Why does my device work on battery but not plugged in (or vice versa)?

Short Answer: Failed DC jack, cracked solder on the power connector, or dead charging IC.

Detailed: This is a classic power path problem.

Symptom	Likely cause
Works on battery, dies when unplugged	Battery is actually dead; device runs only on AC
Works plugged in, dies when unplugged	Dead battery (won't hold charge)
No charging light when plugged in	Bad DC jack, broken solder, or blown charge fuse
Charging light on but battery doesn't charge	Failed charging IC or bad battery management circuit

Fix for loose DC jack: Reflow the solder joints. If the jack is loose mechanically, replace it — they cost $2–5.

Common failure: On laptops, the DC jack is soldered directly to the motherboard. Plugging/unplugging stresses the joints until they crack. Reflowing solder fixes 80% of "no power" laptop repairs.