Sudden (I think) Battery Problem

[RML]

So things were going along nicely, my car was starting OK, then it got a lot colder. The 84 was starting to crank a little slower and I thought it was just the cold weather. I plugged in the block heater overnight and thought all would be good to go in the morning, but the battery wouldn't turn it over. I tried to jump it with my van but it still wouldn't turn over. So I got suspicious. I pushed it into the garage and put the 85 battery in the 84 and it turned over and started perfectly. Not much cranking at all.

I looked in the 84 battery and the water was low. I filled it with distilled water and put it on a battery charger.

This morning I expected to jump in and start it up, but when I turned the key, nothing. No cranking with the 85 battery that was strong the night before. So I assume I have a short somewhere. I am not the best with electrical problems, so where should I start? I picked up a battery tester on my way home today to test each cell of the 84 battery that is charging. The tester is the kind that you suck up some acid into and it gives a reading based on where the guage is floating. Specific gravity, I think it is based on, I guess on the concentration of the acid.

If I can get it started with a charged battery I may pick up a battery terminal cutoff switch so I can prevent it from draining. A friend recommended a good alternator shop that may be able to do some diagnostics for me.

Any other suggestions will be much appreciated.