Boot to safe mode.
- Turn off your Galaxy S6.
- Press and hold the Power key.
- When ‘Samsung Galaxy S6’ appears, immediately release the Power key and then press and hold the Volume Down button.
- Continue holding the Volume Down button until the phone finishes the restart.
- When you see Safe Mode on the lower left corner of the screen, release the button.
Charge with another cable under safe mode, preferable something newer with no signs of wear then see if the symbol stays active. The S6 line had crappy battery life to start with and the batteries started to fail at 12 months so depending on how long you had the phone a new battery is definitely a possibility.