Connecting an external monitor will reduce battery runtime, yes - the system has to render two screens' worth of video.  It will not affect the condition of the battery though, which is what it sounds like you're describing. If the battery is third party, it may simply be poor quality - -  or if it's OEM, it may be the battery is old stock (the system dates from 2017, so the battery could be upward of five years old, even if you recently purchased it.  
Hello, I have a question; Due to my job, I use my computer connected to an external monitor all day long. However, it does affect the battery condition quite a lot. According to some researches, they said that it consumes more battery on the external display, so it is recommended to use the power cable with the power cord plugged in all the time. Ok so far, but I have selected Priority AC Usage over "Dell Power Manager Lite" for the last 2-3 weeks. When I checked today, I saw that the life of my battery has decreased, it has dropped to yellow color levels. I just bought my battery in August. Does the external display have anything to do with it? Or is my Priority AC Usage setting triggering it? Can you tell me your ideas and suggestions about it? How can I optimize this situation? Thank you.
Inspiron 15 Gaming 7567 external screen battery problem