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12v battery question

Discussion in 'Gen III 2010+ Prius Care, Maintenance and Troubles' started by plam0ne, Mar 30, 2012.

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  1. plam0ne

    plam0ne Ninja

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    Five
    noticed yesterday that the right side indicator of the camera is higher than the left one so i went into the service menu and was curious and checked the battery voltage first (acc on mode, in garage overnight) and it read @11.8V tried it on ready mode and it read @13.7V, do i need a new battery? i did not notice any weird symptoms or changes to mileage. any feedback would be much appreciated, thanks.
  2. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    I would plan to replace it at the first weird symptom, but now that you see how easy it is to get to the service menu, I would wait for symptoms.
    elearnaid is both fast and cheap.
  3. plam0ne

    plam0ne Ninja

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  4. css28

    css28 Senior Member

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    I just bought a leftover 2011 Prius III and had been perusing the threads about the service menu so I decided to try it myself. Like you, I looked at the indicated battery voltage and it came up at 11.7 V. Worrying that maybe since the car was a leftover (assembled in October last year) the battery had been neglected as it sat at the dealer's.
    I opened the hood, worked my way to the jumpstart terminal with the intent of attaching my 1 amp charger for an overnight trickle. Before doing so, I put a voltmeter on it and it came up 12.78 V at the terminal. Hmmm. I verified that the voltmeter wasn't reading too high on another of my vehicles.
    So I'm not sure that I'll trust the service menu for this particular measurement.
    It turned out that the other car (a BMW that's only been driven 3 miles in the last month was at 11.7 V, so it's getting the trickle charge for a couple of days instead :)
    After that car's back up, I might give the Prius some charging as well.

    - Chris
    1 person likes this.
  5. macman408

    macman408 Devil's Advocate General

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    Yes, the service menu reads low for at least two reasons:

    1. The car has to be in accessory mode (or IGN, or Ready), meaning the battery will be loaded (except Ready, where it is being charged). A loaded battery will always measure lower than an unloaded one, and the difference depends on how much load there is. For this reason, when using a multimeter to measure a battery to gauge its health, it should always be unloaded, and at rest (e.g. no load and no charge) for several hours beforehand. Even just having the hatch open and interior lights on was enough to drop the battery voltage in my car by 0.2V.

    2. The sensor is probably not located at the battery or the jumping terminal, but rather near or inside one of the computers. There will be some resistive loss in the cables (or maybe other components, perhaps a diode for protection, for example) between the battery and the computers; my car said 11.5V in the service menu, while I measured 12.2V at the battery at the same time. For what it's worth, I did once compare the voltage at the jumping terminal with the voltage at the battery itself; with all doors closed and lights off, the voltages are identical.
    1 person likes this.
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