This simple electronic project on video link below post is consist only with several parts, one transistor BC547, three resistors, 2 small LED, and one dioda 1N4148. Li Ion Battery Level Indicator can be used for DIY Li-Ion based Power Bank, or can test Battery level on several type Li-Ion battery. example test on Li-Ion 18650, battery Li-Ion 16340, battery 14500 type, Li-ion Battery on Smartphone, Li-Ion 26650 type, etc.
Image: Sony Rechargeable Li-Ion 18650 Battery
the kit is works for several project, ie: simple battery indicator for DIY Power Bank, simple battery indicator for DIY Solar Cell Power Bank, Test the Li-Ion battery level condition, etc. the circuit consist only few components, so you can easily built it for your project.
it can detect Li-Ion battery level conditions in Low, Medium, High, and Full level. if battery drop in medium level, red LED become lights up and more brighter if battery in low level condition (arround 3.6 Volt). with my test, the green LED is always on but more bright in high level.
you can found the schematic circuit, demonstration, and description on this link video on Youtube, lets check it out: