Your main enemy is heat. When your battery gets too warm the phone will stop charging it until it cools down. When charging do not leave in direct sunlight. web surfing on the phone for active things like streaming video (YouTube etc) demands more on the processor and that can warm the phone too over time. Tethering via WiFi hotspot is less demanding than surfing on the phone. Also depending on what you are plugged into affects your charge rate. If plugged into a laptop's usb port, the charge rate is slower than the ac adapter and will have a harder time charging a phone that is actively doing something.