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My 12v battery when its connected to car loses charge in 30min

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by alexk345, Feb 6, 2024.

  1. alexk345

    alexk345 Junior Member

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    hi

    I charged by my 12v battery with trickle down charger.
    And left it 2 days in home. Then try to recharge it after 2 days , it said full.

    That mean battery on its own not losing charge and only when its connected to car.

    I have everything off in the car. What is going on?

    Any short circuit?
     
  2. JC91006

    JC91006 Senior Member

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    How old is your battery?
     
  3. TMR-JWAP

    TMR-JWAP Senior Member

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    Does the car work normally when you first install the battery?

    Draining a 12v battery (even the Prius small 12v battery) in 30 minutes could be 2 things
    1. a relatively high parasitic load on the 12v system.
    2. a very weak 12v battery (how old is it? original from 2010? if so, just replace it)
    Test the battery with a real load tester to determine if it has a reasonable capacity. A charger isn't really telling you anything about capacity.
    If the battery tests good, install it and start looking for abnormal loads like lights, or pumps that are running when they shouldn't be. Would be relatively easy if you have an amp meter.
     
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  4. Tombukt2

    Tombukt2 Senior Member

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    Well if you have SKS and it's not turned off under the steering wheel then you're 12 volt battery and remote and all of that every so often is hunting for each other The car wakes up to look for the remote goes back to sleep so on and so forth If the alarm get set off by varmint or whatever at night I have mine turned off it's completely disabled but apparently something can still set it off and make the lights flash It only does this for about 45 seconds and then turns off and Toyota software I have everything turned off at rest no door re-locking no automatic door unlocking nothing but yet still if I go outside and I walk by the car too hard or I don't know what exactly goes on the car will start flashing the headlights and the tail lights like the alarm is gone off there's nothing honking nothing making any noise I even have the beeper turned off under the left front fender or it's unplugged beeper sitting there it's unplugged so anytime those lights go to flashing some energies being used from the 12 volt battery anytime the car decides to wake up and hunt for something 5 hours after you shut the car off in a generation two the heat recovery pump is going to come on move some fluid check temperature and shut off It's in the manual so there are some things that happen they're not really that draining but if your battery is not really up to snuff I guess it could be because whatever you're doing with a trickle charger pretty much has no basis on what happens when it's off the charger in your car If you think things are on in your car hook up a good voltmeter or something that can read milliamps and see if you are in fact losing all this voltage you think you're losing You're 12 volt is probably just going south no matter how new it is or old it is or any of that It's just what happens to lead acid type 12 volt batteries a little better with AGM and gel but still I have very good batteries in my power sports equipment motorcycles four wheelers big bore scooters you'd be shocked how bad and how often the 12 volt batteries go bad in these devices It is unbelievable to imagine today this is still a thing but oh well here we are.
     
  5. Brian1954

    Brian1954 Active Member

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    You created two threads earlier about the same issue with the 12v battery starting in October 2023. Now, you have created this third thread. It is time to buy a new 12v battery!!!!!
     
  6. alexk345

    alexk345 Junior Member

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    I trickle charged my 12V prius battery and left it at home and rechecked after 2 days , it was full charge.

    So i connected to car and started engine. All ok.

    Then 30 minutes later i went back and try to restart car , battery is gone.

    Something eating battery.

    When car is off , i hear some sound coming from engine side when opened front side.

    it almost like something running. I had cigarette lighter before long ago and i removed it long ago.

    So nothing connected. How do i check and resolve this issue?
     
  7. alexk345

    alexk345 Junior Member

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    I do not see its battery issue as i left the battery after charge . Then 2 days later i checked , it was full battery

    My theory is something running in the car when car is offf.

    What could that be.

    Any checklist on me go check?

    All i see is this. When i removed front engine cover , i hear noise of something running when car if off and battery is nearly dead.
    I used the jump start to star the car , it started immediately. Then i switched off car.

    I hear sound coming from engine side something is running. I feel what ever is running is eating up battery.

    can i attach video here?

    My display button broken so i cant check many things without it
     
  8. JC91006

    JC91006 Senior Member

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    Have you answered the question on how old this battery is? If it’s old, a full charge won’t mean anything
     
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  9. Brian1954

    Brian1954 Active Member

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    It is a battery issue!!!

    In a thread that you created in December 2023, you said the car sat idle for 2 years: https://priuschat.com/index.php?threads/245466/
    That killed the 12v battery, and it needs to be replaced!!!
     
  10. dolj

    dolj Senior Member

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    You need to actually pinpoint what is running. Use a mechanic's stethoscope if necessary. Also, reference lefts and rights from the perspective of sitting in the driver's seat looking forward. Somewhat the same principle as how the port and starboard side of a boat/ship never changes.
     
  11. alexk345

    alexk345 Junior Member

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    I cant see anything running but when i opened front trunk , i see some motor is running even car is off.
     
  12. TMR-JWAP

    TMR-JWAP Senior Member

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    weird, sometimes my Gen 2s do that also...something runs when I first get to the car in the morning, then stops running.
     
  13. rjparker

    rjparker Tu Humilde Sirviente

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    A car that sits for two years will lose 90% or more of its capacity to store power. This is the most common issue. A 12v battery replacement fixes it. In the US any auto supply store can load test your battery with a special tester and tell you the remaining capacity.

    All cars including the Prius use a very small amount of power at all times. This is known as Parasitic draw. Normally a Prius will draw 15-30 ma when off. ma is thousands of an amp. Very small. A good battery can take this kind of draw for three or four weeks and still start the car. A mechanic can measure the parasitic draw for you to ensure nothing is eating power fast. In reality you could leave the headlights on for thirty minutes with their very high current draw and still start the car. IF you have a battery with normal power storage capacity.

    The brake master-cylinder will run a pump motor as you approach the car. It is in the front engine compartment behind the big fuse box. It is normal if it runs for perhaps a minute. They do go bad and run longer but will create engine codes a mechanic can read with a scanner. But even this running for thirty minutes would not totally drain a good 12v battery.

    So everything points at your 12v battery needing replacement. Of all the possibilities, a new 12v battery is far less expensive than a Prius Master Cylinder. We are not talking about the 201v hybrid battery, we are suggesting the small 12v battery is old and has low storage capacity left.
     
    #13 rjparker, Feb 8, 2024
    Last edited: Feb 8, 2024