Battery is shot. It has at least one torched cell. What is happening is the charge is 'leveling', where the healthier cells are partially recharging their dead neighbor, allowing enough current to crank the engine. Once the car is running, its pretty much running on alternator alone, as the dead cell will almost immediately drain, shorting the battery internally.
I wonder if this primitive realizes that a battery with a dead cell or two can build up explosive gasses (hydrogen and oxygen) in the dead cells, especially now that it has added straight water to them ?
Probably not...
Listen primitive, what happens to old batteries is called sulfation - over time batteries produce lead sulfate - a combination of chemicals that make your battery a battery, (sulphuric acid and the lead and lead oxides that make the plates) and the lead sulfate has become resistant to separating back out into sulfuric acid.. this essentially leaves behind water in the cell that through electrolysis ( it is a battery after all ) gets broken down into hydrogen and oxygen.
Dumping water into your battery was essentially treating the symptom, and not the cause -- AND -- you managed to reset the possibility of detonation.
Well, thats okay.. Wires, rubber, and most metals love to be hosed down with sulfuric acid..