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Battery Internal Resistance

The batteries also have an internal resistance that in case of large series connection could be significant and could be necessary to take into account. The internal resistant is a function of several factors among them are the design of the battery and thickness of cells plates, materials, electrolytic fluid, and terminals design and materials. Normally the internal resistance of a battery is just few ohms to fraction of an ohm.



How to Calculate the Internal Resistance of a Battery


Lest start by analyzing  the battery internal and the load resistance circuit on the figures

A and B below:











1- Measure the open circuit voltage of the battery with a high impedance meter, let assume  that it is 12.2 Volts, see figure A.


2- The equation that defines the close circuit voltage including the internal battery resistance Rint and the voltage at the load Rload in figure B is:


Vtotal= VRint + VRload;


3-Choose a current for the which the internal resistance will be calculated, always select a current that is safe for the battery current handling capability, lets say 10 amperes for this example.  


VRload = Rload X IB ;  where IB  is battery current flowing in the resistors Rload


Then;  Rload  = VRload / IB =  12.2 volts / 10 amperes = 1.2 ohms



4- Now connect this Rload into the circuit of figure B


5- Now measure the voltage between across Rload and write the result (it is the same if measured at the battery terminals)


The new total voltage is =12 volts


6- Now calculate the voltage drop,  voltage drop = Voltage in open circuit - voltage with the resistor load Rload  connected


= 12.2 - 12 = 0.2 volts


The 0.2 volts difference is the voltage drop caused by the current flowing in the internal resistance of the battery Rint  


Vtotal= VRint + VRload


Vtotal = 0.2 v + 12 v = 12.2 volts, this equation completes the battery voltage circuit with the resistor load.


7- Calculate the internal resistance:


Rint = VRint / IB  = 0.2 v / 10 amperes = 0.02 ohms at the given current




Continue lead  acid batteries tutorial >>

How to Calculate the Internal Resistance of a Battery






Operating Temperature Range


For Charge =  -10 C to + 45 oC

For Discharge =  -20 to + 55 oC at a given current


Charge Retention Also Known as Shelf Life, given at room temperature 20 oC or any other specified


97%       At one month

90%       At three months

82%       At six months



Deep Charge Discharge Capacity Cycles

This concept refers to the number of deep discharge than a battery can tolerate before









Figure A, Open Circuit Voltage

Figure B, Closed Circuit Voltage

S1 Open

S1 Closed

Open Circuit Voltage

 12.2 V

      0.2 V

VRint = Voltage Drop in Rint


12 V