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Battery Disconnect (knife switch) and Battery Tender

Discussion in 'Technical Q&A' started by jcavalie, Mar 15, 2017.

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  1. jcavalie

    jcavalie Formula Junior
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    Jim
    #1 jcavalie, Mar 15, 2017
    Hello -

    I've been keeping my 330 GTC plugged into a battery tender so the battery is always charged and ready to go. Current configuration is that the battery is hooked up in the car (positive and negative terminals connected in the car) and the battery tender quick disconnect dongle (for the tender) is wired to the positive and negative posts also. The tender itself is plugged into the dongle when the car is garaged, and I unplug the tender from the dongle to start the car and go.

    I recently bought a car battery disconnect switch (knife switch) that goes on the battery's negative post - and the car's negative cable connects to the other side of the knife switch so the knife can be closed (connects battery) or open (disconnects battery).

    My question is - where do I now hook up the tender? Is it proper for it to still be connected to the positive and negative terminals of the battery? In this case, it would keep the battery charged but the battery is still connected to the positive side of the car and not the negative side. Is this safe charging this way? Is the "vintage" electrical system in the car de-energized in this configuration or is the battery getting some weird closed energy loop because of the tender being attached across its terminals?

    Alternatively, I could install the tender on the positive side of the battery and the "other" side of the knife switch. In this case, the knife switch would need to be closed to charge the battery. Once charged, I guess I could disconnect the tender and open the knife switch. If I did this, how long does a car battery stay charged with no draw on it?

    Basically, I'm trying to find the safest way to keep the car stored in my garage and keep the battery charged so it's ready to go when I want to drive it.

    Any experts out there who can advise me? Thanks so much!!!
     
  2. vincep99

    vincep99 Formula 3
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    #2 vincep99, Mar 15, 2017
    Connect your tender directly to the battery terminal so you keep the battery charged. This is for example how it is done on the 360
     
  3. jcavalie

    jcavalie Formula Junior
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    #3 jcavalie, Mar 15, 2017
    Thank you!
     
  4. Nuvolari

    Nuvolari F1 Veteran
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    #4 Nuvolari, Mar 15, 2017
    Exactly correct
     
  5. jcavalie

    jcavalie Formula Junior
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    #5 jcavalie, Mar 15, 2017
    Last edited by a moderator: Sep 7, 2017
  6. Ferrrrarrrri

    Ferrrrarrrri Rookie

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    #6 Ferrrrarrrri, Mar 16, 2017
    Looks like a good piece of insurance. Very interesting to see the pics, thanks.
     
  7. Keith360

    Keith360 Formula Junior

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    #7 Keith360, Mar 19, 2017
  8. jcavalie

    jcavalie Formula Junior
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    #8 jcavalie, Mar 19, 2017
    I looked at the Moroso/twist knob style ones and the reviews aren't great because people felt they had a hard time knowing if it was disconnected or reconnected enough when turning the screw. Apparently they aren't a positive-click with a quarter rotation needed or similar. Haven't seen the longacres one.

    I personally wouldn't put any of these on the positive terminal. That seems like a recipe for a short.
     
  9. Nuvolari

    Nuvolari F1 Veteran
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    #9 Nuvolari, Mar 19, 2017
    That knife style switch looks like it could be a little problematic to make a good connection and be susceptible to corrosion. I have used the following switch in many cars over the years with great success. Its cheap and works very well:

    Battery Disconnect Switch

    The downside is that disconnecting the battery requires opening the hood. In the case of the 330 this is pretty easy but leaves the big green knob looking very out of place in the engine bay.

    If you are willing to put in the extra work you can wire in a switch under the dash. I personally use the following unit as it looks totally period and is proven to be totally reliable:

    https://www.pegasusautoracing.com/productdetails.asp?RecID=1469

    Again this is more work but does allow for a disconnection every time you park the car. Seeing as the clock seems to draw an inordinate amount of power, this also extends the time between needing to top up the battery if the car sits for a while.
     
  10. Alden

    Alden F1 Rookie
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    #10 Alden, Mar 23, 2017
    I just purchased one of those but am a bit mystified as to how the cable should attach to the end with just a hole in it.

    Do you simply put a bolt thru the hole and attach the cable via a connector?

    I guess a pic or a description would really help, thanks.
    Alden
     
  11. Nuvolari

    Nuvolari F1 Veteran
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    #11 Nuvolari, Mar 23, 2017
    Very simple , the end with the hole in it is exactly the same dimension as your negative battery terminal. Connect the wire that previously went to the negative battery terminal to this end. Once installed the negative battery cable will now be 90° off in position relative to its mounting had it been on the battery. I'm sorry but I don't happen to have a photo but it's pretty simple once you hold the part in your hand.
     
  12. peterp

    peterp F1 Veteran

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    #12 peterp, Mar 24, 2017
    The green knobbed switches are considered most correct in concours and they work very well. There is no specific "detent" for off, but they are off once you loosen them a bit and you can glance at the interior light with the door open as confirmation.

    The great thing about vintage cars is that there is zero drain, save for the clock, when the car is off, so the battery will stay charged for a very long time. A tender isn't needed if you drive it somewhat regularly, but charging occasionally is a good idea for battery health if you store it over winter or store it for very long periods of time between drives.
     
  13. Streetsurfer

    Streetsurfer Formula Junior

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    #13 Streetsurfer, Mar 24, 2017
    Last edited by a moderator: Sep 7, 2017

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