The two balls 

It will provide a bright green indication if the battery is charged, a clear and bright red indication if the electrolyte level is low.

Advantages:

Determines the battery capacity at a glance.

Basing on specific gravity, indicates stable charging state.

Provides temperature compensation to assure accurate reading at any battery temperature.

Indicate a low electrolyte level

Reduces the demands fpr OCV(open circuit voltage) testing, thereby saving time.

Reduces customer warranty claims.

Improves end-user satisfaction.