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  • Need for DC-DC Charger?

    Posing this question to validate my findings and assumptions - below is a screen shot of the battery condition (206Ah SOK) 9 hours after disconnecting from shore power. About 7 of the 9 hours there was a charge coming from the 7pin wire connected to my '01 Chev PU.

    NO solar (that has been shut off until I have the time to test the new connections to the SOK,) and the 12V Furrion refrigerator/freezer stayed at about 37/6 degrees the entire trip. Outside temps were about 90 degrees, and mostly full sun during the entire drive.

    Unless my settings on the BMV 712 are all wrong (that's entirely possible - I have no idea what I'm doing!) it looks to me like a full day of driving doesn't significantly deplete the battery. Assuming this is correct, and I haven't even added in solar charging yet, I'm thinking there's no reason to worry about a DC-DC charger.

    EDITED - now I'm wondering if my BMV settings are correct. How can the battery be at 97% (in the first photo - taken 2 hours prior to the second photo) but register a higher Voltage at 13.28 than the reading at 99% SOC and 13.20 Voltage? Isn't the Voltage a measurement of the state of charge?
    Last edited by Mike & Rebecca; 08-30-2022, 02:40 PM.
    Mike and Rebecca
    2022 Reflection 150 260RD, October 2021 build date.
    2001 Chev 2500 HD 8.1 liter 4x4

  • #2
    Originally posted by Mike & Rebecca View Post
    Posing this question to validate my findings and assumptions - below is a screen shot of the battery condition (206Ah SOK) 9 hours after disconnecting from shore power. About 7 of the 9 hours there was a charge coming from the 7pin wire connected to my '01 Chev PU.

    NO solar (that has been shut off until I have the time to test the new connections to the SOK,) and the 12V Furrion refrigerator/freezer stayed at about 37/6 degrees the entire trip. Outside temps were about 90 degrees, and mostly full sun during the entire drive.

    Unless my settings on the BMV 712 are all wrong (that's entirely possible - I have no idea what I'm doing!) it looks to me like a full day of driving doesn't significantly deplete the battery. Assuming this is correct, and I haven't even added in solar charging yet, I'm thinking there's no reason to worry about a DC-DC charger.

    EDITED - now I'm wondering if my BMV settings are correct. How can the battery be at 97% (in the first photo - taken 2 hours prior to the second photo) but register a higher Voltage at 13.28 than the reading at 99% SOC and 13.20 Voltage? Isn't the Voltage a measurement of the state of charge?
    Mike,

    In your second pick you are drawing over 9 amps. Check the voltage with no load and you will see a higher voltage at the higher SOC. Also zero your BMV712 under no load when the battery is at 100% so you can see an accurate state of charge based on usage.

    Jim

    Comment


    • #3
      Guest , thanks, as always!! "Under no load" - is that accomplished when I have the aftermarket battery switch to off? I had that switch installed so that it removes all load from the battery. Or do I have to actually remove a cable from the battery to ensure no load?

      Regards zeroing after reaching 100% SOC - if I plug the trailer into the exterior house outlet on the front porch, does the BMV 712 know when it's 100%?

      Can you tell I'm confused? Does the BMV measure actual state of charge? Or is it only capable of measuring inflow and outflow and then "guessing" on the SOC based upon those measurements through the shunt? I guess I'm wondering how the BMV 712 knows the battery is at 100% SOC?

      Regardless - the refrigerator was still cold after a 9-hour drive! So, something is working.
      Mike and Rebecca
      2022 Reflection 150 260RD, October 2021 build date.
      2001 Chev 2500 HD 8.1 liter 4x4

      Comment


      • #4
        Hi Mike, I wouldn't put much faith in the SOC. I watch the current, consumed AH, and cumulative AH drawn. These give you a much better understanding of what is happening with the battery. You have about 206 AH in your battery when fully charged. Watch how many AH you draw down from that point.
        Dave
        2016 Reflection 27RL
        2015 Silverado 2500HD 6L
        B&W Patriot 18k slider

        Comment


        • #5
          Originally posted by Mike & Rebecca View Post
          Guest , thanks, as always!! "Under no load" - is that accomplished when I have the aftermarket battery switch to off? I had that switch installed so that it removes all load from the battery. Or do I have to actually remove a cable from the battery to ensure no load?

          Regards zeroing after reaching 100% SOC - if I plug the trailer into the exterior house outlet on the front porch, does the BMV 712 know when it's 100%?

          Can you tell I'm confused? Does the BMV measure actual state of charge? Or is it only capable of measuring inflow and outflow and then "guessing" on the SOC based upon those measurements through the shunt? I guess I'm wondering how the BMV 712 knows the battery is at 100% SOC?

          Regardless - the refrigerator was still cold after a 9-hour drive! So, something is working.
          Mike,

          I'm not near my rig or I would provide you with detailed instructions in how to do this. Maybe howson Howard is available?

          Anyway, yes turn your battery switch off after fully charging the battery and you will see a couple settings. One is to set zero load and the other is to set 100% SOC. Make sure no draw is being shown on the screen of the BMV.

          Your basically programming the BMV to know what 100% SOC is at zero load and fully charged, then based on usage and having the size of the battery programmed in, the unit will track your SOC.

          Jim
          Last edited by Guest; 08-31-2022, 08:12 PM.

          Comment


          • #6
            Thanks Dave27 and Guest ! I'll need to take your suggestions with me to the storage yard over the weekend. I was at my 48 hours limit the HOA allows for the trailer to be parked in front of the house and I had to take it to storage this afternoon. Jim, I did see the Zero set menu and I got a big warning pop up on the app telling me I needed to remove the cable from the shunt before I proceeded. That was beyond what I was in the mood for this afternoon, so I'll have to circle back on that. Dave, I understand your suggestion and I'll pay more attention to the Ah drawn. I can tell you that I don't recall much in the way of total drawn, even after the 9-hour drive home.
            Mike and Rebecca
            2022 Reflection 150 260RD, October 2021 build date.
            2001 Chev 2500 HD 8.1 liter 4x4

            Comment


            • #7
              Mike,

              I've not seen the warning to remove the cable from the shunt where I can only guess that the BMV was still seeing an electrical draw from the battery. If you kill off any circuits (never kill the BMV power feed) to have no current draw, you will be good to set the zero load. Once this is done and 100% SOC is set, in my experience, the BMV will track well.

              The BMV will not be able to track the natural discharge on the battery and is where some may be disappointed. To overcome this, fully charge the battery before each outing and set the 100% SOC and zero load and you will be good to go. There are several videos available in setting up the BMV as well. Here is one from Victron and AM Solar.

              https://www.youtube.com/watch?v=mEN15Z_S4kE

              https://www.youtube.com/watch?v=KIVeffHqXu4

              Jim
              Last edited by Guest; 09-01-2022, 08:55 AM.

              Comment


              • #8
                Originally posted by Mike & Rebecca View Post
                , I'm thinking there's no reason to worry about a DC-DC charger.
                I'll just answer this. I don't see the need for DC to DC just for travel reasons. I have 400 AH now with no solar (had 200 on our last unit) and a residential fridge. I have pulled as long as 15 hours and never have come close to using the battery capacity. On the high end I'll see a net drain of 3-4 amps per hour on a hot day and loading the fridge right before leaving. On the low end I see net zero lost if I'm pulling in cool weather with a fully cooled of fridge when leaving. Unless the 12V fridge uses more juice than a residential through an inverter you should be fine.

                Jim & Heidi
                2021 Solitude 375RES-r
                2024 GMC Denali Ultimate DRW

                Comment


                • #9
                  Originally posted by Mike & Rebecca View Post
                  Thanks Dave27 and Guest ! I'll need to take your suggestions with me to the storage yard over the weekend. I was at my 48 hours limit the HOA allows for the trailer to be parked in front of the house and I had to take it to storage this afternoon. Jim, I did see the Zero set menu and I got a big warning pop up on the app telling me I needed to remove the cable from the shunt before I proceeded. That was beyond what I was in the mood for this afternoon, so I'll have to circle back on that. Dave, I understand your suggestion and I'll pay more attention to the Ah drawn. I can tell you that I don't recall much in the way of total drawn, even after the 9-hour drive home.
                  The "consumed Ah" will go up or down depending on whether you are net charging or discharging. It will not go into positive numbers if you have charged more than discharged since the zero was set. I watch this number to see what my net change has been since the last full charge. The "cumulative Ah drawn" is like a ratchet, it only goes one way. It only tracks draws, and does not go up for charge. These points are based on the Victron smart shunt, since your interface looks identical, I assume it works the same way.
                  Good luck, Dave
                  2016 Reflection 27RL
                  2015 Silverado 2500HD 6L
                  B&W Patriot 18k slider

                  Comment


                  • #10
                    Mike & Rebecca you've got some good advice here but to answer your earlier question, it can't measure SOC, it gets that by knowing your total capacity and tracking amps in and out of the shunt.
                    Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

                    Neil Citro
                    2018 Reflection 28BH Pepwave
                    2019 F350 6.7L Long Bed Crew Cab

                    Comment


                    • #11
                      Neil,

                      Your correct. Even in auto world we calculate SOC and SOH but it can only be done with an open voltage check (all loads off) after a certain resting period. Many autos today have an IBS (intelligent battery sensor) at the battery on the negative post to also monitor battery condition and is similar to the BMV but far more complex.
                      The BMV can only estimate SOC with the rig under use but its accurate if set up correctly. Its important that the battery is fully charged before a trip and the no load and 100% SOC is set.

                      Once the trip is completed and the battery is drawn down to around 50% SOC for storage and the battery switch has been wired to shut off all loads, the BMV will accurately track SOC for storage.

                      Jim

                      Comment


                      • #12
                        Originally posted by Mike & Rebecca View Post
                        Posing this question to validate my findings and assumptions - below is a screen shot of the battery condition (206Ah SOK) 9 hours after disconnecting from shore power. About 7 of the 9 hours there was a charge coming from the 7pin wire connected to my '01 Chev PU.

                        NO solar (that has been shut off until I have the time to test the new connections to the SOK,) and the 12V Furrion refrigerator/freezer stayed at about 37/6 degrees the entire trip. Outside temps were about 90 degrees, and mostly full sun during the entire drive.

                        Unless my settings on the BMV 712 are all wrong (that's entirely possible - I have no idea what I'm doing!) it looks to me like a full day of driving doesn't significantly deplete the battery. Assuming this is correct, and I haven't even added in solar charging yet, I'm thinking there's no reason to worry about a DC-DC charger.

                        EDITED - now I'm wondering if my BMV settings are correct. How can the battery be at 97% (in the first photo - taken 2 hours prior to the second photo) but register a higher Voltage at 13.28 than the reading at 99% SOC and 13.20 Voltage? Isn't the Voltage a measurement of the state of charge?
                        Mike,
                        fwiw……. I went through all the trouble of running 4 gauge wire to the rear of my truck and put in a DC to DC charger. In the final analysis, I don’t think it was necessary. I only travel less than 6 hours between campsites and even with a residential frig and 1 200 amp lithium battery the SOC doesn’t go down much and the voltage is still up around 13 volts.

                        Comment


                        • #13
                          For the DCDC being necessary there are a couple thoughts. One is to isolate a lead acid battery and charge strategy from the TV to the lithium battery which operates at different voltages. The second is one of the chargers in the rig (Converter, Solar or DCDC) must be capable of fully charging the lithium battery so it will properly balance. Lithium batteries for our purposes will top balance when the battery is approaching 100% SOC. Since standard solar is not reliable to fully charge prior to each trip in order to set no load and 100% SOC in the BMV, this leaves the converter and DCDC. Both of these would be ideal since electrical loads on rigs are increasing all the time for travel as well as camping.

                          Having stated that. will towing without a DCDC harm the lithium battery? Unlikely, but ideally lithium should not be discharged below 20% SOC to maximize battery life.

                          Jim

                          Comment


                          • #14
                            Guest A third thought, is more charge from the vehicle into the RV battery. We find this makes a huge difference in our ability to dry camp indefinitely for drive days, vs either needing enough sun while driving or having to pay for a campground every few days while traveling, where a campground would otherwise not be needed.
                            Forum moderators are not GD employees--we are volunteers and owners presumably just like yourself. Unless specifically mentioned otherwise, we have nothing to gain should you choose to purchase a product or engage a service we discuss on this forum.

                            Neil Citro
                            2018 Reflection 28BH Pepwave
                            2019 F350 6.7L Long Bed Crew Cab

                            Comment


                            • #15
                              Originally posted by plastictires View Post

                              Mike,
                              fwiw……. I went through all the trouble of running 4 gauge wire to the rear of my truck
                              That's a lot of copper to charge with. Personally, I can't see why anyone would go over #10. Thats covers 30a of charging current from the TV. If anyone needs to charge at over 30a, maybe look at what you are expecting your 12v systems to do. Just my opinion. I'm happy enough with the 7a my stock charge setup delivers. I could see in the future stepping up to a 20a dc/dc...maybe... and I'll run #12 for it...but #4? that's good for 80amps.

                              2018 Dodge 3500 6.7 Cummins w Aisin and 9 cup holders
                              Not to brag or anything about my finances, but my bank calls me about every day to tell me my balance is OUTSTANDING!

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