A portable electronic device comprising: electrical energy sources (130A, 130B, 130C); a first current sensor; a second current sensor; an electrical current source; and current monitoring circuitry, the electrical current source, the first current sensor, the second current sensor, a first electrical energy source, and a second electrical energy source being electrically connected to each other and defining a first closed loop and a second closed loop, the first closed loop comprising the electrical current source, the first current sensor, and the first electrical energy source, the second closed loop comprising the electrical current source, the first current sensor, the second current sensor and the second electrical energy source, the first current sensor and the second current sensor being on different current branches, the current monitoring circuitry being electrically connected to the first current sensor and to the second current sensor to sense a first electrical current flowing through the first current sensor and a second electrical current flowing through the second current sensor, the current monitoring circuitry determining the ratio of current between the first electrical current and the second electrical current, comparing the determined ratio to pre-determined ratio values, and identifying a match between the determined ratio and the pre-determined ratio values to control the electrical current source to provide an output current based on the identified match. electrical energy sources (130A, 130B, 130C); a first current sensor; a second current sensor; an electrical current source; and current monitoring circuitry, the electrical current source, the first current sensor, the second current sensor, a first electrical energy source, and a second electrical energy source being electrically connected to each other and defining a first closed loop and a second closed loop, the first closed loop comprising the electrical current source, the first current sensor, and the first electrical energy source, the second closed loop comprising the electrical current source, the first current sensor, the second current sensor and the second electrical energy source, the first current sensor and the second current sensor being on different current branches, the first closed loop comprising the electrical current source, the first current sensor, and the first electrical energy source, the second closed loop comprising the electrical current source, the first current sensor, the second current sensor and the second electrical energy source, the first current sensor and the second current sensor being on different current branches, the current monitoring circuitry being electrically connected to the first current sensor and to the second current sensor to sense a first electrical current flowing through the first current sensor and a second electrical current flowing through the second current sensor, the current monitoring circuitry determining the ratio of current between the first electrical current and the second electrical current, comparing the determined ratio to pre-determined ratio values, and identifying a match between the determined ratio and the pre-determined ratio values to control the electrical current source to provide an output current based on the identified match.