Internal Resistance:

Internal resistance in a chemical cell is due commonly to the resistance of the electrolyte among electrodes in the Figure.

Any current in the battery have to flow by the internal resistance. The internal resistance is in series along with the voltage of the battery, causing an internal voltage drop show in the Figure.

Along With no current flow, the voltage drop is zero; therefore, the full battery voltage is established across the output terminals (V_B).  Load resistance (R_L) is in series with internal resistance (R_i) if a load is placed on the battery.

Figure Internal Resistance in a Chemical Cell

Figure Internal Voltage Drop

Whenever current flows in the circuit (I_L) than the internal voltage drop (I_LR_i) drops the terminal voltage of the battery as shown in Equation (4-3).  Thus, internal resistance decreases the current and voltage both available to the load.

V_L  = V_B  - I_LR_i           (4-3)