How to wire shunt with chassis ground?

[Donkey_in_training]

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

[Sunking]

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.

[Donkey_in_training]

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.

[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.

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.

[Donkey_in_training]

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!

[Sunking]

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.

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?

[littleharbor]

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.

[Sunking]

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.

[Donkey_in_training]

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.

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

[Boompy]

Hey Sunking

I've been pondering this thread.

Could you help clarify this scenario for me?

Assuming I have a vehicle, which is chassis bonded (battery negative tied to chassis - single string of batteries), if I place a shunt between the negative battery terminal post and the cable that ties the battery to the chassis, surely this eliminates any parallel connections and thus provides an accurate current measurement into and out of the battery.

This assumes that there is only one connection point between the battery terminal post and the chassis. It also assumes that any NEG & GND terminals on equipment (charge controller, etc) would ALL be tied to the chassis.

Under this scenario, the chassis acts purely as a giant wire completing the circuit with the battery, maintaining breaker protection also.

Am I on track here?

But yes, assuming you can protect the little wires coming off the shunt, its seems easier to just connect the shunt to the positive terminal.

[Sunking]

Assuming I have a vehicle, which is chassis bonded (battery negative tied to chassis - single string of batteries), if I place a shunt between the negative battery terminal post and the cable that ties the battery to the chassis, surely this eliminates any parallel connections and thus provides an accurate current measurement into and out of the battery.

This assumes that there is only one connection point between the battery terminal post and the chassis. It also assumes that any NEG & GND terminals on equipment (charge controller, etc) would ALL be tied to the chassis.

Under this scenario, the chassis acts purely as a giant wire completing the circuit with the battery, maintaining breaker protection also.

Am I on track here?

Yes you understand the issue. In home wiring however (NEC application), you would never be allowed to add resistance (shunt) to a fault clearing path or Ground Reference. Transducer no problem. Understand?

[joerossjr]

In my vehicle I have nothing tied to the chassis, and everything in the House system going to the negative bus bar. My battery negatives are connected, and that's the only place the house gear is connected to the van's negative. My shunts are connected to the battery connection at the bus bar. All current goes through them. I do think I need a fuse on the load POS coming off the bus bar still. Here's a pic to show. Still needs some work though. Sorry for aspect on the pic.

[Boompy]

Ah fantastic

Thanks guys, glad I can put that one to bed. I really appreciate the clarification.

Sunking, funny you mention not adding resistance (shunt) to the return path. A hall effect transducer would be nice to circumvent this, only they're so darn hard to find in DC form with a 4-20ma output.

Been searching for a bi-polar device (measures current into and out of the battery) that supports 0A - 150A. Seems like they're as rare as hen's teeth.

In the AC world there is plenty to choose from.

Thanks so much for the follow up on this old thread!

[Sunking]

Sunking, funny you mention not adding resistance (shunt) to the return path. A hall effect transducer would be nice to circumvent this, only they're so darn hard to find in DC form with a 4-20ma output.

I bet you are having a hard time because they do not make them with a 4-20 ma output that I know of. They come in two basic flavors:

• Open Loop which has a voltage output in proportion to input current. Example +/- 5.0 volts for 500 Amp Scale
• Closed Loop which has a Current Output proportional to Input Current. Example +/- 50 ma for 500 Amp Scale. Now you might be able to find a +/- 20 ma transducer, but WTF would you do with 4 ma low signal. That is not capable of measuring ZERO Amps or of reversing polarity for current direction. Understand?

Resistor Shunts are voltage devices and they come in 20, 50, and 100 mv outputs.

Hint find a Closed Loop Transducer with say a +/- 2.0 or 5.0 volt output for 200 to 500 Amp Full Scale reading. All you need to read the current is a volt meter.

Example a 2.0 volt / 200 Amp Model:

0 volts = 0 Amps
2.0 volts = 200 amps