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12v Battery Voltage

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by rcard0, Dec 25, 2015.

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  1. rcard0

    rcard0 Junior Member

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    2006 Prius
    My 2006 is on it's second 12v battery. The battery in the car is now 4 yrs old (an Optima Yellow top).
    We only drive short distances daily, 3-6 miles per trip. My avg mileage has gone down to 37-39 because of this. Especially so in the winter. The engine seems to run an awful lot now, so I assumed it was charging the 12v.
    I hooked it up to my battery charger (set on AGM, 2 amp mode) and it charged to full in 25-20 mins.
    Here's the thing - my techy battery charger showed voltage at full as 12.6v. However when I do the voltage check on the MFD, it shows 11.9. Any clues? Thanks Prii people and Happy Holidays!

    2006 Prius with "the works".
    Green Low Roll Resistence e-spec tires.
    51k miles
     
    #1 rcard0, Dec 25, 2015
  2. andrewclaus

    andrewclaus Active Member

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    Vehicle:
    2007 Prius
    I would try a third meter and and test the voltage at the battery terminals, disconnected from the car.

    You can expect some voltage drop from the battery to the sensor read by the MFD, along the lines of 0.2 V, I hear. I assume you're looking at the no-load case, no accessories on (pushing the start button only once).

    Typical DMM accuracies are about 0.5% of full scale. If you're using a 20 V scale, your accuracy will be +/- 0.1 V, even when reading 12 V. So expect some variation between devices.
     
    #2 andrewclaus, Dec 25, 2015
  3. edthefox5

    edthefox5 Senior Member

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    Vehicle:
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    The 12.6 on your techy thing is reading the techy thing charging voltage. The mfd is pretty close and even if its .2 v off at 11.9 its still a dead battery.

    Charging the optima and it reaching full charge in 25 mins means your in the 10 amp charging mode. You should charge the battery in the 2 amp mode. Your battery sounds sulphated and is probably end of life and will leave you somewhere. Its the short drives that is killing it. A car's charging system is not designed to charge a dead battery. Its designed to keep a healthy battery topped off.

    Btw, at 11.9 the dash board has been throwing the red triangle of death on boot up. You never noticed. Its the only warning you get the battery is about to leave you. It only shows up for a second on boot up to ready. Its the only indicator on the dash that doesn't light under bulb check. Its very noticeable as its very bright. At 11.9 mfd voltage its lighting up.

    Get a new battery and install a battery tender permanently under the hood bolted to the jump points. Plug the tender in when not using the car.
     
    #3 edthefox5, Dec 25, 2015
    Ramanjit81 and Andyprius1 like this.
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