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How to wire shunt with chassis ground?

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  • #16
    Cool! Let's get some HE transducers and a raspberry pi and make a battery monitor

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    • #17
      Way overkill as it does not take that much processing power. Nothing more than a DMM is required. Hall Effect Transducer output is milli-volt which you read directly as Amps.
      MSEE, PE

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      • #18
        Hmm. How are you going to win internet points with all of this KISS philosophy? Lol, thanks again, sunking.

        I've got some questions brewing for the possibility of wiring a 24V system into a vehicle, which I think are just questions about how to design a fully floating system, but I have to work out my guesses first.

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        • #19
          Originally posted by Donkey_in_training View Post
          I've got some questions brewing for the possibility of wiring a 24V system into a vehicle, which I think are just questions about how to design a fully floating system, but I have to work out my guesses first.
          Your question is answered in post 4. Note fuses are on both battery polarities.

          Warning the equipment can and will prevent you from making a Floating System because all it takes is one piece of equipment like a Charge Controller or Inverter with chassis bonded to battery negative to force you to use a Grounded System.
          Last edited by Sunking; 04-06-2018, 06:03 PM.
          MSEE, PE

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          • #20
            Originally posted by Sunking View Post
            Warning the equipment can and will prevent you from making a Floating System because all it takes is one piece of equipment like a Charge Controller or Inverter with chassis bonded to battery negative to force you to use a Grounded System.
            Is this something one can make like a second bus bar or a large piece of sheet metal attached to a piece of wood? Or does that defeat the purpose... Because it seems like having a 24v chassis bind might interfere with the OEM 12v system.

            Just to pick your brain; for a floating system, what is the failure scenario for only fusing the (+) and not the (-)? What happens if you don't fuse both runs? Thanks!

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            • #21
              Originally posted by Donkey_in_training View Post

              Is this something one can make like a second bus bar or a large piece of sheet metal attached to a piece of wood? Or does that defeat the purpose... Because it seems like having a 24v chassis bind might interfere with the OEM 12v system.
              All I am going to tell you is you are on to something. Has nothing to do with the vehicle 12 volt system. Reason I am not going to answer is safety. But there is a possible problem. Let's say you have a gizmo like a light, TV, satellite receiver, or something working directly off DC power in contact with the vehicle chassis. The battery return (negative polarity) in your gizmo is more than likely bonded to the chassis.

              Originally posted by Donkey_in_training View Post
              Just to pick your brain; for a floating system, what is the failure scenario for only fusing the (+) and not the (-)? What happens if you don't fuse both runs? Thanks!
              Simple in a Floating System you have to use a fuse on both polarities. If one polarity faults to ground (chassis) in a floating system nothing happens other than you made it a Grounded System. You need a detector to alert you it happened, but other than that nothing happens. Then if the other polarity is grounded or faulted to the chassis, you have a lot of current flowing in the short circuit, and the fuses in both battery polarities are in a race to operate first.

              In a Grounded System you bond one of the polarities to the chassis inappropriately called ground. That bonding jumper is a planned fault path sized to safely handle any fault current imposed onto it so it can operate a fuse very quickly. You want fault current to be roughly 6 times more than the fuse rating so it operates in 1/120th of a second. In an RV with a 1000 watt Inverter you would have a 100 amp fuse between the battery and Inverter on a 4 AWG wire. So you would bond a 4 AWG from the Battery Term Post directly to the chassis keeping it as short as possible.

              So if you are driving along the road and if anything fell on the cable, cut through the insulation or the wire came loose from the Inverter and touched the chassis, you would here a loud pop and the fuse would operate because when the battery touched the frame, and a large current was induced which operated the fuse. You have to provide a path for the fault to find its way back to the battery (source of power) Any power source is a LOOP. Current goes out on positive, and returns on negative.

              Now having said all that having normal power current flowing in a vehicle chassis is not a big deal, and automotive industry has and still does it. Using the chassis saves them weight and money on wiring. However it is absolutely forbidden in your home because it can be extremely dangerous and wreak havoc with electronic equipment. That same havov is why Shunt Current Monitoring systems are rendered useless in autos where yu use chassis as a conductor because a large portion of the current is bypassing the wires on the return circuit.

              Understand?
              Last edited by Sunking; 04-15-2018, 12:30 PM.
              MSEE, PE

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              • #22
                Originally posted by Sunking View Post
                All I am going to tell you is you are on to something. Has nothing to do with the vehicle 12 volt system. Reason I am not going to answer is safety. But there is a possible problem. Let's say you have a gizmo like a light, TV, satellite receiver, or something working directly off DC power in contact with the vehicle chassis. The battery return (negative polarity) in your gizmo is more than likely bonded to the chassis.

                Simple in a Floating System you have to use a fuse on both polarities. If one polarity faults to ground (chassis) in a floating system nothing happens other than you made it a Grounded System. You need a detector to alert you it happened, but other than that nothing happens. Then if the other polarity is grounded or faulted to the chassis, you have a lot of current flowing in the short circuit, and the fuses in both battery polarities are in a race to operate first.

                In a Grounded System you bond one of the polarities to the chassis inappropriately called ground. That bonding jumper is a planned fault path sized to safely handle any fault current imposed onto it so it can operate a fuse very quickly. You want fault current to be roughly 6 times more than the fuse rating so it operates in 1/120th of a second. In an RV with a 1000 watt Inverter you would have a 100 amp fuse between the battery and Inverter on a 4 AWG wire. So you would bond a 4 AWG from the Battery Term Post directly to the chassis keeping it as short as possible.

                So if you are driving along the road and if anything fell on the cable, cut through the insulation or the wire came loose from the Inverter and touched the chassis, you would here a loud pop and the fuse would operate because when the battery touched the frame, and a large current was induced which operated the fuse. You have to provide a path for the fault to find its way back to the battery (source of power) Any power source is a LOOP. Current goes out on positive, and returns on negative.

                Now having said all that having normal power current flowing in a vehicle chassis is not a big deal, and automotive industry has and still does it. Using the chassis saves them weight and money on wiring. However it is absolutely forbidden in your home because it can be extremely dangerous and wreak havoc with electronic equipment. That same havov is why Shunt Current Monitoring systems are rendered useless in autos where yu use chassis as a conductor because a large portion of the current is bypassing the wires on the return circuit.

                Understand?




                Sunking, You definitely have a gift for clearly explaining these things.
                2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                • #23
                  Originally posted by littleharbor View Post





                  Sunking, You definitely have a gift for clearly explaining these things.
                  Thanks, I appreciate it. I have taught a little bit. Hate the job though, no money, no challenge, repetitive, and I do not have the patience for it professionally. Here are a few images in no particular order and many are the wrong way to bond things. So not a lot of explanations to tell you what is right and wrong. But if you understand so far should be able to pick up right and wrong. This is some of the stuff I used teaching. The first Image is exactly what I discussed above.

                  Last edited by Sunking; 04-15-2018, 05:18 PM.
                  MSEE, PE

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                  • #24
                    Originally posted by Sunking View Post
                    I do not have the patience for it professionally.
                    Man, I sure do appreciate that you have the patience to understand and answer all of my questions on here! I'm not surprised, but I should have suspected how true the "you don't know what you don't know" saying applies to electrical wiring. It doesn't help that there are a myriad of van wiring setups out there that are incorrect or downright unsafe to reference.

                    What did throw me off is that reputable companies like Victron sell shunt-based battery monitors for leisure systems. I see that their explanation for this is that the internal resistance of a 12V battery is higher than the resistance of their shunt so it shouldn't affect having negative returns and the bonded ground all going through the shunt. This would make making it not a truly negative bonded system like you're saying. It's all kind of moot due to what you guys are noting about battery meters losing accuracy for lead acid batteries over a short period of time.

                    Anyway, that's a side tangent.

                    Originally posted by Sunking View Post
                    Let's say you have a gizmo like a light, TV, satellite receiver, or something working directly off DC power in contact with the vehicle chassis. The battery return (negative polarity) in your gizmo is more than likely bonded to the chassis.
                    This definitely makes sense to me. For existing systems, not worth the hassle. For a self-designed system, it might be possible to avoid? Especially with smaller systems that might be just an offroading refrigerator in a wooden tray and some aftermarket LED's.

                    Other than that, I haven't fully figured out on my own why a floating system can be more dangerous. I'm reading that it has something to do with the electrical potential of the system being different from the potential of the chassis. So there would be issues with the whole car being made out of metal that can't be avoided? Because there can be failure in a component of the floating system and chassis?

                    Also, I'm having fun figuring out which of your drawings are not the best way to wire a system

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