How to build a nice little cheap, calibrated current sensor.
Start with a peice of pcb, single sided, lets say about 1.5cm x 5cm
Drill some 9/64" holes in ends for our bolt lugs, leave some room on the ends for our sense wires
Bolt in some 1/2" or so bolts for our power lugs.
Solder on some sense wires, put them out of the current path (on the ends).
Using a meter (two is darn handy) run current thru it while watching the voltage drop. gradually cut alternating slots in it to increase the voltage drop till you get to something reasonable.
Label the blooming thing, a few times for good measure.
There in NO step 7, YOUR DONE. You can use an amplifier circuit to... amplify teh signal if you want.