@arthurlutz I'm using a Max17043 to read the State of Charge of a LiPo Battery, It sits between the battery and the charger or the wipy. (https://datasheets.maximintegrated.com/en/ds/MAX17043-MAX17044.pdf) sparkfun has a nice board for it (https://www.sparkfun.com/products/10617) It uses I2C to get the state. Even wrote a library for it (https://github.com/andrethemac/max17043.py). André

I would rather recommend you to have a look at Anker 21W solar charger. In cloud (single port) : 0.45 amps @ 4.75 V In sun (single port) : 1.81 amps @ 5.09 V It recharged the phone even faster than using normal which is using plugging it into a computer USB port. The Anker unit provides 5VDC through 2 USB connections, which keeps the kit compact and light. I can easily carry this in my go bag along with a couple of 10,000mAH battery boxes. Sounds nice, right?