As a kid there were FULL Service gas stations........
Drive in and an attendant or 2 would come skipping out.
One would handle pumping/cleaning windshield and other would pop open the hood...checking belts/hoses and topping off battery levels with a JUG
JUG just like this..
All these years later.....they are still available and easy to use
I have one since 2006 for filling batteries on my 5th wheel and truck
Simple.....holds 2 quarts
I have mine full of distilled water
Pop the battery caps
Insert spout on jug.push down and it fills IF water is needed. Stops when level reaches tip of spout.
Glug, glug, glug, glug, glug, glug........ just topped of my 12V battery.
How hard was that
Any 'auto parts store' have them...O'Reillys , Auto Zone, Kraggen, NAPA (ouch!), CarQuest ---Manny/Moe/Jack
Course there are those 'fill systems'......GREAT for those impossibly tight battery compartments that require one to be a contortionist to blindly reach
But I'm CHEAP and the JUG works great