battery.

    When the battery is getting down to full discharge, not all cells have the same amount of charge. Hence, the weak sister is going to find its polarity reversed when the rest of the cells in the chain still have charge. This is called cell reversal and is Bad News: ye battery may never come back from that. This is one of the reasons that running a car battery all the way down to squat is a Very Bad Idea.

    The "13.5 V" you see in Ready is the voltage coming off the HV-Battery->12V battery charger. Actually, it's much likelier that the Prius regulates an output current, but what the hey: That 13.5 is the HV battery talking. If you turn the car back off and the voltage drops below 12, and I'm not kidding here, get a new battery. No joke. And don't wait.

    By the by: Being a electronics techie by trade, I wouldn't trust any voltage measurement system without a cal sticker if I didn't have a choice. If you don't have one already, chug down to ye local Radio Shack or equivalent and pick up a $20-$30 multimeter. Then, attach that to the battery in back. Then take your measurements and see what you got.

    KBeck
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    donee New Member

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    Hi Kbeck,

    Thanks. I do know all that. I am curious if its a common problem with the 3rd Gen. My car was purchased with less than 10 mi on the clock back in September, 2010.

    Could be part of why I can't make my 60 mpg above freezing I became used to in the 2nd Gen.

    I guess I should brake out the Agilent U1242A (won at a sales event) and double check things....
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    donee New Member

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    Hi Kbeck,

    Ok, the U1242A says 12.65 V unloaded, and 12.0 volts (11.7 Volts) when the NAVI and car are in ACC mode. When the car was on it read 14.9 Volts (compared to 13.5).

    So it looks like a < $20 meter circuit in the Navi. Not great for $2K price addition, ha!

    So the mileage issue is still back with the aerodynamics.

    Oh, if you have one of these Agilent meters, get rid of the Gold Peak batteries they were shipped with, one of mine had leaked.
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    tumbleweed Senior Member

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    Testing your 12 volt battery. I haven't tried this on a car battery but I think it would work, let me know what you think Kbeck. For what it's worth:

    Unfortunately voltage doesn't tell the whole story. You can have a battery with very low capacity and the voltage will still be up, unless there is a dead cell as mentioned above, but it will discharge rapidly under load.

    A fully charged 12 volt lead acid battery should measure at least 12.1 volts with no load after it has set for a while after charging. It should provide it's rated ampere hours (Prius battery is around 35 or 38 I think) if discharged over 8 hours before it falls to 10.5 volts. So in other words, with a new battery, you should be able to draw a little over 4 amps for 8 hours before it falls to 10.5 volts. The discharge curve is not linear but that should be good enough for a test.

    If you were inclined the test could be performed at home. You would need to remove the battery from the car and connect a voltmeter across it. You would then need to connect an ammeter in series with the load. The load would ideally need to be about 2.8 Ohms to start with and decrease to about 2.45 Ohms as the Voltage falls. You could probably just use about 2.6 ohms for the entire test and get close enough. A couple of 200W incandescent bulbs in parallel might work for the load. The test should be stopped when the voltage drops to 10.5 volts or a little before to be safe. The elapse time will be an indication of the capacity, if it takes the full 8 hours it would have it's rated capacity. Capacity drops with age so after several years it might only have 50% or so.

    Of course we can only measure 6 cells, not each individually. So if you have a cell that's ready to reverse polarity, as Kbeck mentions, you may not have a usable battery after the test.

    If I suspect any problems with my battery before 4 or 5 years I will perform a discharge test on it to see what it is out of curiosity.
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    jdcollins5 Senior Member

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    donee,

    Have you read your battery with a DVM connected to the battery to compare to the diagnostic screen reading? The reason that I ask is that when I first read my battery in the diagnostic screen, mine read low as yours did. When I read the battery voltage at the battery terminals and then at the jump post under the hood, the voltage was +0.7V higher with the DVM than displayed on the diagnostic screen.

    Dwight
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    billnchristy Active Member

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    If you drop a non deep cycle battery (like an Optima) to 10v you might as well kiss it good bye because it won't be worth a flip after that. A lot of batteries crap out under 12v.

    Cell voltage should be 2.1v so your idea "idle" battery is 12.6. Most alternators charge from 13.8 to 14.4v but a battery will balance back to its idle state pretty quickly.

    I have never even seen the battery in the Prius but I am curious now as to what it would read.
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    2k1Toaster HID Guru

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    I would trust the 4 digit DVM over the shared ADC "measurement" circuit.

    As you found, this measurement circuit is extremely inaccurate. Unfortunately techs use this to diagnose good and bad batteries. It can be high, it can be low, it can be dead on. From your post we know it is at least a 8% off so it is safe to assume +/- 8%. This basically invalidates all batteries from being tested as good or bad and leaves the options of a) Indeterminate and b) severely bad.

    So when the tech says your battery is fine it's something else throwing codes, do your own homework.
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    J5A Active Member

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    11.5

    I'd like to test with a multimeter for double verification, but not 100% sure how to do this. In process of searching.....
    MPGs dropped from roughly 47mpg to 36mpg since my last fill up. I was thinking it was "bad fuel" but now I'm thinking perhaps the 12V.

    Does it look like I need to call the dealer? Is a new 12V in my near future?
    TIA!
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    2k1Toaster HID Guru

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    bisco cookie crumbler

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    a lot of 2010's sat on the lot so long that the usable life was greatly diminished.
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    J5A Active Member

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    The car has a build date of 3/10, purchased 'new' 1/11 with 30 miles on the odo.
    I know it sat a while, but thought it'd be okay.
    It starts up fine, but the mpgs have not been what I'd like to see.
    I have been running a/c lately.

    I'm wondering how difficult it would be to get dealer to replace under warranty. Never experienced warranty work before on any vehicle...
    TIA!
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    J5A Active Member

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    Made an a.m. appt for tomorrow with the dealer. Hopefully they'll replace the 12v without any issues.

    This morning's results:
    Before start up:11.5V
    Start up: 13.1V
    Driving 25 mph: 13.1V
    Driving with a/c: 13.9V
    20 minute rest after shutdown: 11.6V

    2010: 2800 miles, 5 months ownership.
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    macman408 Devil's Advocate General

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    If those 11.5V measurements were just with the nav system in diagnostic mode, I'd recommend a multimeter instead. The nav system puts a fairly significant load on the battery, dragging the voltage down. When I measured my car's battery when it was a month and a half old, the Nav system told me it was 11.5V as well. I also used a multimeter at the same time, and the multimeter read 12.17V at the same time. With the car off and all doors closed, the multimeter read 12.5V. So the load from the Nav system caused about 0.33V of drop due to the load on the battery, and the inaccuracy of the Nav system's measurement plus the resistive voltage drop in the long wires between the battery and the Nav system probably account for the other 0.67V.
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    J5A Active Member

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    Thanks mac - The dealer's findings were the 12V was good enough to wait and see. I'm okay with that..for now especially since the vehicle's mileage has increased to 50+ mpg since Monday.
    At least they switched the back up beep from many to one, for free. :madgrin:
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    andyprius Senior Member

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    It is simple to test with a meter, neg lead on ground and pos lead on either positive of actual battery or use posts under the hood. Many posts on this subject. Do not confine your search only to 2010. All the models are extremely similar. Do 12V diagnostic check. It is not unheard of for a battery to fail in the first year, it happened on my Prius. You can contact your dealer and they will often check the battery and tell you it is fine, however their check is not very good. By monitoring the battery over a long period of time the owner can get a better idea as to the health of the 12V. Never run any accesories in the accesory mode, this will deplete the battery, always make sure the car is in ready.

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