Samsung S5 phone stuck in a restart boot loop
If your Samsung S5 phone is stuck in a restart boot loop (restarting continuously) while on battery (which may be 100% charged), but works fine when connected to a power source, then the mostly likely problem is a bad battery.
Pick one up on eBay for about $9.99 and replace for a quick solution.
Do not waste time doing a factory reset, clearing out cache, using Samsung Kies software etc.
A failing battery is the most likely culprit.