Cogging...

[What's_nitro?]

I can't find a picture (because they don't make it anymore), but I have an ammeter for my car stereo (made by Autometer) that uses some type of inductive coil around the main power wire to measure the current. It doesn't add resistance. It would add size/complexity/cost, however.

Monitoring voltage drop in order to limit current would be tough due to the variance in C-rating of different cells.