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OT - Adding a 6v capacity to a 12v battery charger

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  • OT - Adding a 6v capacity to a 12v battery charger

    I have access to a Schumacher wheeled battery charger that is 12 v
    only.

    I want to both use its boost feature (200+ amp) on 6 volt systems and to also charge 6 volt batteries.

    Any suggestions as to how to make a mod where the charger will do both
    12v and 6v?

    I have looked and have not found any schematics for the Schumacher charger.

    Thanks for any suggestions.

    And any suggestions as to where else I could post this question?

    Good electronic BBSes seem to be few and far between.

    TMT

  • #2
    What type of battery are you designing for..the battery chemistry will have a major impact on the circuit.

    Rob


    okay..so assuming lead acid... for a 6v battery you will need 7.2v (or 2.4v a cell) ...in low power chargers..the charger circuit will probably have a reference zener diode that could be lowered in value..but without a circuit diagram itd be a guess.
    If yours gives out 200amps it will be more akin to a welding set..with little more than a resistor to control the current..
    If its a SLA battery charger it will be more complex to stop the battery overcharging and gassing.

    If you have another 6v battery you can put them in series and not have to alter the charger at all.


    there are some good datasheets and info on charging lead acids here...
    www.unitrode.com/products/apps_prt.htm
    Last edited by MrSleepy; 11-03-2009, 06:33 PM.

    Comment


    • #3
      Originally posted by MrSleepy
      What type of battery are you designing for..the battery chemistry will have a major impact on the circuit.

      Rob


      okay..so assuming lead acid... for a 6v battery you will need 7.2v (or 2.4v a cell) ...in low power chargers..the charger circuit will probably have a reference zener diode that could be lowered in value..but without a circuit diagram itd be a guess.
      If yours gives out 200amps it will be more akin to a welding set..with little more than a resistor to control the current..
      If its a SLA battery charger it will be more complex to stop the battery overcharging and gassing.

      If you have another 6v battery you can put them in series and not have to alter the charger at all.


      there are some good datasheets and info on charging lead acids here...
      www.unitrode.com/products/apps_prt.htm

      Lead acid.

      The charger I am discussing is a wheeled battery charger that is used to provide starting current for a vehicle that has a dead battery.

      http://store.schumachermart.com/se-2352.html

      So how does one make a 12v 200a charger into a 6v 200a (or 100a) charger?

      TMT

      Comment


      • #4
        Than definately looks like a basic transformer/rectifier version..it will not have much electronics in it ..if any....

        If yours runs on 220v AC...it may be possible to run it off 110v to halve the rectifier output....but the tranformer impedance is designed to limit the current to the battery,so you may need an external resistor...

        Also check if there are any taps to lower the transformer output to the rectifier..

        Rob

        Comment


        • #5
          I have a much less powerful battery charger but it will charge both 6 or 12 volt batteries by selecting a lower voltage tap on the transformer. The diodes and the filter capacitor are the same at either setting. As was suggested above, look for another lower voltage tap on the transformer. If not, see if there's room for another transformer in there with the lower output voltage. Otherwise, you'll need a resistor in series with the 6 volt battery to drop the excess 6 volts. If you plan to jump start something at 200 amps, that will be a hefty resistor. As was suggested above, the simplest way is to charge 2 batteries in series, each will drop 6 volts.
          Ernie

          Comment


          • #6
            Originally posted by Too_Many_Tools

            So how does one make a 12v 200a charger into a 6v 200a (or 100a) charger?

            TMT
            not easily.

            i'm guessing a REAL 12v/6v charger has different taps on the transformer to pull off the 12v or 6v. there are electronic circuits to alter DC voltages, but i doubt they would work at 100A (or at least not cheaply enough to be worth doing).

            andy b.
            The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

            Comment


            • #7
              Originally posted by andy_b
              not easily.

              i'm guessing a REAL 12v/6v charger has different taps on the transformer to pull off the 12v or 6v. there are electronic circuits to alter DC voltages, but i doubt they would work at 100A (or at least not cheaply enough to be worth doing).

              andy b.

              they would definately use taps to switch between 12v/6v in such a crude charger..

              but it would be possible to feed the rectifier output (after removing any capacitors) to an SCR and then use a phase angle delay trigger circuit to give 120hz controlled pulses to the battery.

              1-200 amp SCRs arnt too expensive..

              Rob

              Comment


              • #8
                Originally posted by Ernie
                I have a much less powerful battery charger but it will charge both 6 or 12 volt batteries by selecting a lower voltage tap on the transformer. The diodes and the filter capacitor are the same at either setting. As was suggested above, look for another lower voltage tap on the transformer. If not, see if there's room for another transformer in there with the lower output voltage. Otherwise, you'll need a resistor in series with the 6 volt battery to drop the excess 6 volts. If you plan to jump start something at 200 amps, that will be a hefty resistor. As was suggested above, the simplest way is to charge 2 batteries in series, each will drop 6 volts.
                Ernie
                A hefty resistor would only work at one current. A diode drops a pretty constant .5 -.7 volts depending on type. for 200a you would need some beefy ones with heatsinks but a chain of 10-14 diodes would drop your voltage

                Comment


                • #9
                  voltage divider, guess at 1.5 ohm resistor, metalclad high wattage but google it to confirm
                  mark
                  Last edited by boslab; 11-04-2009, 07:01 PM.

                  Comment


                  • #10
                    Okay...I found a schematic of a similar charger sold by Sears.

                    Does this help explain the design?

                    TMT

                    http://www.searspartsdirect.com/part...5118&pop=flush

                    Comment


                    • #11
                      I've been thinking about using a HF solar battery charger down at the farm. The only problem is the 12V output would be too much for my 6V '47 Farmall Super A. A resistor sounds like it would rob too much current. It would be nice to keep it charged for my monthly visit.
                      BudB

                      Comment


                      • #12
                        Diodes?

                        I'm no electronics whiz, but how about switching a diode into the supply of the primary side of the transformer to make a half wave rectifier. It should reduce the input voltage amplitude by half, and the output should follow. For the solar Farmall charger, put several diodes in series between the PV array and the battery. You'll get a diode voltage drop accross each one. Measure the drop of one and determine how many diodes you need.
                        Davis

                        "Nothing is impossible for the man who doesn't have to do it himself"

                        Comment


                        • #13
                          Originally posted by interrupted_cut
                          I'm no electronics whiz, but how about switching a diode into the supply of the primary side of the transformer to make a half wave rectifier. It should reduce the input voltage amplitude by half, and the output should follow. .
                          transformers wont run on DC, which is what you'll have by using a diode in the primary. if it was a restive load it would work, not so for an inductive load.


                          Originally posted by interrupted_cut
                          For the solar Farmall charger, put several diodes in series between the PV array and the battery. You'll get a diode voltage drop accross each one. Measure the drop of one and determine how many diodes you need..
                          as for the diodes in series that will work as you are just dropping a dc voltage.

                          fred

                          Comment


                          • #14
                            Best I can tell from that diagram is it's a 110vac unit. That means you can't simply make the change from what might have been a 220v input to a 110v input and thereby mostly solve the problem. I say mostly because what will happen is that even though the output voltage from the transformer will drop in half, the rectifiers will still be dropping the same voltage, so what you'd have on the output is considerably less than the full 200 amps.

                            At any rate, assuming you can't turn it into a 6v charger that way, next best might be to tap the secondary winding to a point where you're getting a little more than half the output voltage than the full winding gives. There won't be that many turns on the secondary, so my plan of attack would be- first make sure there's no short on the dc output, and no battery attached, unless the charger doesn't start up without one. Measure the ac voltage across the full secondary. Now make an educated guess at what might look to be about halfway on that winding. You'll probably have to poke through insulation to get a reading, so pick a spot to do this that won't matter if it's bared a little. Your aim is to find a point on the secondary winding where you can make a connection to it, and the voltage is about 1 volt higher than half the full voltage reading. You need that bit extra voltage to ensure that at 6v output, there will be enough current available to be somewhat near the capacity of the charger.

                            Assuming you find that point and can make a solid and permanent connection to it with another suitable gauge wire, the next step is to be able to switch between that wire and the original wire to give the 6v/12v output capability. That's going to require a high current switch, or the ability to do that job fairly easily from the front of the charger. You could mount a terminal strip that you make up yourself with three terminals, and then physically move a connecting bar between them to get the switching you need.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                            Comment


                            • #15
                              If you wire the primary windings to work on 220V that will drop the output voltage.
                              Check that the cooling fan works @ the lower voltage.

                              The best way is if the secondary of the transformer has two sets of windings
                              don't fool w/ the input voltage. If there is two sets of output terminals there should be a short wire between the two inner lungs.
                              Remove that wire.
                              Then make up a set of jumper wires to connect X1 & X3 to one output wire then connect X2 & X4 to the other output wire.
                              You will have a 6V charge @ full current.
                              There are switches made to do this.
                              Install one then you will have both voltages at the flip of a switch.

                              Comment

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