Before taking the risk of doing something that might cause an explosion
it is a good idea to acquire the knowledge humans have gathered about explosions
.... since some of that knowledge came “the hard way.”

On lead acid batteries it is good to learn about:

the tool “battery hydrometer”
specific gravity range of cells
finding “weak” cells
finding shorted cells
when to drain a cell and refill with distilled water
what chemicals can help remove sulfation deposits

in the old days miners and submariners needed to know lead acid battery science

Today a small group of solar panel owners own lead acid battery banks and end up learning the science

Tony does not seem to be
an old submariner,
old industrial battery field representative, or
modern day solar power enthusiast.

I think you mean sulfuric acid, not water

No, I mean refill with distill water, apply a charging current, and measure how the specific gravity increases as chemical compounds dissolve off the plates, then... when to stop, remove fluid from cell and refill with appropriate s.g. electrolyte.

As E-booger noted, voltage is not necessarily the sign of a good battery; only a quality load tester can determine if a battery is "good".

The reason I bought one when I go to junk yard to get $25 batteries (used in my golf cart) haven’t bought a ‘new’ battery since the cost went thru the roof.

100% correct and did I chase my tail for about 5-6 hours a couple of weeks back on this. And I should know better
What tripped me up was I was mowing, jumped off to move a branch and didn't shut the PTO off so it stopped the engine. I jump back on turn the PTO off hit the key and NADA, zip zero. Thought it blew the main fuse. NOPE. Got my other unit towed it up to the shop. VOLTAGE was OK and having just started the unit 30 minutes earlier I NEVER EVEN considered the battery itself. I proceeded to chase my tail thinking something was shorted or one of the safety interlocks was causing the issue.
Chased it from the battery all the way to the solenoid/ starter. Every time i checked the V it was right at 12.5 or 6. I finally got lucky by leaving the meter hooked up and watching it drop to 2.4 volts with a decent load and then recover within about 30 seconds.
In over 50 years I have never had a battery just fail like this one did. As they say you learn something new everyday. i can guarantee you i won't get burnt by this one again

Unneeded, any decent load will suffice. Put your meter on the battery, read the voltage, try to start the engine and read the voltage. That will suffice. Most times just turning on the headlight is sufficient.

I believe in high amp load testing,
and use a 130 amp Mac model number B17300 that has been durable.

PulseTech company says the computer program inside their charger
tests for “Charge Acceptance” to rate battery capacity in a superior way to just voltage.

They list USA Patents
7834592B2
8269465B2
8269466B2

I wonder personally if measuring the
“internal impedance” of a lead acid battery
is of “real world” value.
This can be hand calculated by measuring voltage at two (or more) different amp loads.

Where can you buy sulfuric acid for a battery ?

Most auto parts stores should be able to get it for you. 5 gallon box or 1 1/2 gallon box. Buying locally will save all the pain of paying for shipping.