5V Arduino Pro Mini VCC vs. Raw for my project

I am working on a project which requires the use of a battery pack to power the Arduino. The battery pack is traditionally meant to hold four AAA 1.5V batteries with an output of 6V total; however, I was wondering if I used four NiMH AAA batteries (each with 1.25V) would I use the VCC input? I only ask because I know that batteries vary in voltage over time and without a 5V regulator I am tentative to utilize the VCC input over the RAW.

Vcc input is the correct thing to use; you need ~7v on RAW to get 5v on Vcc due to dropout in the regulator,
so you'd need more batteries to power it through RAW, and that extra power would be devoted to generating waste heat and nothing else. (though the exact minimum dropout may be lower on clones - or it may not be - depending on what regulator they used. The good ones now are down at like 0.3v dropout, but you're not gonna see those in a pro mini ripoff).

To maximize battery life, remove the power LED and regulator from the board.

It's common to run battery powered projects directly off the battery (most often done with LiPo's), this generally improves battery life and reduces the number of batteries you need (by eliminating the dropout that).

If you do not need the 16Mhz, you can change the fuses to 8Mhz and run with even lower voltage–after removing LED and regulator, of course. 3 AA will do fine then. I use them with a 3.3V LDO regulator because the attached nRF24L01 is a little picky. :wink:
I was lucky and bought an Arduino Pro Mini Clone that already had a LG33 LDO regulator on board.

When sleeping, the ProMini wastes <60µA but that depends on the other components I have attach.