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could you use a solar charge controller for as a dc to dc charger

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  • could you use a solar charge controller for as a dc to dc charger

    could you use a 30 amp solar charge controller with its input wired to truck's battery (6 gauge wire) and output to the trailer battery to charge the trailer battery. Obviously this would only work when the truck engine is running allowing the alternator to produce enough amperage. its just 12v to 12v, sure the alternator might be charging at 13.8v-15v, but isn't that what a solar charge controller does, take higher voltage and put out 14.4 for bulk/absorption charging of a battery. I already have a solar charge controller, this saves me from having to buy a dc-dc charger, don't I already have one in the solar charge controller?

  • #2
    Google "Battery Isolator" and no. Your house battery would never fully charge and only expensive MPPT controllers will limit the current were a cheap PWM will overload and let out the magic smoke.

    WWW
    Last edited by Wy_White_Wolf; 03-07-2018, 01:15 PM.

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    • #3
      well i have a Victron MPPT controller so that's not my issue, but after looking at the manual closer the controller doesn't kick on unless it see's an input voltage that is 5v above the battery V. So that kills that idea. The battery isolator just keeps the house battery from draining the truck battery should you forget to disconnect after the engines been shut off, it won't regulate or mange the charging of the house battery like a stage charger does.

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      • #4
        An isolator does allow charging of the house battery and the charge is regulated by the alternator already. So you don't need anymore than that.

        WWW

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        • #5
          the truck regulator is not going to give a 3 stage charge process to the house battery its just going to keep pumping out a high voltage/high amperage charge until batteries return to the regulators cut out voltage. Ok for a starter battery thats barely drawn down and back to full charge in minutes, but not what you want for your deep cycle house batteries.

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          • #6
            Originally posted by dorff View Post
            could you use a 30 amp solar charge controller with its input wired to truck's battery (6 gauge wire) and output to the trailer battery to charge the trailer battery.
            Nope. As others have said, a simple battery isolator works a lot better.
            I already have a solar charge controller, this saves me from having to buy a dc-dc charger, don't I already have one in the solar charge controller?
            No, the charge controller can only reduce voltage. (And if it's PWM it can't even do that - it can just open if the voltage is too high.)

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            • #7
              Originally posted by dorff View Post
              the truck regulator is not going to give a 3 stage charge process to the house battery its just going to keep pumping out a high voltage/high amperage charge until batteries return to the regulators cut out voltage.
              Who cares? Certainly not your battery. That is exactly what you want. 3-Stage is just a gimmick you have fallen for. What you just described is 2-Stage aka Constant Current > Constant Voltage the first 2 stages of a 3 stage charger. You do not need the 3rd Float stage in a vehicle.

              For the last time, get an Electronic Battery Isolator. There is no better solution and the good thing is the least expensive option. Your charge controller will not work unless it has greater than 18 volt input which you do not have.
              Last edited by Sunking; 03-11-2018, 12:07 PM.
              MSEE, PE

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              • #8
                ah......... to be Sunking smeared...... welcome back

                My F350 truck's alternator puts out 220 amps. The two-6v L16 in series house batteries in the 5th wheel trailer @ 440 AH. I thought that FLA batteries should be charged at no more than C/8 or 55 amps in my case. Your suggesting that my 50% SOC batteries will be OK getting the full output of the truck's alternator?

                I thought to properly charge deep cycle FLA the last 20% that you need a 2-stage charger that tapers back the amperage.

                Without the taper in amperage won't the battery dewater excessively if this process is repeated on a daily basis?

                A battery isolator won't do that will it.

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                • #9
                  Since the normal RV regime is Drive 10 hrs & Camp 48, it's not likely you would overcharge the L16's while the group 27 battery under the hood is fine.
                  Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                  || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                  || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                  solar: http://tinyurl.com/LMR-Solar
                  gen: http://tinyurl.com/LMR-Lister

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                  • #10
                    Originally posted by dorff View Post
                    My F350 truck's alternator puts out 220 amps. The two-6v L16 in series house batteries in the 5th wheel trailer @ 440 AH. I thought that FLA batteries should be charged at no more than C/8 or 55 amps in my case. Your suggesting that my 50% SOC batteries will be OK getting the full output of the truck's alternator?
                    You won't get that full 220A to the battery; other loads, cable resistance and battery ESR will see to that.
                    I thought to properly charge deep cycle FLA the last 20% that you need a 2-stage charger that tapers back the amperage.
                    You do.
                    Without the taper in amperage won't the battery dewater excessively if this process is repeated on a daily basis?
                    A standard alternator is a 2-stage charger.

                    Stage 1 - regulator "wide open" charging at maximum rate. (Only happens in most vehicles when battery is very low.)
                    Stage 2 - setpoint voltage reached (13.8 or so) and regulator reduces current output to maintain 13.8 volts.

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                    • #11
                      Originally posted by dorff View Post
                      ah......... to be Sunking smeared...... welcome back
                      Thanks, I appreciate it.

                      Originally posted by dorff View Post
                      Without the taper in amperage won't the battery dewater excessively if this process is repeated on a daily basis?
                      Nope, you just do not understand how battery chargers work. An Alternator is a CC/CV aka 2-stage charger set to 14.4 volts, exactly what you would set Bulk/Absorb in a charger. When the battery voltage is low, the alternator puts out Constant Current because it cannot supply the full 14.4 volts. After thee battery charges up, the voltage rises and reaches 14.4 volts and holds Constant Voltage at which point current tapers off.

                      Read this please for more info. It should help.

                      Originally posted by dorff View Post
                      A battery isolator won't do that will it.
                      Nope the alternator and wiring do that. All the Isolator does is isolates the Starting and House battery. So when you have sat for three days and drained your house batteries until the lights go out, your starter battery is still fully charged so you can start the engine and recharges the house batteries while you idle the engine or drive home.

                      There are some higher end Isolators that offer a selection of battery types and voltages. But the main purpose is to Isolate the House and Starter batteries from each other. Some even offer a Bridge in the event your starter battery dies so you can start the engine with the House batteries in the event of an emergency.

                      Not sure who's L16's you have, but if Trojan I would not worry about to much charge current. The wiring is going to limit current to safe levels coupled with battery Internal resistance. So relax and thanks again.
                      Last edited by Sunking; 03-12-2018, 11:41 PM.
                      MSEE, PE

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                      • #12
                        after looking into battery isolators, it appears that due to voltage drop they are generally considered old tech, and ACR's are now the favorite tech(very little voltage drop) at least in the marine world which seems to be generally better informed about this issue than the RV world. Available in all sorts of amperage levels, so if I limit myself to 4awg wiring due to costs, then with a one way run length of 25 feet from truck battery to trailer bank & with a voltage drop limit of 2% to maximize full charge of trailer batteries, various wire size calculators say this would mean 10amps max. Not going to accomplish much with a 440AH bank at 50% SOC and a daily drive time of 4-6 hours. So why mess with it. Am I generally correct with my math or what?

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