Some info on lead-acid batts here..
http://members.optusnet.com.au/~weza/
I once talked online with a fellow who was making small saltwater cells.He said he could get a few AH capacity from a string of small ~1-2 cubic inch cells.
I was thinking of making a few large cells,from 3-4 foot lengths of pipe (copper and galvanized?),tube-in-tube design,inside some 4-inch PVC pipe to hold each cell.
The cells produce ~0.5V each,so you'd need ~24 cells for a 12V system.
In short,he thought I was crazy,saying the capacity of such a battery would be "extremely enormous".(He was estimating up to perhaps 2000AH(!),depending on the exact details.) And it would no doubt weigh alot,one filled!
I forget exactly what he was using for electrodes,But he said electrode material can make a large difference in performance/capacity. Copper and Zinc could be a good start.
The only problem is corrosion of the electrodes over time.
I was thinking of storing the battery dry,and only filling it (with salt+water) when it's needed (in emergencies,etc.) and draining/rinsing it out before putting it back into the shed.
Saltwater isn't a huge hazard to wildlife,or groundwater,etc. so disposal/storage isn't a huge issue.(Still want to mind where you dump it though.)
Of course,if it's been used alot,it may turn kinda greenish or something..Depending on the dissolved metals/chemicals it contains,you may need to be a little more mindful when disposing of it.
But atleast it's a cheap/easy to get electrolyte.
Something like a (much) larger version of this:
http://dragonet.com/fhp/
Just a random thought I had.