Hey Rydian, I'm not sure if anyone has done a battery life comparison chart but I'm gathering data for one using:
Stock 3DS
3DS + Nyko Power Pak Plus
DSi XL
DS Lite
And I'm going to be getting a 3DS XL in august also, but I am going to be testing these carts under minimum brightness and maximum brightness using Snailface's homebrew DS Battery Timer homebrew. I hope to provide essentially a companion battery life chart for your "Which Flashcart Should I Get?" chart. I decided to start gathering the information because DStwo KILLS battery life on DS Lite compared to the carts I've used for testing that don't have onboard processor (CPU? Co-processor? something like that), and there seemed to be a variation between AK2i & DSTT's maximum battery life results also.
If anyone wants to help, by providing their own battery life tests (for the flashcarts featured on the chart in the first post here) PM me for now. Once I have enough basic data, I can include data from other people to provide a rounder average (and my 3DS's and DSi XL are not brand-new, so they may not reflect maximum battery life.
If someone has done this before and I missed it, let me know, so I can add already recorded info.
(I know there was some comparison data on the DStwo
GBAtemp review, but I want to include more data than that.)
edit: Found Cannonfoddr's thread, I've already PM'd him about it.