The 7805 is a voltage stabilizer IC that regulates 9V to 5V. The 3V voltage difference is consumed by the 7805. Does this have anything to do with 300mA? You can't expect a power supply that can only output 300mA to 600mA.
Don't talk about the problem of low voltage and high current. This refers to transformer or line transmission. It describes that in the process of transmitting electrical energy, assuming that the transmitted power is constant, the lower the transmitted power. the voltage used, the greater the current to be transmitted.
The 9V board you mentioned cannot charge the 9V battery if that is the higher voltage. Look at the battery board settings. For example, the maximum voltage of 12V battery board can reach 18V. The charging condition is The board you are talking about is designed to charge 12V batteries. If eIt is rated at 9V, its maximum voltage will not be 9V, but should be above 12V. In this case, it can charge 9V batteries. Even so, you. You have to watch it. Find out what capacity your battery has. For example, if your battery is 5 Ah, charging with a current of 1A will take at least five hours. it will take ten hours. It's just no loss. Under ideal conditions, charging, but the charging current cannot reach the ideal state and the time will be longer.