Do you want to monitor the battery's voltage using the Mega? You just need a couple of resistors and an analog pin. Look up "voltage divider". However, is anything charging this battery when connected to the Mega? Is it in a vehicle, or connected to a solar panel? If so, some extra protection for the Mega would be a good idea. In vehicle, you can get damaging voltage spikes, and with solar panels, the voltage can rise to double the battery's voltage, or more, if you were to accidentally disconnect the battery while the panel was working.