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Thread: Battery Isolators, what amprage do I get?

  1. #1
    Who am I? HiJackZX1's Avatar
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    Battery Isolators, what amprage do I get?

    Was looking to buy an isolator, but not sure what amprage to get, or even what type.

    The alternator I am getting puts out 200 amps idle. At 1500 rpm, it puts out 260 amps. I was looking on the internet and they only have 200 amp isolators and 500 amp isolators. So do I get the 500 amp one?

    Also it looks like its a Solenoid type Isolator, is that ok?

    http://www.sonicelectronix.com/item_...Bay+IB500.html

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    Ask OldSpark. He could probably tell you…


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  3. #3
    Raw Wave
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    The amperage shall be enough to cover your needs.

    Forget about alternator output. You battery might put out 1,000A in short circuit mode, (my 38AH battery outputs over 2,000A) so do you get a 1kA isolator?


    What are you isolating - a dual battery setup?

    And what are the load(s) of the "isolated" end?

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    Who am I? HiJackZX1's Avatar
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    Quote Originally Posted by OldSpark View Post
    The amperage shall be enough to cover your needs.

    Forget about alternator output. You battery might put out 1,000A in short circuit mode, (my 38AH battery outputs over 2,000A) so do you get a 1kA isolator?


    What are you isolating - a dual battery setup?

    And what are the load(s) of the "isolated" end?
    Yup..... I have no idea..... maybe about 2000 - 2500 watts. Its two computers, 3 amps, and some small accessories.
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    If you don't know, how the heck are we supposed to?

    I wasn't trying to be sarcastic, but I didn't think I had already supplied your answer, namely...
    Quote Originally Posted by OldSpark View Post
    The amperage shall be enough to cover your needs.

    The isolator should carry your typical load current plus enough to recharge your battery.

    But do you really want to run computers off the audio/amp battery?

  6. #6
    Who am I? HiJackZX1's Avatar
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    Quote Originally Posted by OldSpark View Post
    If you don't know, how the heck are we supposed to?

    I wasn't trying to be sarcastic, but I didn't think I had already supplied your answer, namely...


    The isolator should carry your typical load current plus enough to recharge your battery.

    But do you really want to run computers off the audio/amp battery?
    Yea..... the whole point is to save the crank battery. My audio isnt over kill, meaning its what I need to sound to get good sound. No BOOM from 3 blocks away. Its going to use 3 amps, two 4 channel amps and one mono amp. I think the 4 channels amps are like 400 RMS. Then the Mono is 600 RMS.
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    Yet again, YOU have to determine the design parameters.

    I could give worst case... eg 400+600 WRMS = 1kW RMS => 100A at FULL load.

    But if your alternator puts out 200A at idle, then you still have another kW for PCs etc.
    If you exceed 1500RPM, you have 160A spare for the vehicle & PCs. (That's assuming to run the amps to the max.)

    1kW of amps means about 100A. [ Rough rule for 12V systems - divide their power consumption by 10 for conservative-design sizing. ]
    Add 20A or 50A or 100A for charging etc means a 120A, 150A or 200A isolator.
    But you'll have to add in the other 400W amp (40A) hence 160A-250A etc.

    And add other loads - like the computers - unless that are on another circuit or battery. (That's just another interconnection, protection, relay, an battery.)
    And it should be functionality that determines how many "circuits" - eg, the normal main battery & distribution; an additional battery & distribution for PC independence; an additional battery & distribution for audio amp independence or surge minimisation.

    If you want the 2nd battery to be able to crank the motor, then that probably determines your isolator size (eg, 300-400A) though that is usually done as a separate system without protection/fuses.


    BTW - since 500A relays aren't too common or cheap, maybe it's worth grabbing that IB unit.
    The again, if it's rare and not replaceable, it's a failure waiting to happen.

    I have seen 200A - 400A relays for $25 upwards, though a my recent search for the ~$25 200A returned nothing. (I'm sure it was on the12volt if not mp3car). IMO $25 is not much for a continuously rated high current relay - not when 30A is ~$5; 60A ~$10; 140A ~$15 etc (here in Aus).

    And as usual, I recommend auto-resetting circuit breakers for the (2) battery interconnection fuses - unless you have some suitable fuse-blown monitoring system.

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