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?