Exactly right. GA

I do worry that this person had to anonymously post a safety question here to get an answer.

Perhaps there's a war on, Boss.

Yes.

You should also be aware that lead-acid batteries can produce hydrogen gas (explosive when mixed with air). Click here for a list of articles about battery safety.

I've added water to several UPS type batteries that were in different states of charge. I had to pop off the plastic cover, gently remove the rubber caps, and use a hypo I got with ink refills to inject the water.

They all died, none would take a charge. I'm pretty certain I used distilled water.

All cells should of same gravity. If you do not have hydrometer to check, then atleast fill to level all cells with distilled or Demineralized water to the Max level printed outside the cell box.

When filling any lead acid battery, it should be considered that if overfilled, the electrolyte when it expands on charging, will dump the electrolyte acid out into the surrounding area.

If properly delegated YOU will be safe.

Are these wet-cell batteries? Should you be putting any liquid in them at all.

The Majority of new UPS batteries today are VRLA or AGM batteries that are sealed and Do Not require Acid or Distilled Water to be added.

If they are wet-cell batteries... See Crabtree's response..It's spot on. (GA for that one).

Regards,
Sapper