cells

internal resistance

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