> Car fuses tend to run .010 to .050 ohms. If this works you could
> clone the fuse to be a current sensor and save mucking with existing
> wiring harnesses. IF the fuse works for this, the test voltage
> is an offset from 12V (or the battery voltage) which causes some
> processing problems, but the ground connection on many
> on board motors is though the motor mounts or a 'local' in door
> connection and has it's own interconnect problems. The trick
> I would try is a 'current mirror' across the fuse using DIP transistors
> (for Vbe matching) to reflect the signal to being near ground for
> the PIC or whatever.  An opamp isn't much more expensive but
> may have problems in single rail ~12v operations. Some assembly.....
>
This application is ideal for the MAX472 "Precision, High Side
Current Sense Amplifier.
Typical sense resistor at 10 Amps is 0.005 ohms
It is single supply (even though current can flow in either direction
through the sense resistor... a charge/discharge logic level pin
changes state, that's all.)
Check it out at www.maxim-ic.com

I use this I.C. in our smart battery development.
Lance Allen
Uni of Auckland
New Zealand