You will be surprised at how long the batteries will last. I left a Bushnell for 15 months once (it uses 8 sized AAA batteries), and it was still working when I retrieved it. You will not fill 8GB in 3 weeks.
The real issue is your desire to check the results (and, the need to be sure it is working properly). Don't sweat about the batteries or the data card, just check it for the right reason.