RingoPhone battery doesn't work properly.

Dear friends! Please help me to solve the problem. I assembled the phone, the initial test is passed. The telephone works without any issues. However, it works only with a connected cable. If I pul out the cable then there appears a text “battery critical! Turning off…” Herewith, the battery status on screen is fully charged.

       I have been charging it for at least 5 hours, the result is the same. When checking with a multimeter, battery voltage is 4.2V. It means that the battery is fully charged. What could be a problem? 

I also want to add that the telephone has successfully connected to wi-fi and updated a firmware (1.0.5)


Do you measure the battery voltage when it is connected on the phone or the battery itself ?

Sometimes the battery gives the correct voltage when you measure it outside from the circuit but when it is connected ( Load condition ) the voltage drops.