As ATMega328p needs 5V, I think the batteries should suffice. But the Voltage output from AA batteries degrades over time and fluctuates so do I need additional circuitry to control the input to the ATMega328p?
Charged, each battery cell is somewhere around >1.5V but discharged they near 1V. This means that if you go with 4 cells, then the input voltage will drop below 5V when discharging, which makes voltage regulation trickier. It would be better if you were using a modern MCU with 3.3V instead of an antique AVR. So your options are:
- Use 6 cells and then you don't have to worry about the supply dropping below 5V, or
- Use a boost or buck/boost regulator instead, or
- Use a modern MCU with 3.3V instead.
For the relatively huge amount of current you expect, you probably want to go with as powerful batteries as you can for that reason.
The SIM800 needs a 4.2V input. But while transmitting its causes a voltage drop and current bursts up to 2A. For that I am using the power circuit given on the datasheet.
It appears to require a voltage between 3.4V to 4.4V in which case it will at least not affect your choice of batteries and regulator like the MCU does.
Either way, I would consider letting it work on its dedicated power plane. Maybe use one regulator down to 5V and another down to 4.2V.
Overall you should consider the choice of voltage regulator. The one you have picked is adjustable but comes as a fixed 5V version as well. And then linear regulators aren't ideal for battery-powered applications since they have poor efficiency. If you wish to use one as clean reference for the RF part, which is a good idea for EMC reasons, then maybe consider a switch regulator down to 5V and a linear one down from 5V to 4.2V, to power that RF device. The Micrel part you've found seems like it would perhaps suit as the 4.2V one.
Would I need additional circuitry to supply power to the ATMega328 and other components as well or directly from the battery bank should suffice?
I think it is modern enough to have a low-voltage detect feature? Otherwise you need to add an external one.
You will most definitely need reverse polarity protection to protect the board and/or batteries from frying when placed backwards. This is commonly done with a P MOSFET with the body diode in series with your supply, followed by a zener, along the lines of this.