dont know why yours unable to work, lately i have done the same thing in my office, they works too

Once you use more than 50% of the battery capacity, pretty much the battery life cycle will shorthen alot.

who cares, wet car batteries are cheap, and by 50% on 2x NS60, could means hours, even the battery degrade 70%, should still be able to last an hour, still better than the new original battery, unlike car batteries which need to be drain to push starters, need hundreds of amp, you might no longer able to use it after it degrade 30-40%, but for my case, i can use it as long as it can push out like 10 amp. i even can tell all ppl in my office to pass me their old car batteries, and i can put them all in parallel.