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Thread: How to make a cheap isolated dual-battery setup for $50

  1. #11
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    There's a "oops" scenario here. Say you shut off the truck and leave your lights on. Come back later and red battery is dead, but with enough juice to engage the solenoid when the key is turned on. Even though the red battery is still dead and incapable of starting the truck, the yellow battery provides voltage to all circuits (except starter) before you hit 'start' so you don't know red battery is dead. But when you engage start, starting current will blow yellow fuse. It's not such dangerous failure mode as long as the fuse blows.

    The way around this is having a smarter solenoid that only engages at over 12v. That way the yellow battery begins charging only when alternator is charging so you can't try to start with it.
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  2. #12
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    Quote Originally Posted by Tennmogger View Post

    The way around this is having a smarter solenoid that only engages at over 12v. That way the yellow battery begins charging only when alternator is charging so you can't try to start with it.
    Yes, you are totally correct. I think a lot of the full kits have that built into them. I use the 'don't leave your lights on' capacitive cranial neuroreactor to make sure that doesn't happen to me

  3. #13
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    i have pretty much the same set-up, even solar, but i control mine with a manual switch in cab rather than keyed. i recently swapped the ford solenoid for a continuous duty one.

    one of your links is bogus, and a real let down, i cant even cuss you out properly cause i have to go lie down for a bit.
    Last edited by RHINO; 04-28-2012 at 12:08 AM.
    Clay

  4. #14
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    Great design evldave, thanks a lot for sharing it and the links to the pieces to build it

    Quote Originally Posted by Tennmogger View Post
    The way around this is having a smarter solenoid that only engages at over 12v. That way the yellow battery begins charging only when alternator is charging so you can't try to start with it.
    Easier/cheaper than getting a "smarter solenoid" might be simply having an "on delay" relay controlling the solenoid's switched power supply.
    If it were set to switch on power to the solenoid 5 minutes after receiving power (5 min after ignition key turned) then the driver would have discovered if the starting battery will start the vehicle or not. So 5 minutes after ignition key is turned (and typically engine started), the house battery starts getting charged.

    Something like this one : http://www.aliexpress.com/product-fm...olesalers.html
    (although the environmental/electrical specs aren't quite perfect for this application, it's basically what I'm talking about.)
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  5. #15
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    Is there supposed to be a fuse in that solar stuff somewhere?
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  6. #16
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    Dave always has the most entertaining build threads!
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  7. #17
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    As for the scenario of helping your dead starter battery, you could always run another 12g "ON" wire from the + on your rear battery to the 80A Solenoid, but put a switch in the cab so you could enable it only when needed, this way, while sitting in your seat, you could flip the switch, and connect the batteries to start your truck!
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  8. #18
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    Quote Originally Posted by 4RunAmok View Post
    As for the scenario of helping your dead starter battery, you could always run another 12g "ON" wire from the + on your rear battery to the 80A Solenoid, but put a switch in the cab so you could enable it only when needed, this way, while sitting in your seat, you could flip the switch, and connect the batteries to start your truck!
    I wouldn't us an 80-amp solenoid (and fuse) to jump start, or run a winch.
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  9. #19
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    My truck has two batteries under the hood (cranking and deep cycle - both flooded) with a "who knows what" dumb solenoid...prolly 80a and...

    ...NO fuses in the battery cables. Doesn't bother me. The factory battery to starter wire doesn't have a fuse in it anyway.

    Self-jumps just fine. It didn't when I got it - just did the solenoid ratchety-clicky sound. Bigger cables fixed that though.
    ...
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    Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
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  10. #20
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    ok so say i hook the keyed power to a toggle switch, and have it off most the time. would that just mean the house battery wouldn't get a charge? and what would happen if say the switch is left on while the vehicle was not running, would it just drain the start battery?

    just trying to see the difference/importance of keyed on apposed to a toggle switch to turn on, besides the fact you can't forget to turn it on.

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