Hi Fencer,
Somebody mentioned it, but you might want to make a simple shunt to measure current. It's not that hard to make something "close enough" for what you need. The next step is a careful re-learning of Ohm's law. I had some big mis-conceptions myself, when I was starting this hobby, so it's smart to get your brain smoothed out on how Ohm's law works.
Use a piece of #6 or #8 threaded steel rod. About 12 inches will do fine. Using enough nuts and washers to hold everything together, connect the ends of the rod in series with one of the power leads. Make good tight connections, use ring terminals where appropriate. See if you can find some furniture angles or something that will serve as "legs" so that you can attach the whole thing to a wooden board.
Current will flow through the threaded rod. Steel conducts, but isn't great. Take advantage of this fact to make a shunt. There's not much resistance in a threaded rod, but it's enough to make a multimeter show a voltage drop across the ends. The resistance will be too small to measure directly. Expect about 0.01 Ohm or so. You might find that you can adjust the position of the connections on the rod, in or out, to get an exact resistance so that the math is easier, but even if it's something 0.008132 ohms you can use a pocket calculator anyway.
For example, if 10 Amps flows through your threaded rod, Ohms law dictates that you will measure 0.08132 volt across the ends of the shunt. That is:
V=IR
(10A) * (0.008132 ohm) = 0.08132 volt
To calibrate a shunt, you have to know exactly how much current and voltage is present simultaneously in a test circuit. Use Ohms law to work that out. Once calibrated, the shunt will work for a variety of test situations. When you use it after calibrating it, you know its resistance, and whatever voltage potential you measure across the ends is proportional to the current in the circuit.
Eg
if you measure 0.054 volt, then 0.054V / 0.008132 = 6.64 Amps
Good luck.