majhi:
Oh okay, thanks. I thought that since current sensor breakouts exist (like the ACS712), I could just build my own. I also thought that 18650s dropped voltage sharply after a while instead of a smooth decline like an alkaline. Guess not! I'll have to check out my voltmeter (it uses the R1/R2 dealio like you say) when I get home and play around a little bit.
For R1/R2 calculation, learn how to use this javascript application. It can run on my Android tablet or my Linux notebook.
Ray