@AlfaMan @Doc1
So what should you fill your 12 volt battery with - distilled water or hydrogen peroxide 3%?
SmithJ, you need a hydrometer to assess the condition of your battery's electrolyte. I'm not going to explain their use here, as there are tons of vids out there on the subject. I will note that changing your electrolyte with fresh acid - without doing anything else - almost never works. I've seen other people mention hydrogen peroxide in this thread, but I don't have any experience with that. I use a magnesium sulfate solution as part of my reconditioning regime if it seems to be warranted.
Note that you really, really need to able to access the battery's individual cells to be able to gauge its potential for reconditioning. Without that ability, such as in completely sealed batteries, you are at least partially flying in the dark.
Some supposedly-sealed batteries actually have small, removable caps under the decal on the top. Others don't. I greatly prefer those with the caps, but have saved batteries without them in the past by using the 200 amp intermittent charging method I described earlier in the thread.
There is no need - or point to - filling individual battery cells with anything other than distilled water unless they show proof of being bad. Then only partially fill the bad cell with restoration chemicals or additional acid. Usually a battery never needs new acid, but I have encountered batteries where, apparently, the (clueless) previous owner emptied all the acid and just refilled it with water!
There are three ways of assessing an individual battery cell. You can test the voltage of the cell with a common multimeter, you can test the specific gravity of a single cell with a hydrometer and, using my 200 amp charging method, you can see which cells are failing to bubble or are only bubbling weakly.
In my experience, only adding chemicals or only using an electronic desulphater is of limited utility. I'm not saying these things aren't worth doing, but only that they're rarely solutions by themselves.
Hope this helps.
Best
Doc