Many years ago, I and several other people got a bunch of used military BB600 flooded NiCd batteries. These generally last a LONG time, and are very durable.
I've had a large pack of them (from one of the other peoples projects) sitting in my shop for years. I finally realized I might be able to use them to replace the nearly completely dead pack of 6V Golf Cart batteries in my old Taylor-Dunn cart.
I started sorting through the old cells, picking those that looked to be in fairly good shape. (some were bulged, some had partly melted posts, etc...)
I first tried using 30 cells, but they were unable to keep the voltage up when pulling 150A uphill. (Plus, I don't think i re-initialized them properly.)
I sorted through the pile of cells, there were about 40 that looked normal, the remaining 150 or so were all bulged. Since I needed to get 60 good cells, I choose another 35 of the "least bulged" ones to initialize.
Since these cells had sat unused for 15 years, I had to run them through the full cell re-initializion sequence. Put a dead short on the terminals for 12 hours to make sure they were zeroed out. Charge them up, THEN top off the water level with distilled water.
Then, drain them back down to 0, as they are highly likely at different charge levels due to adding water. Then do another full initialization charge, 15A for 3 hours. (they are 30AH cells, so this is a 50% overcharge)
A bunch of the cells (as expected) vented water all over the place. After they had all cooled down again, I cleaned them all off, and let them sit for another day. I then checked every cell of the 75 cells I was working with. I found there were 61 that were almost exactly at 1.37v. The remaining ones were several hundredths higher or lower, and one was at 0v.
I built the new pack of 2 strings of 30 cells, and it seems to be working well now. I also added a cheap little Drok V/A/AH meter to help me keep track of the charge level.
I did actually find the old military BB600 manual to use again.
I just did some digging through my history. We originally bought these (surplus) in 2005. All mine were destroyed in a shop fire in 2006. The ones I have now were from someone else, who used them in a small pickup truck. They eventually stopped using them and gave them to me. (I don't know if he switched to other batteries, or the conversion had an issue, or whatever...) After being somewhat abused by the previous owner, and sitting for so long, they are definitely in rough shape. It's also interesting that the nickel plating on the copper busbars has almost completely failed. What little is left is curled and flaking off.