mrburnette:
No!You want to track the voltage under load.
Every chemistry has a particular curve. Buy your batteries, fully charge them, use an Arduino with A0 and a voltage divider. Write a sketch where millis() is used to track A0 from the R1/R2 divider (very high resistance) when your original knock-knock is connected. Read every 15 minutes and store or output serially the time interval + A0 voltage.
Your data should closely match the manufacturer's curve. Put your curve in EEPROM or PROGMEN. Project finished.
Ray
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.