The ADC inside the PIC is ratiometric, meaning the reading is a ratio based on Vdd. A 7805 is not very precise, so your battery voltage reading can be only as accurate as the voltage produced by the 7805. If you care about accuracy, then get a precision voltage reference in lieu of the 7805.
If you are discharging a deep-cycle lead-acid battery, you shouldn't go below about 10.5V. If it is an automotive starting battery, you shouldn't be going below 11.5V (at room temperature). Under charge, these batteries can go as high as 14.5V. If the task is to read this range, if you use just a resistive voltage divider, you will only be using a small part of the PIC ADC range. I used an external opamp which has the appropriate gain and offset to create a signal that goes 0-5V while the battery voltage goes 10 to 15V.