You can use a battery hydrometer to measure the specific-gravity of the electrolyte.
As you can see from the diagram, the "float" rises in the "battery acid" when the battery gets charged "more-and-more." This means the acid gets "heavier" not lighter, when the battery is fully charged - as the denser fluid pushes the float higher out of the water, just like a ship floating in salt water compared to fresh water (plimsoll line - or load line) .
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