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battery is at 12.5 volt when "charged"

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  • battery is at 12.5 volt when "charged"

    hey guys. i really need help here. in my car i have two batteries ( battery 1 and battery 2. ) recently, i upgraded an installed a battery isolator. now i have the following problem:
    when my car is not running. the battery1 is at 11.9 volt, BUT the battery2 is at 7.8 (pretty dead).
    when the car is running, the alternator outputs 13.8v (as normal), BUT cause of the battery isolator, the other two battaries read 12.2 volt each... so, i guess they do not charge at all?? what can i do to solve this?? better alternator maybe??

    p.s don't bother answering " you should change alternator, go buy one that costs 1.000.000 euro so it has no voltage drop" or anything like that, cause i really dont think it as "help" ...

  • #2
    Sounds like your batteries are funky or on the way out.
    link to my CTS-V project:


    • #3
      the battery 2 (that with the car off is at 7) yes. but the other one is good. besides, i did not have the slightest problem before the isolator


      • #4
        Isolators have a diode or two and that will result in a 0.6 volt decrease (minimum) available to charge the battery.

        A typical lead acid requires 14.2 minimum to charge correctly and fully charged, open circuit should read 13.6 volts, 2.2 volts per cell.

        The regulator in the alternator might be funky also.

        I won't say alternator as you don't want to hear that.
        link to my CTS-V project:


        • #5
          my car is prety old (1992) so i guess it has a "normal" altarnator, without any adaptive regulation. is it possible to install a "clever" or whatever its called regulator, to increase the altarnators output?


          • #6
            Originally posted by settra View Post
            my car is prety old (1992) so i guess it has a "normal" altarnator, without any adaptive regulation. is it possible to install a "clever" or whatever its called regulator, to increase the altarnators output?
            Here is one way:


            You might look around for a rebuilt alternator or have the regulator in yours replaced (the diodes usually get replaced also).

            IMO replace the diodes first, that is rebuild the alternator.
            link to my CTS-V project:



            • #7
              The 7v battery is bringing the other down, disconnect the bad battery and toss it, it's only doing damage to the other parts of your system. here is how you should have it wired. (excuse the stick drawing) SNO
              Click image for larger version

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              • #8
                but i have only 2 batteries? also, i would agree that the bad battery sucks all the ampere from the alternator, but i dont think it is getting charged at all (cause as soon as i turn off my car, my inverter starts beeping (meaning it has no power on it). ) and even after long trip, the 2nd battery is dead after i turn off..


                • #9
                  Ok so you only have 2 batteries and 1 is bad you already have your answer the isolator is isolating your main battery from the bad battery when car is off your bad battery will not come back to life it is done like dinner and if you keep driving with that bad battery you will have to replace your alternator sooner than later. SNO


                  • #10
                    A "bad battery" usually will not charge as that is one of the faults they can show, besides being a drain.

                    The bad one is just loading the system to the point neither battery is getting charged and one never will.

                    You may have already damaged the alternator.
                    link to my CTS-V project:



                    • #11
                      so... the battery2 , not being able to hold charge, and drooping at 7 volt, is a sign that is completely dead? seems prety legit since battery2 is very old. BUT isint the fact that they charge at 12.2 volt, also a problem? i still dont know how to solve it..
             seems pretty solid , but i would prefer not to mod it..


                      • #12
                        Read this:


                        IMO you have one battery that is definitely bad.

                        The other battery may not be far behind.

                        The alternator may have been stressed.
                        link to my CTS-V project:



                        • #13
                          Your 7.8V battery is definitely bad. Get rid of it.
                          Your 11.9V battery is almost certainly also bad. It's definitely stuffed if it's an AGM.

                          All alternators are regulating. The only difference with vintage is that they progressed from external electro-mechanical to electronic to internal (integral) electronic voltage regulators, and they increased alternator outputs from 13.8V to a max of 14.4V - usually 14.2V.

                          If you have a diode type isolator, I suggest you get rid of it and get a relay type.
                          If you have a single wire alternator (with only a D+ or L wire (excluding the heavy +12V output wire and the alternator's GND) AND a diode isolator, either change to a relay type isolator or change to a 2-wire alternator - ie, with an S = Sense wire that connects straight to the battery +12V terminal. Having both means accelerated destruction of both batteries.

                          From a battery POV, the preferred alternator in all situations is one with an internal regulator and a Sense wire. That way it doesn't matter how big the voltage drop is from the alternator to the battery, the regulation will aim for the correct voltage. (This has qualifications and caveats, but assuming good cabling and grounding between the alternator and battery and grounds and loads, it's the best set up. And if cabling has problems, fix that before trying to fix anything else or before electronic loads blow.)

                          If you have a Sense type alternator, then diode type isolators might be acceptable from the sensed (main) battery's POV but not necessarily from the 2nd battery's POV, but diode isolators simply are not used anymore for a plethora of reasons which I will not repeat (again).
                          If your system has the sense with diode isolator, then as stated before (by heavymetals), the bad battery may be taking all the alternator's charging output. If the better battery is near new and hasn't been undercharged for long (ie, a few days), it may come good again. If longer or if older it'll be on its way out.

                          If you have a relay type isolator, then removing the bad battery should mean the normal 13.8V charging voltage. But if only 13.8V, do a battery maintenance - ie, charge separately with heavy current at 14.4V or higher and preferably give it an equalisation cycle (if not AGM). But otherwise, a normal 14.2V charge voltage might recover the battery somewhat.

                          A relay type isolator can cost as little as a relay. And if you have an older external electo-mech regulator, driving that relay should present no problems (see my posts re the "UIBI").
                          I'd assume you don't have a voltage sensing isolator as all that I know would have dropped out with battery voltages below 12.5V. Voltage sensing isolators are inferior to charge-light controlled relay isolators. That includes so-called smart and intelligent battery isolators. (Both smart and intelligent are misnomers, hence the "UIBI" - the Ultimate Intelligence Battery Isolator. And claims of priority charging etc are misleading if not outright fraudulent.)

                          Your alternator should not have suffered any "strain". They should be self limiting etc, but I know of bad designs (like older Bosch and apparently some newer GMs) that cannot handle over-current - they blow their main diodes. And rewound alternators can fail under stress for a variety of reasons (usually inadequate thermal design).

                          If the alternator's brushes are worn, the alternator's output will progressively drop with time or increased load.
                          Other alternator faults (shorted or bad turns, blown diodes, etc) can be determined by a knowledgeable person or sometimes a cheap "alternator" tester or clever use of a DMM.
                          Voltage regulation can be checked with a voltmeter at low & high RPM with low & high loads.
                          Electro-mech regulated alternators can usually have their voltage adjusted.
                          For Sense-type electronic alternators you can insert diodes in the Sense line to boost the output by a voltage equivalent to that diode drop (eg, 0.3V, 0.6V, 1.2V, 1.5V, etc) to raise alternator output from (say) 13.8 to 14.2-14.4V.
                          A large voltage dip at the alternator's output with increasing load means it's reached its output limit at that RPM.
                          Last edited by OldSpark; 06-21-2013, 03:05 AM. Reason: minor edits (better reading)(??)


                          • #14
                            my original point was not really if the battaries are dead or what. IF tommorow i go and buy 2 new batteries. and i install them. at the end of the day, will the fact that they are being charged at 12.2 will be a problem? and if yes, how to solve it? any way i could modify the alternator to be "Sense" type ?


                            • #15
                              Chances are if you put 2 good batteries, your voltage should go back to normal, if you haven't damaged your alternator already. You already said if you disconnect the bad battery you are charging the good battery at 13.8volts, your 12.2 volts is when the bad battery is connected putting a drain on the good battery and the alternator. SNO
                              Last edited by SNOtwistR; 06-21-2013, 08:00 AM.