Old School Tech has just posted an article about off-grid lead acid battery principles. While lead-acid batteries are not the optimum choice for long-term use, they are relatively inexpensive for a two- or three-year solution as long as they are well-maintained and properly used.
Another advantage of lead-acid batteries is that, in certain circumstances, there may be lots of them lying around (albeit starting batteries rather than the preferred deep cycle variety). However, thanks to modern keyless entry and other high-tech gizmos that constantly burn power when the ignition is off, most of them will be rendered worthless in a few months, or weeks in some cases, if not harvested early.
While deep cycle batteries are preferred, starting batteries can still be used effectively if managed properly. First, make sure all the batteries in a string are of the same model and relatively similar charge and discharge characteristics. A future article will piggy-back on a similar OST article about what this means. Also, make sure the entire starting battery array is composed of healthy batteries. Finally, only use the starting battery array to about 10% to 15% of its capacity, unlike the 35% recommended in the OST article for a deep cycle battery array.
Not mentioned in the article is a battery reconditioning legend that a mix of the right chemicals will restore a dead battery. This is a complete myth; the only thing that these techniques do is fool the hydrometer and voltmeter tests into registering the battery as viable when it is not, typically by bumping up the surface charge effect mentioned in the article. A usage test will reveal this deception. Lead acid batteries typically fail because the plate material has fallen off is lying on the bottom of the cell as unrecoverable sludge. No amount of magic chemicals is going to make that sludge jump up and reconfigure the plate.
In some circumstances, a weak battery has a buildup of undesirable compounds on the surface of the plates, and these techniques remove those materials, exposing viable plate material beneath. The remaining battery will have some useful life left, and would be better than nothing, but your long-term plans in general should not count on storing reconditioning chemicals. You would be better off storing additional brushes, filters and fuel lines for your generator, and figuring out how to make fuel for it.