Auxiliary battery happy voltage

Discussion in 'Gen 2 Prius Technical Discussion' started by Docholo, Feb 18, 2018.

  1. Docholo

    Docholo Junior Member

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    I drove about 800 miles over two days and the no-load voltage on the auxiliary 12 v battery is 12.72 volts measured right at the battery terminals with the leads off. This battery is about 5 years old.

    Is this a reasonable state of charge for this battery? The engine seems to be running a bit more than usual and the traction battery is above 75% full.


    iPhone ?
     
  2. bisco

    bisco cookie crumbler

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    did you measure it after the surface charge wore off?

    it's hard to put a number on it. low 12's is getting iffy.

    there are a million reasons why the engine might run more. best thing to do is track your mpg's at every fill up.
     
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  3. padroo

    padroo Senior Member

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    What have the temperatures been?
     
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  4. Docholo

    Docholo Junior Member

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    Outside temperatures have been between 45 and 78 F. (southern AZ and SoCal).


    iPhone ?
     
  5. bisco

    bisco cookie crumbler

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    southwest heat is hard on all batteries.
     
  6. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    That sounds fine, but: 5 years old, I'd replace it anyway, leave on a high note.
     
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  7. padroo

    padroo Senior Member

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    I always try to squeeze all I can out of a 12V battery and my wife says just change it. I started listening to her, that way I can never be wrong again. DAH
     
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  8. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    I hear that emergency vehicles, ambulances and the like, change batteries every two years. Overkill, acknowledged, but just a decision they've made.
     
  9. padroo

    padroo Senior Member

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    The contract for my whole house generator wants you to change the battery every four years, mine only lasted three.
     
  10. Prodigyplace

    Prodigyplace Senior Member

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    So did you dutifully wait one more year?
     
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  11. edthefox5

    edthefox5 Senior Member

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    Measure it at the jump points. A lot of work to measure it at the battery and that really doesn’t matter what
    The voltage is there what matter is what’s getting to the main fuse box then the inverter. Got a few fuses and a long heavy duty cable before it gets there. That’s the true working voltage. At the jump points.
    After 11 years I have never measured the battery voltage at the battery.

    Measure there with the car off then measure again with the car off and turn on the headlights in high beam to check it’s capability to deliver under load. Measure with high beams on car NOT in ready.

    And I’d be way more Concerned about the traction battery’s health than the 12 volt.
    12.7 is pretty good. Unless it dips down to 10.7 under headlight load,
     
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  12. Docholo

    Docholo Junior Member

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    Yeah, it had to be replaced. I got a new one from the dealer, charged it up on a high quality charger and now the prius runs like a champ.

    I took the old battery and charged it on a desulfating/pulsing charger and discharged it several times. I got an electric load cell battery analyzer that let me discharge into a calibrated 12 watts of load until it discharged to 11.5 volts and stopped.

    In total the old battery could put out 179 watt-hours or, approximately, 14.5 amp hours before hitting 11.5 volts under the (tiny) load. I think that new the battery should be about 45 amp hours when drained at a 27 watt load (?to get discharge 45 amp hours in 20 hours?).

    Anyhow, the old battery is crap. Seems like it has lost about 70% of the capacity relative to the spec when new.

    Thanks all.
     
  13. edthefox5

    edthefox5 Senior Member

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    For future readers.

    Other day I measured my 2 year old optima yellow top.

    12.63 resting volt at the front battery jump points.
    12.1 with car off and hi beams on.

    So if you see more than .5 voltage drop at jumps with headlights on hi beam and NOT READY start giving your 12 volt battery the stink eye.
     
  14. valde3

    valde3 Senior Member

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    Acceptable voltage drop changed a lot depending on conditions. If your battery is cold and especially if it’s not charged full (happens easily if you drive short trips in cold temperature) bigger voltage drop is still fine.
     
  15. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    If you've got deepish pockets and want in-depth assessment of the 12 volt, pick up a Solar BA5.
     
  16. greasy

    greasy Junior Member

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    I've had great luck using cheap solar panel trickle chargers to extend the life of marginal batteries, and the Prius seemed a perfect candidate for this solution – expensive batteries, lots of current-draining electronics, slow recharge rate. I learned the hard way that leaving a door open (even with no lights), or leaving the key fob in the car can quickly drain an older battery. With a replacement battery costing at least $150, these chargers can easily pay for themselves:

    This being my first post, Priuschat won't let me include links to Amazon. But you can do a search for these products:

    Low output, but very easy to install:
    ALLPOWERS 18V 5W Portable Solar Car Battery Charger

    Higher output, but requires regulator (and maybe an in-line fuse, to be safe):
    ECO-WORTHY 10W Solar Panel
    Y-SOLAR Solar Panel Controller PWM 12V/24V
    EPAuto ATC / ATO 10 Gauge 30 AMP In-Line Fuse Holder
    I hooked one of these directly to the battery, and leave it in the “trunk,” catching sunlight through the windows. This regulator also has usb outlets – handy for charging your phone on a camping trip…
     
  17. padroo

    padroo Senior Member

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    I hope you haven't been holding your breath for a reply but I changed that battery last year, about 15 days out of warranty.
    Sam's Club said it had a dead cell. My electrical service took a hit by lightening and I think that cooked my battery, I lost a 20 Amp circuit breaker at the same time for the charging circuit for my generator. It didn't seem to hurt the generator, I heard it doing it's weakly test run today about 11 months later.
     
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