Thx guys, I've done my research on this one
Sealed lead acid AGM / deep-cycle marine batteries (100Ah +) will set you back
$250 + but you'll also need a specialised Deep Cycle Charger, $100 +
I've looked into those small generators (GMC, Homelite) 720W etc,
$150 + but I hear that they're messy, noisy, and a dirty source (voltages spike - not good for ol' laptop).
I went with the laptop battery option. For
$100.00 shipped I got two new '9 cell laptop batteries' of Ebay, genuine Dell brand (easy to charge, no specialised 12v deep cycle charger needed, and no fuel / oil mixtures).
Which brings me to my above questions - how to minimise laptop battery drain
Logie.