[Kevin Sevcik]

Lead acid batteries are, in fact, remarkably good at dying. That is, if you don't store them properly. See, SLA charges are stored in sulfate ions. When you charge the battery, you force the ions into solution in the electrolyte. Conversely, when you discharge the batteries, the sulfate ions come out of solution and bond with the lead plates, forming lead sulfate and giving up their extra electrons to create current and all that. So far so good. The problem comes when you leave a highly discharged battery just sitting around. All the lead sulfate originally forms as a very loose highly divided mass. But left to its own purposes, it slowly coalesces and forms lead sulfate crystals. Which are highly stable and which won't dissolve back into the electrolyte just because you'd like them to. This is called sulfation, and every bit of sulfation your battery undergoes removes that little bit more of its capacity.

So, in summary, SLAs should be stored well charged. Preferably on something putting out a float charge to combat self-discharge.

As for your particular problem, in addition to bad wiring it, your SLA could be suffering from sulfation. However, it could also have a shorted cell or an open. There's plenty of this that can go wrong in there that you won't be able to see.