I am working on a simple data logger that connects the SD card shield directly to the pins of a ProMini/RocketUltra/Clone board: http://edwardmallon.wordpress.com/2014/07/01/a-10-diy-data-logger-is-born/
So far they are working really well, but the big weakness of the system is what happens when the battery voltage falls below the 3.4 v needed by the voltage regulators. Usually the system goes into a kind of loop, constantly restarting due to "draw down - brown out - battery rebound - restart". These units are already monitoring the power supply voltage via a divider, but I need to add some way of having the logger completely disconnect itself from the power supply when it falls low, but I would like that to be controlled by the Arduino rather than by passive components.
The reason I want the Arduino to control the shutdown is that the new file creation event on the SD cards is the single biggest sustained load and I am afraid that a passive component system would see the voltage drop from that, and pull the power just when the SD card is in the middle of a write process, effectively nuking the card, and all the data on it. If the Arduino handles the decision to power down, then it can wait till all the SD card operations are safely complete. Once the shut down occurs, I don't want the arduino waking up again until there has been a full disconnect (ie new batteries are put in) because I am sure there will be voltage rebound on the power supply after the disconnect happens.
I have found this: http://www.pololu.com/product/751
But I wondered if there were any suggestions for something simpler? I am hoping for an approach that is as minimal as the rest of the logger design.