Hi,

If your phone only works when connected to the charger, most probably the battery is failing to hold its' charge and will need to be replaced. Even rechargeable batteries have a life span.

Here is the Ifixit guide on how to replace the battery.

Samsung Galaxy S5 Active Battery Replacement

Here is a link to just one supplier of the battery. If this supplier doesn't suit you there are others, just go online and search for Samsung Galaxy S5 Active battery.

https://www.amazon.com/BELTRON-BLT-GS5-R...