Example product in the 300 watt-hour range:
http://www.batteryspace.com/polymerli-i ... arger.aspx
Menu:
http://www.batteryspace.com/12v-132vreg ... ypack.aspx
These are being used in an off grid application to significantly extend the battery life of a couple of laptops, some USB chargers, Iridium phone and a BGAN.
Don't buy the "consumer" battery packs that look like fat laptops, you can pay really horrible prices... Like $299 for a 75 watt-hour battery. Buy something serious and match the voltage yourself, then use a good quality DC to DC power supply to regulate it to your laptop's needs. BGANs and other things can run directly off 12V.
As an example of battery life, the latest generation 15" macbook pro runs about 6.5 to 7 hours on its own internal 75 watt hour battery:
http://www.anandtech.com/mac/showdoc.aspx?i=3580&p=1 . The new 17" model has a 95 watt hour battery and gets real world figures above 7 hours unless there's constant heavy CPU use (Transcoding mpeg4 video, whatever). You can run a fast laptop literally for days on a pack in the 300 to 400 watt-hour range.
If you have a bunch of low power laptops and not terribly hungry power gear, a six pound lithium ion battery pack in the 300 watt-hour range will last you a LONG time.
