I want to set up a power failover system
I am looking at using a external laptop battery (http://www.global-batteries.com.au/external-batteries-cPath-10-products_id-1329.html)

how would I setup a power failover system?

What exactly do you mean with a "power failover system"?

For which device, application?

What exactly do you mean with a "power failover system"?
when I lose gird/main/AC power I want to be able to switch over to battery power, like a UPS

For which device, application?
the a 12v elphel camera, 12volt netbook

The simplest way is to go buy a cheap UPS from your local computer shop and plug a 240 to 12 Volt power converter (whatever is supplying the 12 volts for your two units) into one of the UPS mains outlets.

Plug the UPS into the mains and if you get a power cut it will automatically switch to battery.

Hey presto.


CS

1 hours+ on UPS is a bit tall order

based on guesstimate (6 watts apox[1] camera only) i can get 22 hours on a 133.00 Watt/Hours battery[2]

[1] 353 in a nutshell - ElphelWiki (http://wiki.elphel.com/index.php?title=353_in_a_nutshell#Power_Supply)
[2] External Batteries (http://www.global-batteries.com.au/external-batteries-cPath-10-products_id-1329.html)

please fact check

I have just reread the manual of the battery and forgot about the on-board battery charger
AC power in thought the battey charging circut , DC power out of the battery circut. D'OH! :)