Reading Battery Voltage - Alternative to Divider + ADC vs. Gas Gauge IC?
1 cell LiPo battery guage $0.15/ea for IC only DS2782

Precision Voltage, Temperature, and Current Measurement System
Accurate Stable Internal Time Base
Absolute and Relative Capacity Estimated from Coulomb Count, Discharge Rate, Temperature and Battery Cell Characteristics
Accurate Warning of Low Battery Conditions
Automatic Backup of Coulomb Count and Age Estimation to Nonvolatile (NV) EEPROM
Gain and Tempco Calibration Allows the Use of Low-Cost Sense Resistors
24-Byte User EEPROM or 16-Byte User EEPROM and Unique 64-Bit ID
Industry 2-Wire Interface with Programmable Slave Address
Tiny 8-Pin TSSOP and 10-TDFN Packages Embed Easily in Thin Prismatic Cell Packs