Electrical – Adaptability of DALI Click Board for DALI Power Supply 18V DC, 250mA

At 2.5V nominal supply voltage, you have to limit the clock frequency to maybe 7MHz (depending on the tolerance on your 2.5V supply). I'm not an Arduio aficionado, but I believe at least some incarnations run at 16MHz. See this:

As you noted, the BOD has to be either set to an appropriate value or disabled (and if it's disabled you'd probably want to use an external supervisor chip).

Note that with the setting 0x06 (the only possible setting where the BOD is enabled) the BOD is not guaranteed to work properly, even with clock frequency of 4MHz because it won't necessarily be active until the supply voltage drops to 1.7V.

So, if you want a reliable design, you should disable the BOD and use an external supervisor chip, such as the ADM811 and limit your clock frequency to the lowest possible voltage trip of the supervisory circuit.

In the above case, it's 2.25V so around 6-7MHz (or less) would be safe.

This is all fairly rough but based on your numbers and some past experience:

• 40 mA for the GPS receiver is probably a good figure to use for a start, although for most GPS receivers the maximum current is running at the maximum update rate which looks like 5 Hz for that module. You can probably save some power by setting it for a slower update rate even though you might still want the update rate to be higher than the transmission rate to capture maximum speeds for example.

• You'll probably find the GSM module may spend about five seconds in transmit mode per minute or say 12% of it's time at 200 mA which would give an extra 24 mA on average. Maybe round that up and say the GSM module will take 50 mA. GSM modules are always a little trickier because it can also depend on how many retries are required and the transmit power needed to reach the basestation.

• One problem I see is that page you've linked says while running an Arduino Uno the current is 46.5 mA without a load, the I/O current limit 0f 40 mA isn't related to that so you need to add on that current required by the Arduino itself. But the Nano is simpler and after a quick look at the schematic 20 mA would probably be closer the mark.

So I'd estimate as 40 + 50 + 20 = 115 mA but don't forget to take into account any conversion losses if somewhere along the line you're using a switching converter for the power supply. It's probably better for a prototype to add in say a 50% extra margin and then do some actual testing on current draw. If you don't have much in the way of test equipment just using a battery of a fairly known capacity should give you a good idea.