The electrolyte should only be topped up when the battery is
pretty much fully charged and still on the charger. The level will rise
while charging, and you can overfill a battery by bringing the acid to
the bottom of the fill neck and then charging it. Results in spilled
acid. Spilling acid and replacing it with distilled water reduces the
SG and shortens battery life.
Charging a battery at high amperage heats it up, too, and can
wreck it.

Many batteries now really ARE sealed, and have stuff in them to
take the gases and convert them back to liquid, so they never need
topping up. It's called recombinant-gas technology. Often found in
emergency lighting, backup power systems, and we are encountering them
in aviation. Wouldn't be surprised if a few autos have them now. Cut
those open and you ruin them.

Dan