SIM battery life depends on several factors, eg how well exposed the
solar panel may be, sunshine hours and so on. Also worth
double-checking that the solar panel connection to the SIM board is
definitely firmly in place. But there is also some significant
variation in battery life with individual systems and, as someone else
commented, quite a number of users get into the habit of replacing the
battery once a year, as a matter of course.

The lithium battery itself (CR123A) is widely available but in some
places eg Tesco's it's located in the aisle with photographic bits and
pieces rather than general batteries (since this battery is most
commonly used in digital cameras).

Archive interval won't have any effect on SIM battery life - how could
it in a wireless system? [The frequency of data download from the
logger to a PC can shorten the life of batteries in the _console_ if
the console is run on batteries only but of course that's a completely
different issue.]

Feel free to drop me an email if there are any further questions.

John Dann
www.weatherstations.co.uk