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Thread: High Current Relay questions (for dual battery)

  1. #1
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    High Current Relay questions (for dual battery)

    Hi there, Im just curious about setting up a relay between my 2 batteries, the starter and the deep cycle.
    Relays can be rated to certain current, most of the heavy duty ones being 80Amp, 100amp, and even 200amp. My stereo system and computer are hooked up to the second aux battery, and taht battery currently is hooked up straight to the starter. My alternator is new and cranking out 110 amps

    Question: What size relay? would 80 amp work? I mean, with my amp running itll be pulling mabye 50-60amps, plus all the things running in the car will no doubt take a lot of the amps straight from the alt.

    What happens if the current going over the relay exceeds its rating?

    Thanks for the help!

  2. #2
    Stank Cheese n8scstm's Avatar
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    Be careful doing that. One common problem is that when you turn the key the batteries will be parralell. If your aux is dead, your main batt. will be attempting to start your car and charge the aux at the same time. So it may not work for you.
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  3. #3
    Maximum Bitrate Altimat's Avatar
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    The old Ford starter relays are cheap and work well for large amounts of current. I'm not sure how to measure exactly what your draw will be but the contacts can weld closed if you exceed the capacity of the relay.
    Fabricator

  4. #4
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    Quote Originally Posted by Altimat
    The old Ford starter relays are cheap and work well for large amounts of current. I'm not sure how to measure exactly what your draw will be but the contacts can weld closed if you exceed the capacity of the relay.
    I have never had any experience with starter relays myself, but i have been warned by several sources that they do not hold up well to being contuniously energized and have very high failure rates under such conditions


    I have fried quite a number of relays in a project lately and it seems that relays usually fail in any of several common ways. Some of them result in the relay being stuck open and some of them stuck closed. I wouldn't rely on them being stuck closed when they fail.

  5. #5
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    Use a continous duty solenoid rated for your application. If you're just doing a tank setup, a $25 100A solenoid should work. If you're running a dual battery setup with Optimas/Orbitals then you need to have a solenoid that can handle the amount of current your alternator puts out.

    eBay seems to be the best source now a days for solenoids. They work like a giant relay. Just hook them up to like your ignition wire and it'll prevent your carputer and inverter from rebooting due to low voltage on the starter battery.

    Ford starter relays have too little resistence on their cores and cannot handle continous current. Like a starter battery, they are not designed to handle vast amoutns of current for long periods of time.

    I bought a 90A continous duty solenoid last week and installed it for my 10AH tank circuit battery and it works like a charm. The 80A ANL fuse for the solenoid never blows either, even though my alternator is 160A fullspeed/100A idle.

    The nicest thing about a solenoid rather than a Diode isolater is the fact that solenoids DO NOT cause a voltage drop like Diodes do. My amps cause enough of a voltage drop already lol!

    -LAKORAI

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