While doing my monthly truck maintenance - drain water separator, check fluids, tire pressure, etc. I looked at the battery hygrometer eyes and noticed one was dark.

Looking again it suddenly occurred to me that these are not maintenance free / unmaintainable batteries Popping the caps I see all the cells could use some water and a few cells are down to exposing the top of the plates.

Fortunately I have distilled water on hand so I topped them off. With the proliferation of the unmaintainable batteries in so many vehicles, I suspect I'm not the only one who missed this little detail.

 

Glad to know that..hadn't paid attention to mine while going over all the other stuff!