nice. no instruction in the package. two switches to select output V, 3.6, 5, 6, 9V and if you wish to output more than 12V, you should switch "T" on other side and switch favorable positon, 12, 15,16,19 and 21V. these output use 2.5mm output, USB output only effect if you choose "5V". three or six 18650 batteries are needed to use. short no PCB 18650 is only acceptable.
Tested with batteries and works without any pronlems. I'm using it as UPS for Raspberry Pi 4B.