The battery going down while charging can happen due to several reasons:

• Heavy usage during charging causing power consumption to be higher than the charging input.
• Using an inappropriate or low-output charger or damaged charging cables.
• An aged or weakened battery that cannot hold charge properly anymore.
• Device or battery overheating which reduces charging efficiency.
• Unstable electric current quality from the power source.
• Software issues such as improper battery calibration or system bugs.
• Hardware problems including damaged or loose charging port, faulty power IC, or sub-board.
• Background processes like system updates or syncing consuming power during charging.

To fix this, use the correct charger and cable, avoid heavy usage while charging, keep device temperature stable, clean the charging port, update software, calibrate the battery periodically, and consider battery replacement if it's old or degraded. Hardware issues may require professional repair or replacement.