All you need is a 4 ohm 5W power resistor or similar and a multimeter.

• First, measure the battery's voltage when charged. This is V1
• Next, connect your multimeter leads to each side of the resistor and briefly connect the resistor across the battery. Note the voltage reading, this is V2.
• Measure your resistor to get its precise resistance, this is R

The battery's internal resistance (Ri) is calculated with the formula:

Ri = (V1-V2)*R/V2

18650 Ri values:

• 75-150 mOhm - Excellent
• 150-250 mOhm - Good
• 250-350 mOhm - Marginal
• 350-Up mOhm - Bad