After 6 months, why does my phone charge erratically?

[Windows Central Question]

After 6 months, the phone charges erratically

It rarely charges fully to the green light and sometimes discharges when plugged in. I must shut it down and turn it back on, when I get the "low-battery" warning. I then plug it in again briefly and it appears to be nearly fully charged. I can't determine a pattern for how it drains.