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2nd car battery

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  • 2nd car battery

    Hi I am running 2 car batteries in my car. I have this problem ever since I installed a Redarc Battery Isolator SBI12. This isolator is supposed to first charge my starting battery while isolating my 2nd car battery. Then once the starting battery gets its charge, it will isolate the starting battery and charge the 2nd car battery.

    The problem is like this - once I started driving, the isolator will switch over to charge the 2nd battery. So I presume the starting car battery is fully charged. But when I stopped at a traffic stop, the isolator will switch over instead to charge the starting car battery. Once that happen, within 5 secs, my car pc losses power. This is because my car pc is wired to my 2nd car battery. It is weird that the 2nd car battery cannot hold the charge for more than 5 secs before losing power.

    I have listed a couple of things that could be at fault here :

    1) The Redarc Battery Isolator SBI12. It is brand new so it is likely the fault. I dont have this problem with my 2nd car battery before I install this device.

    2) The 2nd car battery. I have been using this battery for like could be a year. But the display indicator still shows green - indicating that the battery is still working fine.

    3) Wiring connecting the Redarc Battery Isolator SBI12 to my starting car battery and 2 car battery

    4) My car pc is draining too much power from the 2nd car battery and because the distance I am driving is short to fully charge the 2nd car battery

    The above list is just what I think are the likely problems. Anyone who is experiencing or experienced the above before, please PM me how to solve this problem.

    Thankx you so much!

  • #2
    it might be that your pc is drawing to much power and the battery is not charged enough. but what i suggest, if you can is get rid of the isolator. they drain power and charge seperatly your bateries, and you will always have that problem. instead get a relay. a heavy duty realy from an auto electric store. hook that up and it will charge both bateries at the same time.
    all you do is hook a line from the starter battery to the relay on one side, then hook the other to the second battery. hook a ground to the relay and then a remote line from your stereo or such, so the relay kicks on when your car turns on thus letting a charge go to the second battery aswell keeping them both nice and full. thats how i set mine system up and im useing 3 optima yellows and only need the one relay to charge them all, havet had a problem for over a year now.

    you can buy one online. its a 200A relay or more if you can, oznium has this one but its kinda spendy. i got mine for 25 buck at an auto electric place and i sugest u get one like that too.

    This 200 amp relay can be used as a battery isolator. When you use more than 1 battery in your car, you want to isolate them when the engine is not running. This prevents excess wear on the batteries because they will not be able to drain themselves.
    " from there site

    and i put pictures to show u mines so u can see that it works for more than 2 batteries and it works great and its cheap

    and i ran only a 4 guage from the front to the back. so its easy cheap and efficent.
    try that. it shuld solve your problem.
    My name is Ryan and im a CarPC....

    Check out my post 94 Cadillac Seville

    If your in Oregon hit me up. lets get a meet going. or let me know of any.


    • #3
      Hi there, I thought that the isolator IS a relay that separates and charge the batteries?

      Do enlighten me on the differences between the isolator and the relay?

      Also, is it possible to confirm that it is the car pc draining too much power and that the 2nd car battery is not fully charged?


      • #4
        well the difference between an isolator and a relay is in the words. isolator does what is says isolates. it charges your batteries but seperatly and keeps them seperate.
        i·so·late (s-lt)
        tr.v. i·so·lat·ed, i·so·lat·ing, i·so·lates
        1. To set apart or cut off from others.
        2. To place in quarantine.
        3. Chemistry To separate (a substance) in pure form from a combined mixture.
        4. To render free of external influence; insulate.
        5. Microbiology To separate (a pure strain) from a mixed bacterial or fungal culture.
        6. Psychology To separate (experiences or memories) from the emotions relating to them.
        7. Electricity
        a. To set apart (a component, circuit, or system) from a source of electricity.
        b. To insulate or shield.

        this means that your batteries never connect so if your pc is running off one and your car the other power is being drained from both batteries at the same time, but only being charged to one. or atleast enough power is being charged to one.
        the isolator also has a drop in power from capacity because it is a giant heat sink so it gets hot and stores energy and blah blah blah

        Diode Based Isolator:
        This type of isolator is basically two very high current diodes in a heat sink. The diode based isolator has the advantage of long life and trouble free operation but many of the diode based isolators cause a .4 to .6 volt loss which means that the batteries will not charge to as high a voltage as when they are connected directly to the alternator. The diagram below shows the connection for the diode based isolator. Some diode type isolators have a fourth terminal which controls the current flow through the device.

        the relay is nothing but a giant electric switch. all it does is turn on when power is supplied and lets the current flow from the front to the back. no drop and no pull. it connects both batteries simultaniusly to your charging system so that they get charged evenly and the same power at the same time. now when power is being drawn from your pc, its almost like running off the alternator. and then as thats happening the batteries are staying fully charged so when you turn the car off you still have power to run things for a while.

        now on how to confirm its the pc and to tell if the batteries are low, hook up a voltage meeter when nothing is on and see what it reads at. if it says 12v then the battery is probably charged ok. it could have low amps but if its reading 12v it shuld be able to run the pc just fine. then turn everything on and hook up the meter again and see what it reads. on both batteries. if the front says 12-14 its being charged and running great, if the back says any less than that then you know the pc is sucking power and your not getting it charged back up while its doing that.

        hopefully that helps and solves your problem. let me know if you need anything else. ill try my best to help you out.
        My name is Ryan and im a CarPC....

        Check out my post 94 Cadillac Seville

        If your in Oregon hit me up. lets get a meet going. or let me know of any.


        • #5
          in lamens terms, an isolator is just a huge diode that allows electricty to flow only one way. that allows your alternator to charge the batteries, but your 2nd battery wont pull a drain back off the 1st. the downside to isolators is the loss of voltage (1v+ lost)

          a relay is a mechanical device that physically disconnects your 2nd battery when not in use. they are preferred by audio installers since they allow full voltage to reach amps. for carpc's, the input voltage doesnt matter much so the isolator is a cheap and reliable way to go. op's isolator sounds weird though, they do make ones that charge both batteries at the same time.


          • #6
            xmatrix the readarc you have is actually a solenoid . Quote "Basic Operation The Smart Start is a microprocessor controlled Smart Battery Isolator. The Smart Start is designed
            specifically for use in multi battery applications as a solenoid priority system to protect the start battery from being excessively discharged by auxiliary loads, whilst still allowing the auxiliary battery to supply non essential loads. Put simply, once the start battery is charged by the alternator, the Smart Start will connect an auxiliary battery to the charge circuit. Similarly, if the start battery voltage drops too low, the Smart Start will disconnect any auxiliary batteries or loads to conserve charge in the start battery."
            I have used the same one you have for years with a 20 a/hr deep cycle battery to power my computer. i put my computer in standby so is ready to go in seconds.Take a look on the redarc website on wiring info,these units are very widely used among 4wd circles,at least here in Aus.
            The reason it is called an isolator is because it isolates the second battery from the start battery when you are drawing current from the second battery thus maintaining charge in start battery, to start vehicle


            • #7
              see what millsy said is why for your purpose you dont want to use an isolator because it will kick off when the first battery needs charging leaving your second battery to die because that is not its main concer. the relay allows both to charge at the same time and to full capacity so you never worry about either battery dying on you.
              My name is Ryan and im a CarPC....

              Check out my post 94 Cadillac Seville

              If your in Oregon hit me up. lets get a meet going. or let me know of any.


              • #8
                Originally posted by migitcheetah View Post
                see what millsy said is why for your purpose you dont want to use an isolator because it will kick off when the first battery needs charging leaving your second battery to die because that is not its main concer. the relay allows both to charge at the same time and to full capacity so you never worry about either battery dying on you.
                most isolators allow current from the alternator to charge both batteries at the same time. they are also easier on a charging system compared to relays that snap on and off with high spikes. they are also inexpensive for good ones. it sounds like op's isolator is a fancy one that is designed for a limited charging input current. most car alternators can charge two batteries at once without issue.


                • #9
                  caravan towing relays are the way forward
                  2001 Passat 130TDi Sport - Via Epia 15000, 1gb, 100gb, 220w psu, 7" lilliput motorized touch (chopped and in dash as a fixed), bluetooth, bu-303 gps, 7 port usb hub, tiny keyboard, netgear 54mb wifi, optima yellow top, ryder rt1170x, freecom dvb


                  • #10
                    A new product from Painless Performance.


                    It is both a battery isolator and it will only let you drain down one battery, but if you have more charge in the backup battery, it will automatically connect that to the starting battery.

                    So like an isolator diode, and like a battery relay...don't know how well it works, but seems like a good idea.

                    ...but is almost $200.
                    current projects


                    • #11
                      if anyone is interested in knowing more about comparing complete dual battery management system, i just did lots of research into this topic:

                      and the whole thread where that post is has valuable info:


                      • #12


                        • #13
                          that does look awesome, bratnetwork. $100 here-

                          and they have a 200a version for $50 more. this looks like a VERY solid way to go.


                          • #14
                            I've just skimmed, but AFAIAConcerned, those "current sharers" etc are a croc of crap and are unecessary.
                            I have discussed this ad (my) nauseum on other sites. Nobody has yet explained HOW the systems determine full charge etc. (When somebody does finally answer that, I will ask "why?" and under what circumstances such an approach is useful. LOL!!)

                            Your system is best replace by a simple relay which is energised when the system (alternator) is charging.
                            This can be done via the charge-lamp circuit from alternators (L or D+ terminal), or an over-voltage sensor where that is not practical (eg, marine stator systems).

                            There are then ways that the system can be made to manually disconnect, connect, or "continue" after charging ceases. (Most involve merely diodes an switches or push-buttons added to the relay - very simple stuff. The only complexity is if an undervoltage cutout is desired to preserve the 2nd battery.)

                            I might have posted a sample circuit on this site, else maybe

                            The basic circuit is the additional relay as per:

                            For more info, see - adding a second battery-page 5 which also refers to ( - trailor alarm wiring) Split charge systems etc (~7th post).

                            There is also Smart Battery Isolator on this site.


                            • #15
                              ive tried the relay thing, and it didnt work months ago, unless I did it totally wrong. The blue wire on the psu is not 12v it goes to ground when the carputer powers up. I need the amps to delay