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Thread: Diode versus relay (solenoid) battery isolator

  1. #1
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    Aug 2005

    Diode versus relay (solenoid) battery isolator

    Has anybody had any experience with either type of isolator? They both have disadvantages as per

    The diode isolator has a voltage drop to the batteries .4-.6 volts and the actual isolator I want states a .7 volt drop. What kind of problems would this cause in a car with a carpc?

    The relay (solenoid) isolator is said to be less reliable and I'm a bit confused as to its implementation. The above mention web site shows a wiring diagram using two solenoids.

    The site also talks a bit about using two batteries with no isolator, but doesn't go in to detail or list negatives for that set up. I know that one negative would be the batteries discharging in to each another so I don't think that's an option I want to explore.

    Just got an email from sure power and they stat their voltage drop to be .8-1 volt. I know the CarPC will run off the alternator, but how will this 11-11.3 volts affect starting up the PC?

  2. #2
    Join Date
    Sep 2005
    The diode type isolator will not affect your boot up. You still get 12v because the voltage drop is from the alternator to your batteries. It will only affect your charging voltage. All your equipments are still connected to the battery terminal which is always 12v when the car is not running.

  3. #3
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    Aug 2005
    So I shouldn't have any problems using a diode type isolator with a carpc and amp ? I know I just added the amp to this little scenario, but just trying to cover my bases before I throw cash down.

    Why is the reduction in volts a "negative"? I guess what I'm asking is in what set-up would it be a problem?

    Oh and thanks for the first reply

  4. #4
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    Mar 2004
    London, UK
    Quote Originally Posted by Silentbob343
    Why is the reduction in volts a "negative"? I guess what I'm asking is in what set-up would it be a problem?
    Because this voltage drop might mean that there isn't a high enough voltage to charge the batteries completely. Whilst everything will still work, because you won't have a full charge you won't be able to run your computer/amps for as long with the engine off. Also continually undercharging the battery can cause premature failure. Alternatively the alternator might sense the voltage drop and automatically increase its voltage output to compensate. However this might cause premature failure of the alternator.

    If your charging system is in good condition it might not be a problem, but the voltage drop could be compounded by cable resistance, poor connections or weak/old alternator etc.

    I'm using a relay controlled by the accessory line, and after I got all my cable and connection problems sorted out it's working well for me.

    Check this battery FAQ for more info about charging batteries.

    I recommend that whichever method you use, that you also do the Big 3 upgrade.


  5. #5
    Variable Bitrate billmee's Avatar
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    May 2005
    Since my battery is not fully charging using a diode isolator I'm considering the following two options.

    1) No diode iso : I don't think I need it since my second battery is relay disconnected from the alternator/battery node. I control the relay from the taillight so i'm always reminded to switch it off upon leaving the car.

    2) Bump it up : bump the alternator voltage to 15 volts using a DC-DC converter, I'll need one that can do 6 amps to power my carpc and also charge the battery.

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