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Battery Isolator Question

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  • Battery Isolator Question

    So I am installing a deep cycle battery in my 2005 suburban and I'm trying to figure out how to separate it from the starting battery to prevent them from depleting themselves when the vehicle is off (also, I want to use the deep cycle to run my burbanputer when the car is off.)

    From my understanding, diode based isolators will prevent the batteries from depleting each other but have a voltage drop. This voltage drop stops the batteries from getting a full charge and is a no-no.

    They have solenoid based isolators, these combine the batteries in parallel when the vehicle is on but separate them when it is off; they can also be designed as voltage regulated isolators that isolate when starting battery is below a threshold. When operating a deep cycle battery with the vehicle off it can decrease the voltage across the battery (duh) but when the solenoid turns on a surge of power will run from the starting battery to the deep cycle. This can short the batteries and fry electronics (this is also a no-no).

    I have seen some solidstate isolators but the ones I have seen have been in excess of $400, this seems like quite a lot considering what I am getting out of it.

    What have you guys done for battery isolation and where did you get them? O.o

  • #2
    I use a solenoid with a smart monitoring circuit. Eventually I'm going to replace that smart circuit with an even smarter arduino.


    • #3
      no reason you should ever use a diode isolator... it has absolutly no benefit, over the totally cripling loss of voltage...

      as to when you connect the main battery to a very discharged deep cycle. what happens is not a "short circuit" , cause they are anyway, always sorted . what happens is that the discharged battery, draws allot of current from the system. Because of this, the alternator gets stressed, and instead of 12V, outpouts 5-6v for a very brief time... its those 5v, that might damage your electronics... but :
      1) most electronics,nowadays, work anyway with 5v, not 12v, so no prob there.
      2) most new electronics have either their own regulating PSU's, or some kind of protections against those things...
      3)in order for the alternator to get stressed, you WIRING must be able to carry that much current... if your wiring is 4m of 4g or 8g i BEALIVE(not tottaly sure) that it will itself act as a "current limiter".... (Again, tottaly not sure).

      in my car i use just a plain 80A relay. nothing special.and so long i have no probs with it... BUT if you have access to, and afford to buy an isolator with control over such situations, then this is definitely the best option.


      • #4
        The wiring from the aux battery to the isolator (whatever style I end up going with) will be 2 gauge, wiring to the isolator is 4 or 6 gauge (cant remember what I ran right now). Regardless, I'd like the alternator to be able to push out enough current to charge the batteries without limiting it through thin wires.
        Anywho, I agree on the diode isolation being a no-no but so far the only isolation systems I can reasonably find are diode, which are inefficient, and solenoid, which are unsafe for the trucks computer.

        What setup are you using that_kid? I'd like to see where you purchased it from & a few pics if you have a minute


        • #5
          I'm using this smart controller but not their relay because I needed a bigger one. I use a pac-200 relay which I don't like at all. For some reason they had a resistor inline with the control circuit which didn't allow the relay to fully close. This caused arcing between the contacts which pitted them which even after resurfacing didn't perform well. My alternator puts out 190 amps so I wanted to make sure I had a relay that could handle the current. So now i'm searching for a replacement relay.


          • #6
            3. Q: What is the principle of operation?

            A: In the normal state the main and auxiliary batteries are separated by the isolator.

            The control box will constantly monitor the main battery voltage until it is charged to 13.6 volts and stays there or above for 15 seconds. The isolator will then connect the two batteries in parallel through the solenoid contactor to allow both batteries to be charged.
            This is what I am concerned about, with the batteries being in parallel this can cause the current draw that settra explained in better detail.
            Last thing I want to do is end up stranded with blown truck electronics.


            • #7
              Doing some more reading (i've been at this for about 3 days now) I flipped through quite a few forums and found on another forum detailing the "surge current" that I was worried about. Basically the current is really a non issue due to the voltage that batteries die at in comparision to the voltage of the system.

              Incase anyone ever digs this post up.... is a link to a nice walkthrough of how to build your own dual battery system. My concern wasn't brought up till page 28 but its a pretty good read regardless.

              I will ultimately go with the "smart" solenoids that wait for a set voltage.

              Thanks all ^_^


              • #8


                • #9
                  I went with one of these... installation is painless... it uses a high current relay so no voltage drop like a diode would have.



                  • #10
                    I took a look at the one Rickk used, I expect it is indicative of most smart isolators out there. I really like the premise of what these do and I may want to add one however one question:

                    First let's reference the attached drawing from their website, let's go with PB=primary bank, SB=secondary bank.

                    OK here's the scenario:

                    Truck is off so PB & SB are isolated.

                    Isolated accessories are used and SB is significantly run down.

                    Truck is started, alternator charging V+ exceeds 13.2v so PB & SB are connected in parallel. Won't PB instantly offer a large dump of current to SB to try and equalize the 2 banks?

                    Assuming the vehicle stays running for a period of time allowing the Alt to recharge everything, no issue, however what happens if the vehicle is shut off shortly after the 2 banks are connected together and a large current dump is taking place? Won't PB take a hit on charge state?

                    Am I missing something here or would this specific scenario compromise the charge state of your charging battery for the next restart of the vehicle?
                    Attached Files
                    My 2007 Ford F350 Work Log located HERE


                    • #11
                      Hi Phil....

                      Probably not.

                      I would be amazed if, at a normal regulator output voltage that the charging current is over 10 amps per battery. The alternator output voltage is higher than the float state voltage of either batter while the engine is running, so current will only go into the batteries... none would flow from primary to secondary.

                      As soon as the alternator stops working, the primary battery terminal voltage would drop to a "float state". At that voltage, the secondary battery wouldn't draw a heck of a lot of current.

                      Plus, a second or so later the relay opens and it is all over.

                      At least, that is my theory and I an stickin' with it.


                      • #12
                        That does sound like it makes sense. I'm extra sensitive to battery condition due to issues with a diesel and low battery power.
                        My 2007 Ford F350 Work Log located HERE


                        • #13
                          This past weekend I finally installed my auxiliary battery and have power in the area of my new console.

                          I made a few voltage readings under different conditions after it had a while to charge the auxiliary battery.

                          Both the primary and the auxiliary battery measure about 12.6 volts when the engine is off and the battery isolator is "off".

                          When the engine is running (and it is about 75-80 degrees, which is important to state), the alternator brings the battery voltage up to about 14.2 volts. I have a DC clamp-on ammeter but I forgot to drag it out and measure charging current. I assume it was somewhat low.

                          When I shut the engine off, the isolator opens pretty quickly (withing a few seconds) and the battery terminal voltage drops fairly fast to high 12's (12.6 - 12.7-ish).

                          When I start the truck, the alternator "walks" the voltage up in small steps (probably a microprocessor in it). At some point within 10 seconds or so the isolator is closed and both batteries are at around 14.2 volts (at 75-ish F).

                          The isolator is a painless install.. connect each battery to one of the two relay terminals (fuses in each direction as close to the batteries as possible I would recommend) and connect the bottom push-on terminal to chassis ground and life is good.

                          Last edited by Rickk; 06-09-2014, 09:17 PM.