Why is only one "wet indikator" red

It is most likely not water damage to the entire logic board. Probably a small amount of water from rain, or another source got into the phone. Since the water damage indicator near the battery was red, the water probably got into battery connector. This would short the pins in the connector causing rapid power cycling ultimately killing the battery. Even though replacing the battery, in this case, would fix the problem, I would recommend inspecting the logic board for any other corrosion before continuing use. Doing this will ensure you don't have any issues further down the road.