Schematic diagram of a simple battery and resistor circuit, showing that the DC resistance of a power source (Rs) is negative and equal to the positive resistance of the attached load (RL).  P is the power produced by the battery and consumed by the resistor.  From the passive sign convention power entering a component is defined as positive while power leaving a component is negative.  Therefore due to conservation of energy the power flow of the battery is the negative of the power flow of the resistor.  Thus the static or "true" resistance of the battery is the negative of the resistor.
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Chetvorno