Battery as reference voltage
Don't use a battery as a voltage reference.  To quote my firmer boss and mentor: "Battery voltage is a nonlinear function of everything. [That includes temperature.]"

Zener voltage reference, or a bandgap voltage reference would work for you.  They are available in a variety of voltages.

Keep in mind also that most A/D converters inside microcontrollers have a built-in voltage reference, which can be enabled from code.