The original design at the end of last year used a 3.7V LiPo and USB charging, this was always my preferred way and still may resurface if I do a version 2 with touch screen, however it got lost along the road to reducing the BOM and version 1 so the device is now powered from 2 x AA rechargeable batteries. 

Because of this I need to boost the 2.4V to 3.3V.  I originally envisaged using the RN-131 in built boost converter for this, however I've gone through the manual and asked on the forum but cannot get 3.3V out from it, the second option was an external boost converter. Plus I may want to use a different WiFi module if the tests pan out.

I have quite a few ON Semiconductor NCP1410 lying around from previous projects so I never thought anything of reusing this, however looking on the web site these look to becoming obsolete in favour of the NCP1421 which is a much better device for about the same price. ($0.76)

According to the datasheet it supplies a lot more current up to 600mA at 3.3V as opposed to 200mA for the old one, but realistically this is lower as the batteries will go down under 2V when discharging (see  the eevblog episode on boost converters which is quite interesting!)

Also as the system does not have a power switch and to preserve battery life I wanted to ensure it would power off automatically after an hour or so of not being used. The NCP1410 had a shutdown but this was useless as it still discharged the battery through an internal diode, the 21 has "true power off" which means if I put it into power save mode it will disconnect the battery effectively allowing me to replace the MOSFET I currently have for the job.

I will therefore be modifying the circuit to add this in the near future and put the update in for others to comment.