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Auto battery isolator question

Discussion in 'Electronic Basics' started by Keith, Jul 19, 2004.

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  1. Keith

    Keith Guest

    I have set up an inverter for 120-volt use in a service van. I have
    used a commercial (RV supply house) battery isolator to separate the
    inverter's battery from the vehicle's battery. The problem comes from
    the vehicle's alternator needing to see battery voltage to begin
    charging - in other words the alternator is self initiating, and only
    has a "status" out wire (for lack of a better term :) and the high
    amperage terminal. Now, because the isolator, umm, isolates everything
    from everything, the alternator won't charge unless I jump it to the
    vehicle's battery. This means, however, the inverter also will draw
    from the vehicle battery, which defeats the purpose of the second
    battery and isolator. I know I can play with contactors etc, to connect
    the charging system as needed, but any ideas on how to do it "right"
    using solid state stuff? Have I got the wrong type of isolator?

    Keith
     
  2. Dbowey

    Dbowey Guest

    Keith posted:
    << I have set up an inverter for 120-volt use in a service van. I have
    used a commercial (RV supply house) battery isolator to separate the
    inverter's battery from the vehicle's battery. The problem comes from
    the vehicle's alternator needing to see battery voltage to begin
    charging - in other words the alternator is self initiating, and only
    has a "status" out wire (for lack of a better term :) and the high
    amperage terminal. Now, because the isolator, umm, isolates everything
    from everything, the alternator won't charge unless I jump it to the
    vehicle's battery. This means, however, the inverter also will draw
    from the vehicle battery, which defeats the purpose of the second
    battery and isolator. I know I can play with contactors etc, to connect
    the charging system as needed, but any ideas on how to do it "right"
    using solid state stuff? Have I got the wrong type of isolator?
    I believe your isolator may be mis-wired.

    In my motorhome the isolator does not affect how the vehicle system operates:
    The vehicle battery is always available to vehicle load and the alternator.
    When the engine is not running, the isolator isolates the rear battery from the
    vehicle battery. When the engine is running, it connects the rear battery in
    parallel with the vehicle battery.

    Don
     
  3. Rodney Kelp

    Rodney Kelp Guest

    What about when your boondocking generator is running? Does it also charge
    all the batteries?
     
  4. Bill Bowden

    Bill Bowden Guest


    You can make an isolator with single diode and just put
    the diode is series with the spare battery. The only problem
    is the diode will drop about 0.7 volts so the spare battery will
    never fully charge. The regulator keeps the voltage of the main
    battery around 14 volts, so the spare battery will limit to
    around 13, but that may not be a problem. It should be fairly well
    charged (maybe 75%) at 13 volts.

    You need a high current diode rated at 25 amps or more. An
    old diode out of an alternator should work. Connect the
    cathode of the diode to the spare battery (+) terminal and
    the anode to the (+) terminal of the main battery. Current
    will only flow one way through the diode, so the only discharge
    path will be through the inverter.

    -Bill
     
  5. Bill Bowden

    Bill Bowden Guest


    You can make an isolator with single diode and just put
    the diode is series with the spare battery. The only problem
    is the diode will drop about 0.7 volts so the spare battery will
    never fully charge. The regulator keeps the voltage of the main
    battery around 14 volts, so the spare battery will limit to
    around 13, but that may not be a problem. It should be fairly well
    charged (maybe 75%) at 13 volts.

    You need a high current diode rated at 25 amps or more. An
    old diode out of an alternator should work. Connect the
    cathode of the diode to the spare battery (+) terminal and
    the anode to the (+) terminal of the main battery. Current
    will only flow one way through the diode, so the only discharge
    path will be through the inverter.

    -Bill
     
  6. Keith

    Keith Guest

    Therein lies the rub. I do not want the vehicle battery to feed the
    inverter at all. I do want the vehicle charging system to charge the
    inverter's separate battery. With a diode, or the isolator (which is I
    suspect just 2 90-amp rated diodes in a heatsink case) using just 1 of
    it's diodes, the inverter will pull current from the vehicle's battery
    through the diode, yes?

    I realise I should have described the isolator in my post. It is just a
    simple 3-terminal metal encased unit. Alternator output goes to the
    center termainl, and each battery positive goes to one the other
    terminals. It's rated at 90 amps continuous.

    Keith
     
  7. Rich Grise

    Rich Grise Guest

    Find an RV show. A guy I used to know years ago sold battery isolators
    specifically designed for RVs. They have all kinds of stuff that will
    do exactly what you need, probably better than what you (or most of
    us, for that matter) could come up with. There's some really cool stuff
    at these shows - but it's a niche market, so you don't hear much about
    it. Stuff like satellite antenna aimers - a motorized swivel mount with
    elevation, to get satellite tv in your RV. I think they even have them
    that will track the satellite while you're in motion. A fun place to
    visit anyway - same class of people as hamfests and computer flea
    markets and stuff. (or the county fair! :) )
     
  8. Jamie

    Jamie Guest

    Diode?
     
  9. Bill Bowden

    Bill Bowden Guest

    Yes, that's right, the inverter would pull current from the
    main battery as soon as the spare battery voltage dropped.
    I think your isolator with 2 diodes should prevent that.
    It should be working correctly now since the inverter cannot
    pull current from the main battery.

    Why do you think it's not working?

    -Bill
     
  10. Keith

    Keith Guest

    It will. But...
    It also prevents the main battery to flow to the alternator to enable it
    to start charging. The alternator is self-exciting, and needs to see
    battery voltage at it's output terminal to start charging. Older
    alternators used a separate field wire, made live when the key is on.
    On this Dodge Sprinter Van, AKA Mercedes Benz, the alternator has no
    explicit input to supply initial current, as far as I can tell. As zoon
    az zhes aspinnin, zhes a chargin. :)
    *Sound of claxon in background* "Warning! Battery is not being charged
    by alternator!"

    And by voltmeter check of charging current.

    Thanks!

    KR
     
  11. Don Kelly

    Don Kelly Guest

    The isolator should keep the batteries independent. It is designed to do so.
    The isolator will have 2 diodes feeding the two batteries from the common
    alternator terminal. Where is the "status??" wire normally connected? Is it
    normally connected to the hot of the alternator (or, with no isolator- to
    the same place- the battery terminal? You should have no problem with this.
     
  12. Keith

    Keith Guest

    The status wire is connected to a separate terminal on the alternator.

    Yes, the isolator will keep the 2 batteries separate. It also prevents
    current flow back into the alternator from either battery. This would
    normally be a desirable thing, but this alternator needs to see battery
    voltage at it's output terminal before it will charge.

    Hmmm... I'm wondering if it needs to see full voltage. Maybe I could
    use a small resistor to provide a little current to get it charging. I
    mean like really small, in the order of 1K ohms. Then if it would start
    charging, the power dissapated by the charging current through the
    resistor would be very small.

    More expirimenting is called for this weekend! :)

    Keith
     
  13. From my understanding, the high current diodes used in isolators are
    schottky, so the voltage drop is not very much, maybe only a half volt
    or less depending on the current.
     
  14. Don Kelly

    Don Kelly Guest

    Have you tried connecting the "status" wire to the alternator terminal. I
    would suspect that it is a field wire and the voltage/current controller is
    built into the alternator.
    Otherwise your resistor idea might work- If the status wire is not connected
    to the main alternator output in any way then a resistor should not be
    needed. Automotive wiring is a mystifying thing- often with little logic
    except- "Oh gee, if I make this shortcut, we can save a penny per vehicle"
    I lost two alternators in a '75 VW van due to such a shortcut.
     
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