rc-pilot-freak:

I think you're saying your're going to reference tha A-D to a stable and accurate 2.5V source (the reference diode). If that's the case, your A-D input range will be 0 to 2.5V. If you put a simple voltage divider at the battery, designed so that the max battery voltage is divided down to 2.5 volts, you should be OK

Joe