Replaced phone li-po with dc-dc converter problem

[Android Central Question]

A few days ago i replaced the li-po battery from my old phone with a dc-dc converter set to output at 3.75v ... ..no problem ... but i see something strage... the voltage is constant 3.75v monitored by ampere amplication and a multimeter ... but the percent value of the battery decreases without any reason... when i power up the phone ...percent level is 53-55 % ..but in a few hours ... decreases at 15% and then the phone tell me to connect the charger, but the voltage power supply is constant at 3.75v... i can;t understand how this is possible ...
Do you have any idea about this ? Thx for all of your answers

 

[Rukbat]

Since the "percentage" is actually just a measure of the terminal voltage of the "battery", it should show the same constantly - but I'd set the converter to 4.1 Volts (or as much as 4.25 Volts, depending on how the phone handles it). 3,75 Volts is almost discharged.