Can water be added in the large removable "eye" or is it necessary to break the seal on the individual cells and add water ? Thanks for your help.

 

thru the individual cells

 

Yes, the eye unscrews. I have added water to mine this way. Use distilled water. Fill it up until the plates you barely see inside are about 1/4 inch submerged. It will take a few minutes for the eye to indicate any change once it is re-installed.