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?
Best Answer
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.