USB charger. It is necessary to adjust the regulator to 5V, supply it with a USB port and provide power to the charger. I use a radio-model lithium-polymer battery purchased in China, which provides 5 ampere-hours at 11.1 volts. This is very much - enough to charge an ordinary smartphone 8 times (without taking into account the efficiency). Taking into account the efficiency, it will be at least 6 times. Do not forget to close the D + and D-jacks of the USB connector to tell the phone that it is connected to the charger and the transmitted current is unlimited. Without this event, the phone will think that it is connected to a computer, and it will be charged with a current of 500mA - for a very long time. Moreover, this current may not even compensate for the current consumption of the phone, and the battery will not be charged at all.