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  • Battery charging problem

    I have a secondary battery in the trunk thats not charging correctly.
    Its fed directly from the Alt, via an 80A fuse, 30A relay (only closes when the ignition on), to the battery. The charging cable is 4GA copper.

    I start the car, then turn on the system, let the car run up and away I go.
    But when I stop the car and look at the battery voltage in the trunk, its only a few volts. It looks like the system is drawing from the secondary battery, but not allowing the battery to charge.

    The Alt is giving out enough current (rated at 90A) and the system is drawing 10A to run the Mastero 4.5.

    Its got me stumped guys. Any ideas?
    1981 VW Scirocco, 2L 16v turbo. EEEPC 1000 running Mega Squirt and music.
    Http://www.menloparkrandd.com/scirocco

  • #2
    Do you have it wired up like this?
    Attached Files
    Only true postwhores remember everything they post

    REP CLUB IS DEAD
    LONG LIVE REP CLUB

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    • #3
      Exactly. That should work right?
      1981 VW Scirocco, 2L 16v turbo. EEEPC 1000 running Mega Squirt and music.
      Http://www.menloparkrandd.com/scirocco

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      • #4
        Have you checked the relay and fuses?
        Disconnect the battery, put a meter on the connectors and start the car. Got power?

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        • #5
          If I run with out the computer the battery level goes up and the current on the ammeter shows normal current drain.
          When running the computer, the ammeter shoes a much heavier current draw, but diminishes the secondary battery.
          I am going to recheck the internal power settings and draw. I don't think I made a mistake though.
          1981 VW Scirocco, 2L 16v turbo. EEEPC 1000 running Mega Squirt and music.
          Http://www.menloparkrandd.com/scirocco

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          • #6
            In my opinion you should have a battery isolator. That way the secondary battery is charged after the main. And if your secondary battery is drained by lets say the pc being left on, the main battery will not be dead when you try to start your car. You can get them in different sizes but these are some general ones. Ive done many setups like this in audio installs and it works great every time. http://www.jcwhitney.com/autoparts/B...+10201/c-10101

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            • #7
              Wow. 10A doesn't seem like that much. Does the battery ever charge up full? Maybe it's going bad. If that's all good, I bet something (wire or relay) is getting hot.

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              • #8
                Originally posted by Jcarere
                In my opinion you should have a battery isolator. That way the secondary battery is charged after the main. And if your secondary battery is drained by lets say the pc being left on, the main battery will not be dead when you try to start your car. You can get them in different sizes but these are some general ones. Ive done many setups like this in audio installs and it works great every time. http://www.jcwhitney.com/autoparts/B...+10201/c-10101
                Isn't that what the relay does? Surely the connection between the two batteries is cut off when the alternator is idle (engine off).

                Julie, is the second battery in good condition?
                Only true postwhores remember everything they post

                REP CLUB IS DEAD
                LONG LIVE REP CLUB

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                • #9
                  Battery is a mid sized lawn mower battery that can withstand deep discharge.

                  I'll deal with it later, I am about to drive 10 hours (there and back) to get a door for my car. This winter, complete chassis strip and rewire after a nice fresh coat of paint!
                  1981 VW Scirocco, 2L 16v turbo. EEEPC 1000 running Mega Squirt and music.
                  Http://www.menloparkrandd.com/scirocco

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                  • #10
                    I think most batteries can take a deep discharge, but the longer they stay undercharged the worse off they get. Like if it was at 6V or less for a month it would be very weak and couldn't hold much anymore.

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                    • #11
                      You sure that lawnmower battery is a 12v and not a 6v?

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                      • #12
                        Yup its a 12v. Its still doing it too. After a 10 hour drive (2 being lost in Mid Canada) the battery was at about 3v.
                        The computer drained the alt batt and ran off the cars alternator.

                        I'm stumped guys...
                        1981 VW Scirocco, 2L 16v turbo. EEEPC 1000 running Mega Squirt and music.
                        Http://www.menloparkrandd.com/scirocco

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                        • #13
                          Originally posted by Juliemac
                          Yup its a 12v. Its still doing it too. After a 10 hour drive (2 being lost in Mid Canada) the battery was at about 3v.
                          The computer drained the alt batt and ran off the cars alternator.

                          I'm stumped guys...
                          Me too. The only thing I can think of is either the computer is sucking the life out of the battery or the battery itself is shot. It might be worth getting the battery checked out by an auto electrician if you haven't done so already. If its okay and the wiring is right then I guess its down to the computer. Is the starter battery holding its charge ok?
                          Only true postwhores remember everything they post

                          REP CLUB IS DEAD
                          LONG LIVE REP CLUB

                          Comment


                          • #14
                            I was thinking you didn't give it enough time to charge, but that trip should have done it. Maybe the relay gets hot and starts opening and closing?

                            How can the battery get down to 3v and the PC still runs off the same wire? Does the battery just create a high resistance while the supply voltage is still at 12+ or would it drop it down lots of volts like a huge 1 ohm resistor?

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