I designed a shield for a customer and found I shouldn't have mixed the analog and digital the way I did. I put a software serial on pin 14, analog input on pin 15 and another software serial on 16,17.
14 - serial lcd (9600 every loop writes to lcd) 15 - check voltage of battery 12v---switch---47K 1%---AN1---10K 1%---GND 16 - Serial rx (little used). 17 - Serial tx (little used).
Is there a way to shut the serial lcd serial port off, let things settle on the analog pins and then take an accurate reading? Once the battery switch is closed, I could display "WAIT" on the lcd, stop writing to the lcd and take a measurment. With battery check switch open, I am reading 1.25vdc on the AN1 pi while the lcd serial is sending data.