issue solved....
Bat+ connected to PC0 (ADC0) which could provide a sneak path from Bat+ to Vcc,
giving you the 2.7V when USB is disconnected.
Try putting a 10K resistor between PC0 and Bat+.
(Because of the high impedance of the ADC input, the reading of the
Bat+ voltage should not be significantly affected by the 10K resistor.)
my thanks to my all friends who spend time to support me thank you very much....