Dead or Dying Battery, or?

Electrical and Ignition

  1. DartGTDan

    DartGTDan '71 Dart GT Fan

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    Saturday I was out driving my car ('71 Dart GT 318) around town at speeds up to 50 mph. As the vehicle speed increased I noticed the voltage would also increase to just over 16V (which I do not recall seeing it ever go that high before). When I would come to a stop, with the transmission in Drive, the voltage would drop down to ~12V. During this trip I was not using anything electrical (wipers, headlamps, blower fan, etc...).

    Sunday, my wife and I took a drive to the cemetery (the 5th anniversary of her dad's passing), then to a local ice cream parlor. During this trip the voltage appeared to be normal (~12V at a stop and ~14V while at speed).

    This morning I drove it to work (as I had an appointment to drop it off at the body shop directly after work) with the headlamps on. Again, the voltage appeared to be functioning normally (~12V while stopped and ~14V at speed).

    After work, the volt gauge was reading just a hair under 10V no matter what speed I was traveling. Fortunately, I made it to the body shop without incident.

    Unfortunately, I cannot remember when I purchased the battery, but I'm sure it has to be at least 5 years old. The last time the battery died, it was a "sudden death" (as in, I shut the car off, and it would not re-start 5 minutes later). I've never experienced a "dying battery".

    Am I correct in assuming the battery is dying? Or?
     
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    • Locomotion

      Locomotion Well-Known Member

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      Could be the alternator or regulator.
       
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      • TrailBeast

        TrailBeast AKA Mopars4us on Youtube

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        It could be the battery, but from the description of the symptoms I would check all the connections first.
        Alternator, battery cables and even at the regulator plug.
         
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        • Dana67Dart

          Dana67Dart Like a fine wine, only getting better with age! FABO Gold Member

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          I would check the voltage to the field ( from the voltage regulator) while it is not charging well.
           
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          • 67Dart273

            67Dart273 FABO Gold Member FABO Gold Member

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            THIS IS WHY (guys and girls) "we" need to learn to troubleshoot, unless "you like" throwing money at it

            Possibilities..........green field connection or that end of rotor is grounded causing "full field" charging
            VR is UNgrounded
            VR is actually defective
            Harness voltage drop---the "usual" problem
            I dont see a bad battery causing those symptoms, but a bad battery can cause things like never returning the ammeter to zero

            Start car run at fast idle, monitor battery voltage. Disconnect VR. If voltage drops to below 13, alternator rotor/ field/ brushes are not shorted

            Remove VR scrape mounting bolts and remount making certain it is grounded

            If voltage is still high, check for voltage drop in harness

            If voltage drop tests are OK replace VR

            To check for voltage drop, turn key to run, engine stopped. Hook one end of meter to "key" side of ballast, stab the other into the battery POS post. You are hoping for a reading below .3V (3/10 of one volt) If reading is a volt or more this explains the over-charge

            To check "ground" side, run engine at fast idle, make the following check first with accessories off, and again with lights, heater, etc running

            Stab one probe into battery NEG post and stab other into VR mounting flange, make sure to stab through paint, rust, etc. Once again, the lower the reading the better, zero is perfect.

            The voltage drops measured above will result in the addition of charge voltage, that is IF the VR is correctly working at about 14V and you have 2V drop in the above measurements, the system will charge at 16V
             
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            • 70dart340

              70dart340 Well-Known Member

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              67Dart273 is spot on. I've followed his advice over the years, and always found the culprit without "easter egging." Do you still have the original VR? I'd start there after making sure ALL grounds are correct. If you have a buddy with an O'Scope, that can be an immense help, as you can see things that won't show up with just a DVM. Please keep us advised.
               
            • Demonracer

              Demonracer 71 Demon 00 Ram 16 Chrysler 300S 05 Caravan FABO Gold Member

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              As stated, could be a variety of fixes. Diagnostics, diagnostics, diagnostics. DO NOT rely on your local fix it shop to find the problem, most do not understand non computer cars.
               
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