FIG. 1 shows a conventional charging device 11, which has an end connectable to an electrical energy generating device 10 and another end connectable to a capacitor battery 12. Electrical energy output by the electrical energy generating device 10 can be charged via the charging device 11 to the capacitor battery 12, so that the capacitor battery 12 can supply electric power to a load 13 for the same to work. The electrical energy generating device 10 can be a renewable energy generator or a power supply unit.
The conventional charging device 11 includes a transformer 14, a control circuit 15 and a rectifier diode 16. The transformer 14 is able to increase or decrease a voltage of the electrical energy output by the electrical energy generating device 10. The control circuit 15 is used mainly to control and maintain the electric power transmitted by the charging device 11 at a constant current and a constant voltage. The rectifier diode 16 can convert the transmitted electric power to a direct current (DC) of no frequency response.
The transformer 14 shown in FIG. 1 functions to regulate the voltage of the electrical energy output by the electrical energy generating device 10 and then outputs voltage-regulated electric power. Therefore, the transformer 14 can be considered as a power supply and it has an internal resistance r. As opposed to the transformer 14, the capacitor battery 12 can be considered as a load and it produces resistance R in the process of being charged. In the course of charging, electric power is charged into the capacitor battery 12 in the form of voltage, so as to increase the electric potential level of the capacitor battery 12. Therefore, the capacitor battery 12 will produce heat in the course of being charged. When the transformer 14 outputs continuous electric power to charge the capacitor battery 12, it will encounter the condition proposed by the maximum power transfer theorem (MPTT). That is, when the resistance R of the capacitor battery 12 is the same as the internal resistance r of the transformer 14 in the electric power transmission path, a maximum electric power output can be obtained. That is, Poutmax=½ Pin. Therefore, one half of the electrical energy is consumed in the circuit to result in very poor charging efficiency.
When using the conventional charging device 11 shown in FIG. 1 to charge the capacitor battery 12, a considerably long time is needed to fully charge the capacitor battery 12 due to a capacitive reactance thereof, i.e. a static electricity that resists the voltage change at two ends of the capacitor. Further, when the transformer 14 outputs electric power of a relatively large current, the capacitor battery 12 is subject to burnout and the charging work could not be completed.