power useage while docked

[indocus]

i have a question from time to time i find that i need to share my phone internet connection but when ever i do have the need to do this i find that my phone stops charging the battery it dose not use the power supply from the usb connection as a alt power source and begins to rapidly drain my battery
to the point with a full charge im lucky to get 3.5 maybe 4 hours of use
this also happens when i use my GPS or stream internet radio in my car for those long road trips
i have "when device is turned on,do not charge the battery when connected to a PC" UNCHECKED


my question is this
is there a ROM or setting that would make my phone
a:charge no matter what app or service i was using
b:after the battery is charged to draw power from the USB supply instead of a constant draw in the battery

also is this normal for the phone to switch from charging when the shared connection function is used