Here are some ideas on controlling/optimizing the range:
A simple thing to do would be to have multiple voltage references for your microcontroller ADC and switch to a lower voltage reference for better accuracy/resolution at lower input voltages. So, the idea could be to start with VREF = 5V for example. Then read the voltage. If it's low for 5V VREF, switch VREF to a lower voltage, for example, 3.5V and then continue doing this until you reach the lowest allowable voltage range.
Another method you could use is to have a voltage amplifier stage where the microcontroller controls the gain of the amplifier stage and increases the gain if it decides that the input voltage is too low and needs to be amplified.
In each of these cases, make sure you have some sort of protection scheme - clipping circuit - so that the input voltage doesn't damage the microcontroller ADC input.
Hope this helps.
Tahmid.