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Gerald55's avatar
Gerald55
Explorer
Dec 02, 2015

Switch for alternator to coach battery disconnect

I would like to be able to sometimes turn off the engine alternator charging of the house batteries on my rig. To do that, I'm thinking of putting a switch on the positive chargeline running from the alternator back to the batteries. This line is already accessible from inside since it passes through a 120A DC breaker in a cabinet in the coach.

I'm not sure what kind of switch is appropriate here. I gather that I can't just grab any SPST switch off the shelf at home depot, since these are mostly designed for typical AC voltages of 20A or lower, and I think up to 100A or so may be passing through the switch during charging, and I also understand that DC current is harder for switches to turn off, particularly for inductive loads (I don't know if the alternator/battery circuit is an inductive load, however).

Any tips on what kind of switch would work?

18 Replies

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  • wolfe10's avatar
    wolfe10
    Explorer
    Dec 03, 2015
    grizzzman wrote:
    wolfe10 wrote:
    NO, you do not want to interrupt alternator B+ to battery-- you will blow diodes in the alternator if it "charges an open circuit".

    There are some very simple solutions to your issue. The simplest is a Marine manual ON-OFF switch.

    Alternator B+ and chassis battery to one lug and house bank to the other lug.

    So, when you drive:

    Switch off: chassis battery charged, house bank not charged.
    Switch on: both banks charged. Can also be used to "combine" batteries for starting. But, if dry camping, it if left on, would discharge both banks!

    Here is a common switch:http://www.ebay.com/itm/SEACHOICE-PERKO-MARINE-MAIN-BATTERY-DISCONNECT-SWITCH-ON-OFF-11551-/231715714054?hash=item35f3553406:m:mBUmc0K8QfB1Ob07kh36phw&vxp=mtr


    As long as there is a engine battery that stays hooked up it will not hurt a thing to disconnect the coach batteries.


    Correct. But, from OP's statement below it is not clear where he wanted to interrupt the B+ to battery feed. Both of us are saying the same thing-- ONE battery bank must stay in the circuit.

    He said: ", I'm thinking of putting a switch on the positive chargeline running from the alternator back to the batteries..."

    And, the how to achieve this depends on whether he has a solenoid based isolator (easy to do by interrupting ignition hot signal) or diode based isolator which would be more complicated.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Dec 03, 2015
    Hi,

    I added a 2nd charging circuit and have manual control of both charging solenoids. It was done by switching the activation of the solenoid. Amperage draw from the relay is 0.88 amps which means just about any switch will work.
  • haddy1's avatar
    haddy1
    Explorer II
    Dec 03, 2015
    And the winner is DrewE.

    Simply switch the ignition wire to charge solenoid if you have one.
  • grizzzman's avatar
    grizzzman
    Explorer
    Dec 03, 2015
    wolfe10 wrote:
    NO, you do not want to interrupt alternator B+ to battery-- you will blow diodes in the alternator if it "charges an open circuit".

    There are some very simple solutions to your issue. The simplest is a Marine manual ON-OFF switch.

    Alternator B+ and chassis battery to one lug and house bank to the other lug.

    So, when you drive:

    Switch off: chassis battery charged, house bank not charged.
    Switch on: both banks charged. Can also be used to "combine" batteries for starting. But, if dry camping, it if left on, would discharge both banks!

    Here is a common switch:http://www.ebay.com/itm/SEACHOICE-PERKO-MARINE-MAIN-BATTERY-DISCONNECT-SWITCH-ON-OFF-11551-/231715714054?hash=item35f3553406:m:mBUmc0K8QfB1Ob07kh36phw&vxp=mtr


    As long as there is a engine battery that stays hooked up it will not hurt a thing to disconnect the coach batteries.
  • enblethen's avatar
    enblethen
    Nomad
    Dec 02, 2015
    You should be able to interrupt the control circuit on the charge relay.
    You would need to get wiring diagram from manufacturer.
  • landyacht318's avatar
    landyacht318
    Explorer
    Dec 02, 2015
    I like Blueseas products:

    https://www.bluesea.com/products/6006/m-Series_Mini_On-Off_Battery_Switch_with__Knob_-_Red

    Thicker longer studs, higher continuous amp rating and smaller physical size than my previous 'guest' brand switch which wore out and got hot passing 50 amps.

    As long as the starting battery is not switched off with the engine running, turning off the house bank from alternator current 'should' not present an alternator load dump situation and fry it, but I'd wait for Mexwanderer to confirm this.
  • wolfe10's avatar
    wolfe10
    Explorer
    Dec 02, 2015
    NO, you do not want to interrupt alternator B+ to battery-- you will blow diodes in the alternator if it "charges an open circuit".

    There are some very simple solutions to your issue. The simplest is a Marine manual ON-OFF switch.

    Alternator B+ and chassis battery to one lug and house bank to the other lug.

    So, when you drive:

    Switch off: chassis battery charged, house bank not charged.
    Switch on: both banks charged. Can also be used to "combine" batteries for starting. But, if dry camping, it if left on, would discharge both banks!

    Here is a common switch:http://www.ebay.com/itm/SEACHOICE-PERKO-MARINE-MAIN-BATTERY-DISCONNECT-SWITCH-ON-OFF-11551-/231715714054?hash=item35f3553406:m:mBUmc0K8QfB1Ob07kh36phw&vxp=mtr
  • DrewE's avatar
    DrewE
    Explorer II
    Dec 02, 2015
    A switch for this sort of power would probably be marketed as a battery shutoff switch, and may not really be intended to be switched under load. Auto parts stores etc. might carry such a switch.

    However, if you have a solenoid based battery isolator, you can simply switch the control wire for the solenoid that connects the batteries. That's a much more readily available (and cheaper) switch, and doesn't require routing big heavy wires to and from it. The wires you need to switch may already be at the dashboard if you have an emergency start switch (depending upon exactly how the emergency start is wired on your motorhome).

    If you have a diode based isolator, I'd suggest that you replace it with a solenoid-based one and proceed as above.

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