I have a charger, I do not want another. I just want to test the current/capacity with the Arduino.
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