# Downsides of using an existing length of wire as a shunt

I have an isolated electrical system composed of a couple of solar panels, 4 x 6V batteries and a 12V -> 120V inverter.

I'd like the measure the current flow in and out of the batteries in order to implement a DIY battery monitoring system. To that end, I've purchased a 200A shunt (75mV full scale, so 0.375 mOhm), which frankly looks pretty junky, and proabably is for the price I paid. When the system is running at 200A, the shunt will be consuming 15W, which also kind of sucks (I guess it will get pretty hot at that load and I'll have to careful how I mount it).

While I was contemplating adding the shunt into my existing system, it occurred to me that perhaps I could just use any existing length of wire as a "shunt". For example, I have a ~2 foot length of 00 gauge wire coming off of the battery terminal, which based on my calculation should be about 0.156 mOhm, fairly similar to my shunt. Rather than complicating my system (and adding resistance) with this shunt, what if I just measured the voltage drop across this length of wire instead?

Of course, I'd have to calibrate the measurement with some external source (e.g., run 100A though it and measure the drop) - but looking at the not-so-high-precision construction of my shunt I suspect I might have been doing that anyways.

What are the downsides of this approach?

• The main downside is inaccuracy due to the temperature coefficient of copper. Shunts are made of (relatively) temperature-invariant alloys. You could get a Hall sensor or fluxgate based transducer that you pass one of the wires through for zero (ish) loss sensing Apr 26, 2018 at 23:57
• Thanks @JohnD - yeah I looked into Hall Sensors, but (a) I cannot easily obtain them in my current location and (b) 200A-capable ones are not that common anyways and (c) they have a zeroing problem which might be material for me since the system will often be at or close to zero. Apr 27, 2018 at 0:05
• You wouldn't pass 200A through the Hall sensor, you'd pass 200A through a wire and have a Hall sensor next to the wire. Apr 27, 2018 at 0:14
• @immibis - correct, and I don't think I implied otherwise? Hall sensors generally surroud the wire and so there are still current limits based on the diameter of the hole (for those that are printed on a PCB) and the sensing circuitry. That's what I meant by "200A-capable". Apr 27, 2018 at 0:16

There is no downside for your job. You just have to make sure you organise it as a proper 4 wire connection. i.e. the two sense wires have to attach to the wire itself, i.e. be inside any connectors.

You can strip the insulation off the current carrying wire, and wrap the sense wires around it.

You can also get a big chocolate block type screw connector, and pass the bare current carrying wires straight through the whole sleeve. Then slip the sense wires in, and screw it up. Only use these chocolate blocks for the sense wires, the current connection must be beyond them.

Stainless wire. or a thin strip cut off some stainless sheet is perfect. Stainless bicycle gear/brake cables are common in my workshop. Actually I have seen these used as shunts in commercial equipment before.

Copper has a poor temperature coefficient, but you aren't trying to make a precision measurement, so it doesn't matter.

Interestingly, all the pure metals have about the same tempco 0.4%/deg. (so you can use any pure metal wire to make an RTD sensor)

Resistance alloys are 20x better, but ordinary stainless steel is 4x better than copper, has much higher resistance, so can be shorter.

That said, a long , low resistance, piece of copper wire won't self-heat, so its temperature will be more stable, and the nett change less, than a short piece of stainless that gets hot.

Ordinary mild steel is not very different to copper.

• Why does it matter how the sense wire connections are made, e.g., directly to the inner conductor versus say the outside of the ring connector crimped to the wire? Certainly it will change the resistance, but more or less resistance doesn't really matter here since the point is to calibrate the system with the measured resistance so I don't see how the exact connection matters, unless say it has a worse coefficient of temperature when hooked up that way? Apr 27, 2018 at 1:24
• the point is that you can't be measuring voltage across any connection. the r is so low that coonections can be far greater, and are totally unstable resistance Apr 27, 2018 at 3:53

That's economically perfect to use wires,for 100a meter,I used 2x awg10 two feet long, to distribute heat,coil them 1 turn to lessen space and calibrate them with good ammeter for accuracy.