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Thread: need to tap into "true ignition source", have diagram, wondering what fuse is best?

  1. #21
    Maximum Bitrate Mickz's Avatar
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    Quote Originally Posted by doncarbone View Post
    What about once the vehicle is running (after cranking) - they will again be in parallel?

    The isolator is rated at 200amps
    Sorry for the previous post- I added a more correct answer

    Yes, they will again be in parallel.

    Do you want the batteries in parallel when cranking though?
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  2. #22
    Constant Bitrate doncarbone's Avatar
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    No I am not too concerned about that. I do not draw enough power to require both batteries to be paralleled for cranking.

    With regard to the PSU, would it make more sense to have that tap into an accessory line so the PC will not shut off if I'm still in ACC? Then if I have the isolator connect to a IGN 12V source, when in ACC only the aux battery will be used for the PC?

  3. #23
    Maximum Bitrate Mickz's Avatar
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    Yes, the control line to the PSU is normally powered from the ACC line. A PSU for vehicle use normally has a “hold on” time to keep it ON if it’s powered up. This stops the PSU switching off during cranking.
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  4. #24
    Constant Bitrate doncarbone's Avatar
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    Awesome, well, I will wait on my 'add-a-circuit' to arrive and tap into likely the radio line on this diagram here:



    This is the fuse box located under my hood...easier to run my line to this rather than through the firewall to the fusebox under my steering wheel.

    Of the available fuses to tap into on that diagram, do you still think the radio one is best, or would another perhaps be better suited?

  5. #25
    Maximum Bitrate Mickz's Avatar
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    Radio circuit should be fine for PSU control.

    I would see if you can find a circuit that is +12 with IGN and drops during cranking for the relay though. There are easy ways to get around this if you can’t find a suitable circuit. Otherwise you need to get the manual and find out the (worst case) cold cranking amperes, the problem will come if the main battery starts to fail and the AUX has to supply high current to the starter. Max current drawn from the AUX also depends on the fuse you use and the cable gauge you run to that trunk mounted AUX battery.
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  6. #26
    Constant Bitrate doncarbone's Avatar
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    Quote Originally Posted by Mickz View Post
    Radio circuit should be fine for PSU control.

    I would see if you can find a circuit that is +12 with IGN and drops during cranking for the relay though. There are easy ways to get around this if you can’t find a suitable circuit. Otherwise you need to get the manual and find out the (worst case) cold cranking amperes, the problem will come if the main battery starts to fail and the AUX has to supply high current to the starter. Max current drawn from the AUX also depends on the fuse you use and the cable gauge you run to that trunk mounted AUX battery.
    I have no idea what would be +12 with IGN/drop during cranking...do I just have to test a light on different fuses while cranking to check that?

    0 gauge cable with 200amp fuses

  7. #27
    Raw Wave
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    Quote Originally Posted by Mickz View Post
    ...a circuit that is +12 with IGN and drops during cranking for the relay ...
    IE - ACC. ACCessories drop out during cranking.


    doncarbone - keep in mind that how the battery isolator is powered has NOTHING to do with your PC or whatever is powered from the 2nd battery. That is a separate issue.

    The 2nd battery will continue to power its loads unless you turn the loads off, or the battery flattens, or you add some other control - eg timer controlled relay, or a low-voltage cut-off relay (aka battery protectors etc - same as a voltage-controlled battery isolators but set to a lower voltage and usually longer delays).

    Most people do not want loads (PCs) cut off during cranking or even when IGN or ACC is cut off.
    They usually prefer manual control, but if automated, then UPS-type functionality - ie, a signal that shutdown is imminent so the the PC can gracefully shutdown before the power is cut.
    Since many use automotive 12V designed PC PSUs, they have that functionality (ie, it's part of most 12V ATX supplies).
    Others use PC programs that detect IGN or ACC off and - after a delay - commence shutdown.
    In the above cases, the PC or PSU can disconnect the aux-battery to PC relay. But refer to others for such setups - other mp3car contributors know more about that.


    Again, many I know use the charge-lamp to control the battery isolator - ie, the UIBI which FYI stands for "Ultimate Intelligence Battery Isolator (yes, a direct slur on voltage-controlled or "smart" isolators EXCEPT where suitable "charging" or not charging" circuits are not available).
    They then let their 12V-ATX PSU or other methods handle their PC shutdown and disconnection of the 2nd-battery loads.


    Quote Originally Posted by ...a wise person
    “Smart isolator” really is a stupid name for a dumb device, what’s so smart about voltage tripping a relay and adding a bit of hold time?
    Hence the UIBI's name.
    IMO "Smart" refers to the marketing hype for voltage controlled isolators - they turn the disadvantages and problems when using voltage sensing for battery isolation into "desirable features".
    You only have to review a few different "smart" isolator specs and copy to begin to see the contradictions between different manufacturers or models. Failing that, the different voltage thresholds should sound warning bells.
    And if you consider the various time delays and when they occur, and consider certain common scenarios, you may begin to see some glaring problems...


    I often ask the (IMO) simple question "What is a smart isolator trying to do?" or more fundamentally, "What is a smart isolator trying to detect?"
    Answer: If or when the vehicle is charging. (It then interconnects the batteries.)
    Well, as witnessed by their own propaganda, that is difficult to do.
    And what does a charge light do? (Or rather, an alternator's D+ or L circuit?)

    Incidentally, the type of battery shouldn't matter as long as it's lead-acid (eg, wet or flooded cells, gel-cel, AGM, etc - but NOT LiPo, NiCad etc).
    Ignore stuff about not mixing different types (AGM & wet) or different sizes (1AH wth 100AH etc) or different ages and brands. That is all irrelevant when interconnected when charging.
    Leaving batteries paralleled when not being charged or used is a different issue, but that shouldn't even be done with identical batteries from the same batch with same age, history, condition, etc. Not that the UIBI does that (though it is easy to trigger manually), and most "smart" isolators will not do it for long (though some will until the batteries are 10% to 20% discharged!).


    PS - few fusebox fuses met the "off when charging" etc criterion. Such circuits are usually downstream of (ie, after) the fuses.
    Last edited by OldSpark; 08-07-2012 at 04:55 AM. Reason: PS...

  8. #28
    Constant Bitrate doncarbone's Avatar
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    I do see the advantages of tapping into the UIBI (charge lamp) circuit, but I must admit I have next to no idea where this would be located in my vehicle and how to safely and correctly tap into this circuit

  9. #29
    Raw Wave
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    I thought you'd be able to find your alternator.

  10. #30
    Constant Bitrate doncarbone's Avatar
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    lol ya but the particular circuit you are describing...not sure

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