It’s actually quite easy to calculate DC internal resistance.
The problem is that you need a battery holder that does not have piss poor resistance like you see with the cheap ones on eBay and Aliexpress.
If you just want, you just need some power resistor of a determined load, a multimeter, a soldering iron, and some 18AWG wire, and perhaps some brass buttons.
1. Solder one end of 30cm 18AWG wire to 2x brass buttons for nice contacts.
2. Solder the other end to the legs of a 5 Ohm resistor.
3. Wire a switch through the wires.
4. Clamp the brass buttons to the cell with the switch off.
5. Put your multimeter on voltage reading mode.
6. Measure the open circuit voltage of the cell. For example, it will be 4.2V.
7. Flip the switch, and measure the voltage of the cell instantly, then flip the switch again. The noted voltage will be 3,9V
8. Do math:
ΔV = voltage delta = 0,3V
R = 5r = resistance used
V = cell voltage = 4,2V
VL= Voltage under load = 3,9V
C= current flowing
V-VL = ΔV= 4,2V-3,9V = 0,3V
C= VL/R = 3,9V/5t = 0,78A
Internal resistance = ΔV/C = 0,3V/0,78A = 0,38R = 380 milliohm
Yeah, that is the real internal resistance of a cell, and how you calculate it.
