I have an Arduino MEGA 2560 ,which is connected to a couple of sensors and a GSM/GPRS shield mounted on the Arduino itself.The idea is to read the data from the sensors and send it to MQ through GPRS connection.The set-up is working properly and it is right now powered via a 12V adaptor.
The maximum current required for the entire set-up, calculated roughly is 200-250 mA. As this is a truck data logging project,the expectation is that the set-up must long for a few months straight.So ,right now I am trying to connect a battery to power the entire set-up.Got a 12V ,7Ah battery trying to interface it to the set-up.So,looking for some general guide lines while connecting the Arduino to Battery power.
My maths indicates that your worst case operating time is only 28 hours.
This is because the linear regulator in the Mega2560 will take at least 250m at 12v to deliver 250mA at 5V to the board.
Therefore 7AH / 0.25A = 28 hours.
To run for 1 month on a 7AH battery you'd need to consume less than 10mA of current
i.e
0.010 A * 24 hours * 30 = 7.2 AH
So you'll either need a much bigger battery, i.e several hundred AH and you'd need to reduce power consumption a lot.
This sort of question gets asked a lot, and generally the answer is that you'd need to power down the GSM shield (and possibly the sensors) when they are not in use, and also use the watchdog timer to suspend the operation of the Arduino if its not needed to be run continuously e.g. if you only need to read data once a minute or once an hour.
BTW. Also don't use the internal linear regulator on the Mega, use a buck converter regulator its more efficient.