Battery disconnect

Discussion in 'Shop Talk' started by Ed Norman, Apr 29, 2006.

  1. Ed Norman

    Ed Norman Well-Known Member

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    We have a '90 Dodge one ton 15 passenger van that get driven very seldom. It has one of those fancy gel batteries that is supposedly bulletproof but lately the battery has been dead after not starting it for a month or more. I can't see anything like a light that stays on but there's a leak somewhere. I'm thinking of putting in a battery disconnect switch. Does it go in the positive or negative cable? Thanks.
     
  2. wy_white_wolf

    wy_white_wolf Just howling at the moon

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    They are generally put in the positive, but either will work.
     

  3. beorning

    beorning Well-Known Member

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    Ever try one of those solar dashboard trickle chargers? I used to have a lot of friends in the Navy who would go out for months at a time, leaving their cars in a parking lot on the base. They swore by those chargers.
     
  4. Naughty Pines

    Naughty Pines Active Member

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    I had four of them. :nerd:
    I gave ont to the fire station 4 blocks away for their Dodge red ram truck snow plow. :angel:
    I used one of them to charge the battery on my sears garden tractor. :hobbyhors
    Seems to work pretty good. :cool:
     
  5. Beeman

    Beeman Well-Known Member

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    I would first try and diagnose to see if there is a parasitic draw on the battery. A draw of more than 30 milliamps is too much and should be isolated and repaired. Disconnecting the battery will wipe out clock and radio presets and possibly block learn on the PCM which could effect performance.
     
  6. Ed Norman

    Ed Norman Well-Known Member

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    How can I check what it is drawing? I've thought something has changed that is draining the battery, but it isn't anything obvious.
     
  7. VA Backwoodsman

    VA Backwoodsman Active Member

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    There are two things that you can do. First is remove the negative cable from the battery, then take a 12V test light and hook the clamp to the neg battery post. then hook probe end to the neg. cable. the light will come on if there is a draw. now go to the fuse box and remove 1 fuse at a time and check your test light each time. the light goes out when you find the circut that is drawing current. this tells you where to go look for the problem. Engine/transmission computers and radios have a small draw to hold the memory. The next thing is get a amperage meter to test how much draw you have. Beeman is right about his 30 milliamp recommedation. also put the disconnect in the ground cable if you chose that route because in the positive side can cause a voltage pulse that can damage some electronics when you flip the switch on. the test light method works on most automotive systems - farm equip., boats etc.
     
  8. Beeman

    Beeman Well-Known Member

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    If installing a disconnect it doesn't really matter which post you turn on or off, both will stop current flow and both will cause an arc or surge if there is a current flow when connecting/disconnecting.

    The only positive way to check for a parasitic draw is with an digital ammeter with a milliamp scale. The ammeter must be hooked up between the battery cable and the battery to get a reading of the current that is flowing. Because of electronics in newer vehicles you can't just disconnect the battery and then hook in the meter. you must have the meter hooked in and the cable connected at the same time and then remove the cable leaving the meter as the flow route for the current. The reason for this is that when the battery is disconnected electronic items discharge, when the battery is reconnected it takes time for them to recharge, this recharging will give you a false current draw reading. This is why the test light method gives false readings on newer cars. The light will glow while electronic components are powering up and lead you to believe ther is a draw. Pulling fuses then will cause you furthur confusion. The absolute ideal method is to use one of those battery shutoff switches and bridge across it with the ammeter, when you open the switch it allows the current to flow thru the ammeter thus never actually disconnecting the power from any circuit.

    While checking with an ammeter you might want to start on it's highest scale as it is most likely fused and if the draw is more than the fuse value you will blow the fuse in the meter. once you feel you can drop to the lower scale do so just be sure not to have a helper open a door which would turn on the interior lights and overload the fuse in the meter.
     
  9. Northman

    Northman Well-Known Member

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    I agree with Beeman on his explaination of checking for draw but might wanna checka manual to see acceptable draw he mentioned 30 milliamp this is generally true but some vehicles can be higher. some 50 milliamps.

    there can be many things that can draw and not be easy to see like radio (internal clock) or alternator.