Running from the car ought to be alright for short periods - like Andrew says, don't run it flat!
When your financial situation improves, you can afford a decent deep cycle battery solution and avoid those over-priced (for what you get), under-performing starter packs. There are many threads on batteries on IIS - here are a few I recall:
http://www.iceinspace.com.au/forum/s...d.php?t=115976
http://www.iceinspace.com.au/forum/s...d.php?t=106902
http://www.iceinspace.com.au/forum/s...d.php?t=103581
http://www.iceinspace.com.au/forum/s...d.php?t=103067
http://www.iceinspace.com.au/forum/s...ad.php?t=94663
HTH