How to map adc count to voltage when your ADC reference is a battery?
As you have "an internal fixed voltage reference of 2.048V" then your ADC reading should be:
Vadc/Vref * 2^N   i.e.   Vadc/2.048 * 1024

So Vadc = 1V => code 500

Unless I've misunderstood your question, the battery does not matter (within reason) because the reference Voltage is stable at 2.048V