Additionally, if you have any other voltages to measure, connect your 2.8V to AN3. You can change Vref on the fly. Set Vref to AN3, and you can measure the other voltages accurately without regard to the battery voltage. Then set Vref to Vdd and measure the battery voltage, switch it back and measure your other voltages...

I use this technique often when reading absolute voltages (which must be compared against a constant reference) and ratiometric sensors, whose output varies as a percentage of their supply voltage (such as Hall-effect current sensors).