Here's a really cheap, easy, and accurate method - that gives zero loss to the circuit!. It was in an old Practical Electronics, back when the LM3914/5 came out!.
You've got two wires connecting from the battery to the load, motor or whatever, this wire, thick as it is, will still drop a voltage across it depending on the current through it (it's essentially a shunt). Use an opamp to amplify the voltage dropped across it, this gives a voltage output directly related to the current through the wire. Calibrate it by known loads, or by using a current meter and adjusting the opamp gain to read the same.
BTW, the original article was for a car ammeter, and it monitored the voltage across the battery earth strap - giving both charge and discharge readings, including the starter motor current!.