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Two battery banks: I'm assuming the negatives need to be on disconnects?

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  • Two battery banks: I'm assuming the negatives need to be on disconnects?

    I may have trashed my four year of Trojan L16s. I looking into the Discovery 48v 130ah LifeP04 as a replacement. Assuming I can salvage some significant capacity from my Trojan bank I'm thinking of using it as my battery bank when the place is vacant, which is a significant amount of time. I would charge the LifePO4 to 80% and turn it off for storage. When we are there one breaker off, one breaker on, change the charge settings and I'm in business. At least that's my thinking.

    I have a spare 175 amp breaker. On the small chance that somebody had both breakers engaged at the same time wouldn't my system just see two parallel 48v banks? Do I need to put the negative cables on disconnects?

  • #2
    Perhaps a marine battery switch (rated for 48v) would be the easiest thing, and glue a block of wood over the BOTH setting
    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

    Comment


    • #3
      Originally posted by Mike90250 View Post
      Perhaps a marine battery switch (rated for 48v) would be the easiest thing, and glue a block of wood over the BOTH setting
      Thanks Mike. Thinking about this further I could place the LifePo4 battery directly under my Conext PDP and run a set of short battery cables. So both banks would have their own cables. The banks would be in parallel, right? I would never use both banks at the same time so I'm not worried about one bank draining the other or different battery types.

      Would I need to disconnect the negative of the bank not in use? The positive cables of each bank would be on separate 175 amp breakers.

      My system is not negative grounded. My current Trojan bank does have fuses at the positive and negative terminals.

      If I need a way to disconnect the negatives I could put a Blue Seas fuse on the negative terminal of the LifePO4. Only takes a few seconds to remove a fuse.

      The switches would add extra cables which would be messy.

      Anything I'm overlooking here?

      I'll start a separate thread in the off grid battery section about the Discovery AES battery.

      Comment


      • #4
        No need to do anything to the - cables

        Look up "Generator interlock kit" or "Circuit breaker interlock"

        You would need to mount your 175A breakers in panel that allows the installation of interlock gates so that only 1 breaker can be ON

        https://en.wikipedia.org/wiki/Generator_interlock_kit
        http://www.interlockkit.com/
        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

        Comment


        • #5
          Originally posted by hammick View Post
          I may have trashed my four year of Trojan L16s. I looking into the Discovery 48v 130ah LifeP04 as a replacement. Assuming I can salvage some significant capacity from my Trojan bank I'm thinking of using it as my battery bank when the place is vacant, which is a significant amount of time. I would charge the LifePO4 to 80% and turn it off for storage. When we are there one breaker off, one breaker on, change the charge settings and I'm in business. At least that's my thinking.

          I have a spare 175 amp breaker. On the small chance that somebody had both breakers engaged at the same time wouldn't my system just see two parallel 48v banks? Do I need to put the negative cables on disconnects?
          What you are describing is what some people in the DIY EV world have called a Frankenpack. It can be done but you need to be constantly monitoring voltages. Those two chemistries have different charge/discharge curves and different resting voltages. One set would discharge into the other depending on the State of Charge. In addition LFP does not like to be trickle charged and could be ruined more easily than if it were maintained at a partial state of charge. The voltages of the batteries will be equalized by the parallel connection but the SOCs will be different. I would definitely consider an AB switch on the negative terminal. If there is an A+B setting on that switch I would block it out similar to the suggestion by Mike.


          Comment


          • #6
            Originally posted by Ampster View Post
            What you are describing is what some people in the DIY EV world have called a Frankenpack. It can be done but you need to be constantly monitoring voltages. Those two chemistries have different charge/discharge curves and different resting voltages. One set would discharge into the other depending on the State of Charge. In addition LFP does not like to be trickle charged and could be ruined more easily than if it were maintained at a partial state of charge. The voltages of the batteries will be equalized by the parallel connection but the SOCs will be different. I would definitely consider an AB switch on the negative terminal. If there is an A+B setting on that switch I would block it out similar to the suggestion by Mike.

            Thanks. I would never have the Trojan L16 bank and LifePO4 powering the inverter at the same time. My only real question is whether I need to disconnect the negative terminal of the inactive bank (positive disconnected via the 175 amp breaker but negative battery cable with no disconnect). Does having the negative of an inactive bank connected cause any issues?
            Last edited by hammick; 02-10-2020, 12:57 PM.

            Comment


            • #7
              Originally posted by Mike90250 View Post
              No need to do anything to the - cables

              Look up "Generator interlock kit" or "Circuit breaker interlock"

              You would need to mount your 175A breakers in panel that allows the installation of interlock gates so that only 1 breaker can be ON

              https://en.wikipedia.org/wiki/Generator_interlock_kit
              http://www.interlockkit.com/
              Thanks Mike. I am familiar with the interlock. My Conext PDP came pre-wired with the breakers and interlock for the generator bypass. So I'm assuming I can just the get the interlock from Schneider to work with the 175 amp breakers.

              Comment


              • #8
                Originally posted by hammick View Post
                Does having the negative of an inactive bank connected cause any issues?
                None that can think of. My suggestion for an AB switch was more of a risk management issue. Having those packs connected for periods when you are in control of loads or charging could have some advantages. I would do the math first about the voltages in case the disparity is great. The risk of an arc or rapid charge of one and discharge of the other is not worth the benefit if the differences are greater than 1 volt on a nominal 48 volt system.

                Comment


                • #9
                  Originally posted by Ampster View Post
                  None that can think of. My suggestion for an AB switch was more of a risk management issue. Having those packs connected for periods when you are in control of loads or charging could have some advantages. I would do the math first about the voltages in case the disparity is great. The risk of an arc or rapid charge of one and discharge of the other is not worth the benefit if the differences are greater than 1 volt on a nominal 48 volt system.
                  Yeah the engineer at Northern Arizona Wind and Sun couldn't think of a safety issue of having two both banks connected at the same unless one bank was on a very low SOC and took a big rush of current from the other bank. I would never allow that to happen. The charging voltage of the Discover LifePO4 is only .4v higher than the float voltage of the Trojans. So no benefit to charging both banks at the same time. When the Discover is plugged into the Conext Xanbus system it is supposed to automatically configure my charge settings on the inverter, CC and AGS and even if something gets changed it sets it back It also appears as a shunt based battery monitor on my Conext equipment. Pretty cool assuming it all works well.

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