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Thread: Battery Isolator Question

  1. #11
    Constant Bitrate Rickk's Avatar
    Join Date
    Feb 2013
    Western Massachusetts
    Hi Phil....

    Probably not.

    I would be amazed if, at a normal regulator output voltage that the charging current is over 10 amps per battery. The alternator output voltage is higher than the float state voltage of either batter while the engine is running, so current will only go into the batteries... none would flow from primary to secondary.

    As soon as the alternator stops working, the primary battery terminal voltage would drop to a "float state". At that voltage, the secondary battery wouldn't draw a heck of a lot of current.

    Plus, a second or so later the relay opens and it is all over.

    At least, that is my theory and I an stickin' with it.

  2. #12
    FLAC PhilG's Avatar
    Join Date
    Mar 2009
    Ontario, Canada
    That does sound like it makes sense. I'm extra sensitive to battery condition due to issues with a diesel and low battery power.
    My 2007 Ford F350 Work Log located HERE

  3. #13
    Constant Bitrate Rickk's Avatar
    Join Date
    Feb 2013
    Western Massachusetts
    This past weekend I finally installed my auxiliary battery and have power in the area of my new console.

    I made a few voltage readings under different conditions after it had a while to charge the auxiliary battery.

    Both the primary and the auxiliary battery measure about 12.6 volts when the engine is off and the battery isolator is "off".

    When the engine is running (and it is about 75-80 degrees, which is important to state), the alternator brings the battery voltage up to about 14.2 volts. I have a DC clamp-on ammeter but I forgot to drag it out and measure charging current. I assume it was somewhat low.

    When I shut the engine off, the isolator opens pretty quickly (withing a few seconds) and the battery terminal voltage drops fairly fast to high 12's (12.6 - 12.7-ish).

    When I start the truck, the alternator "walks" the voltage up in small steps (probably a microprocessor in it). At some point within 10 seconds or so the isolator is closed and both batteries are at around 14.2 volts (at 75-ish F).

    The isolator is a painless install.. connect each battery to one of the two relay terminals (fuses in each direction as close to the batteries as possible I would recommend) and connect the bottom push-on terminal to chassis ground and life is good.

    Last edited by Rickk; 06-09-2014 at 10:17 PM.

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