I have a 2011 CR-V EX (23,000 miles).
After a 800 mile trip I noticed the built-in hydrometer on my battery was not showing green. It was showing dark. So I checked the fluid with a hydrometer and it was OK. I tested the battery with a digital volt meter and it read 12.66 volts, again OK. I brought it to an auto parts store and they tested the battery with a load, again OK.
My question why is the built-in hydrometer not showing green?
If you have an hydrometer and know how to use it ignore the green eye. It’s a sales gimmick. You could shoot a hole in the 5 cells and the green eye would still indicate that the battery was good to go. How useful is that?
I agree with Rod Knox that it’s a sales gimmick and that it’s also not accurate. The prior AC Delco battery in my oldest son’s car showed a green eye and it was so weak that it wouldn’t even start a lawn tractor or hold a charge for half a day.
The green eye of a car’s battery is not a hydrometer, as many people assume. It merely indicates the level of the electrolyte. It works by the principle of light refraction. A dark eye indicates a low level of electrolyte.
This gimmick might be useful for batteries in which you can add electrolyte. It serves no purpose on maintenance-free batteries.
Note that a battery with a dark eye might still be perfectly OK, and that a battery with ample but weak electrolyte will still show a green eye.