How to measure Lithium battery voltage with the nRF51 ADC
The ADC samples the voltage on the ADC input pin and outputs the result in the ADC->RESULT register. The example outputs the result to pins 8-15 as an 8-bit binary number. If you have the nRF51822/nRF51422 development kit and the nRFgo motherboard, and you have the default setup of the motherboard where PORT1 pins are connected to LEDS pins, then you should see the 8 bit number on the 8 leds on the motherboard. You can also put a breakpoint in the ADC interrupt handler in the example to see the number in the ADC->RESULT register.
So how would I deal with the number that the register outputs? The ADC->REGISTER gives me 160 which should correspond to a battery of 4.2V and the input voltage should be about 3.46V when the voltage divider is taken into account?
I suspect that the answer to your questions are more or less in my comments to Lucas questions later in this thread. If you are using 8-bit sampling then the resolution of the ADC is 256. The voltage on the ADC input when the Lithium battery if fully charged: 4.2 * (2.2M/(2.2M+10M))=0.76V which should output 0.76/1.2*255=161 which is in harmony with your findings.