You need to use a 7V supply minimum to the Mega's barrel jack, it will automatically switch to USB power when it is there, I would use a 6 AA battery holder and rechargeable NiMH cells (8.4V, full charge), not a 9V "transistor" battery.EDIT: I misspoke saying the Mega would auto switch to USB power, but there are fairly simple circuits to do that.
Many parts on a Mega can't be put to sleep.That makes a Maga a poor choice to run on battery backup.Not sure how much you can get it down (from 70mA), but be prepaired to recharge the 6* NiMh AA pack every two or three days.Leo..
My project is quite simple, The board is in sleep state all the time. It wakes up only when a low voltage is detected on the interrupt pin and writes a value to EEPROM.The circuit is connected to a PC via USB. At any point, the PC can ask Arduino for the stored value.
Is there any other board that you can recommend that is more efficient and has USB connectivity on the board?My needs are not cpu intensive. I could go with a simpler chip, if needed.