That's great if you can provide an option to store on SD card as my old tablet is really short on internal memory and I have tons spare on the card!
I've been looking into this tonight, and it turns out, quite surprisingly to me really, that this is actually not possible on Android before KitKat in any normal and best practice way.. There simply isn't anything in the SDK that lets me store anything on the SD card. I looked into other ways of doing it, but no matter how I've tried to force it, it wont give me access to do it. I could try some other obscure ways, but honestly, i don't feel like it can be made good enough.
I will say though, that the application is made so that it only stores 7 days back in time as default. All data before that is cleared, so that each day only takes up a single row in the database with min/max values.. If you have the default 16 seconds pr update you really shouldn't have any trouble. My test tablet, have been running with the app non stop and has data as far back as the beginning of may, and it currently takes up 8 mb.
I would have loved to have the option for external storage really, and it probably is possible in some obscure way on Android < 4.4 (its rather easy to implement on newer devices though)