The current-voltage characteristic (IV curve) of an ideal battery, showing negative static resistance (red) in its normal operating range. The battery is modeled as a voltage source in series with a resistance R_s representing the battery's internal resistance. V_oc is the battery's open-circuit voltage and I_sc = V_oc/R_s is the short-circuit current. In its normal operating range the current through the battery is in the direction of increasing electric potential, so its static resistance R_static = V/I is negative, and it produces power which is consumed in the external circuit. Applying a voltage greater than the open circuit voltage or less than zero to it will result in a current in the direction of decreasing potential, so the battery consumes power.
The battery's static resistance at a given point on its IV curve is equal to the inverse slope of the line from the origin to the point. The battery has positive differential resistance, equal to its internal resistance.