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Thread: battery isolator vs relay

  1. #11
    Low Bitrate
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    So is this setup correct. I know nothing about power, but I have this problem.

    First of all, my batteries are connected using a relay that switches on only when the car is running. If the car is off, the relay is open. That part I think is correct. Is that the case?

    The problem is that the batteries are not holding their power very well. I can actually see the power drop as I stop at a stoplight, especially at night.

    Does this mean that the alternator is just not producing enough juice to run the setup?

    Would upgrading the alternator solve the problem or is there a bigger issue with the batteries and the way they are connected. One of them is a brand-new Optima yellow-top and I don;t want to mess it up!

    What brand of alternator should I consider for a '95 chevy tahoe?

    Thanks in advance for your replies!

  2. #12
    Variable Bitrate stimps's Avatar
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    I used to have a relay setup, where the relay connected the two batteries together, only when the engine was running (ignition relay) and the second battery was only connected to the stereo amp. Didnt have any problem with it, worked for years. I have an isolator on the second battery, that connected the two batterys together, incase the vehical batery was flat. Got me out of a flat battery a few times, ( get in car, flat battery, doh, get out, close isolator, start, drive away)
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  3. #13
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    So would I be better off with an isolator an not the relay? I found a great one with very little resistance (diode effect).

    Kind of expensive though.

    http://www.hellroaring.com/bic75150.htm

    (UPDATE) This device is actually a combiner/isolator and can be configured for a secondary battery to be the aux startup battery if your main battery is handling the load or as an aux battery controller if your loads are connected to the second battery.

  4. #14
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    here is a good place to find a guide for alternators, etc.

    http://www.balmar.net/Page13-alternators.htm

  5. #15
    FLAC DodgeCummins's Avatar
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    I also had the dual battery through the relay (actually a ford starter solenoid) in my old Blazer. Same deal, if the ignition was on, the relay was closed and both batteries got charged, with the switch off...(or even during the start cycle I think) the batteries were isolated.

    So I could run my stereo with the engine off, and still make it out of the woods.

    The isolator has no moving parts, and always keeps the batteries isolated, with the relay the batteries are tied together after the engine starts.

    There are different isolator sizes depending on the batteries...

  6. #16
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    Well I did it. I just had a custom 250 AMP regulated alternator into the truck. The difference is amazing. For a while the laptop would go to sleep because the voltage would drop below what the dc adapter could handle. Now the system is generating 13.9 volts with the car at idle in drive and with the AC, stereo, high beams and computer all running at once.

    I had a little problem at first in that the output was great until I stopped at a light. A smaller pulley fixed that.

    I kept the relay setup, but both batteries are much better charged now, so I don't think I'll need the isolator for now.

    thanks to all for your suggestions and information
    More to come as I use it....

  7. #17
    Constant Bitrate
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    Well it is time to revive the thread again....

    I found a cool gadget from the RV world where you ALWAYS have two batteries; the car battery and the house battery. Both can typically be charged from the car alternator.

    The device is called a ‘Isolator Relay Delay’, and that is a fairly good description of what it does. This gives you a combined tank solution (as you don’t use both batteries to crank the engine with), and a smart charging solution (including a stress solution for your alternator).

    From the manufacturer:

    “It operates by sensing the voltage on the main 12 volt system. When this voltage goes above 13.3 volts for approximately 12 seconds, as happens when the engine is running normally (normal alternator output voltage is approximately 14.4 volts), it will close the isolator relay providing charging current to the auxiliary battery. When the ignition switch is turned off, the relay will open immediately.

    If the voltage should drop below 12.0 volts for more than two seconds while the engine is running, the relay will drop out. This might happen when the alternator is not able to supply sufficient current to all the loads and charge the auxiliary battery.

    When the main battery voltage rises above 13.3 volts again, the relay will again close in about 2 seconds to retry charging the auxiliary battery. The resultant flicker of the lights will alert the driver of the system overload.”

    Here is the manufacturers data sheet:
    http://www.intellitecsve.com/pdf/5322132.pdf

    Sounds exactly like what I need.

    What do you guys think?

  8. #18
    Constant Bitrate
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    you checked that the second battery is grounded properly?

  9. #19
    FLAC DodgeCummins's Avatar
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    Interesting toy!

    Of course this is not the same as a diode isolator...however...

    The big difference between that and a normal setup is that your main battery always has first dibs on the alternator juice.

    So if your amps and things start pulling down the alternator to the point where the primary battery is not getting charged, then it will disconnect the second battery.

    It means if the aux setup craps the bed, or you get a short etc, the main system keeps going and ignores the aux system.



    If you find your setup overloading the alternator so your lights dim when the bass thumps, then this might be a good idea for you.

    Or if you are doing a 'round the world expedition...

    But I think for the casual users like most of us, that setup is a bit overkill...(unless it is the same cost as a diode isolator)

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