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I need a method to measure the load resistance attached to the terminals on a battery. I want to determine this load before I turn mosfet on to let battery pack discharge. Most important I want to check for a short at the terminals before turning battery on. I have a 5 volt cpu to control system. I am thinking of trying to use a low ohm shunt and current sensing ic along with a current limiting resistor. Issue is trying to measure difference between a 0.01 ohm load (1500 amps) vs Short yields only a few uA difference, which will be hard to measure be that precise with circuitry. Without limiting resistor even a 0.001 ohm shunt yields hundreds of watts at high loads.

Is there another method I could use?

schematic

simulate this circuit – Schematic created using CircuitLab

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You need to rethink what you're trying to do, or how you're describing the problem.

I think you're trying to make a current limit on your switched supply, so that it will supply up to X amps, but not more than Y amps. Judicious choice of X and Y are required. If Y = 1.1X, you have a difficult sensing problem. If Y = 2X, then it's somewhat more straightforward.

There are ways to sense the resistance of the load before full voltage is applied, however few interesting loads are constant resistance. An input capacitor filtered load will draw a high inrush current as the capacitor charges. When cold, a filament lamp load will be a tiny fraction of its hot resistance. Any active load may draw little if any current at low voltage, before it's reached its startup input voltage level. All of these loads will need knowledge about the load, in addition to any measurements made before powering them fully.

One option is to study the SOA graph for your particular switching device, use a current shunt in series with it, and devise a time sensitive trip, so that you can power the load on fully, and trip off again if the SOA gets near to being exceeded. This won't necessarily work with loads that have inrush, which will require a more limited current output if you want to start them successfully with what is arguably an under-sized power supply switch.

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  • \$\begingroup\$ I already have a "run time" current sense that can monitor system for variable load changes after startup. What I really need is to be able to sense direct short before turning on. \$\endgroup\$ – user1895526 Nov 20 '18 at 13:23

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