A real battery with electromotive force \(\mathcal{E}\) and internal resistance \(r\) is connected in a complete circuit with a variable load resistor. When the current in the circuit is \(1.5\text{ A}\), the terminal potential difference across the battery is \(11.4\text{ V}\). When the load resistance is decreased such that the current increases to \(4.0\text{ A}\), the terminal potential difference decreases to \(10.4\text{ V}\). What is the internal resistance \(r\) of the battery?