I got a cheap powerbank, which didn't work at all, so decided to look inside it. There is a control IC AP5900, what I found about it is a chinese website: http://www.chipown.com.cn/en/case.asp?id=24 They included a reference design there with schematic: Now I measured the battery voltage in turned off state, which was 1.7V...pretty unusual for a Li-ion battery, expected at least 3V. So I tried to charge it and measured the voltages during charging. The control IC gets 4,9V measured on pin 6 (referenced to GND pin 8), but the voltage on the pin 5 (so the charge voltage) was only 0,9V (referenced to pin 8). The battery voltage was still 1.7V, so I measured the voltage between pin 5 and battery positive and there was the 0,8V differnce, so its not connected on the PCB as on the schematic.
Can you give me any tips what could be wrong with it or how to try to find it out?