How to calculate internal resistance of a 12V/ 12 Ah battery?
This won’t be too bad. You will need to take one measurement though and do a little math. What you need to do is measure and record the open circuit voltage of the battery, Then you need to load it with a resistor. Use one that draws a good bit of current (50 ohm, 100 ohms, etc) and measure and record the voltage again. The difference between the two readings is the number we need for the math below. (You might have to do this second measurement fast so as not to burn up your resistor unless you have a big power resistor. Remember that P=V^2/R which for 50 ohms would be 12^2/50 or almost 3 watts; so be fast and careful if you use a resistor with a smaller power rating). OK now the math. The internal resistance of the battery actually froms a resistive voltage diveder with the external resistor you load it with. This is what causes the voltage drop. The standard equation for this voltage drop is: Vdrop = Vbatt*(Rbatt/(RL+Rbatt)) Where: Vdrop is the difference in your two readings above
