Internal short in the lead plates, time for a new battery.
If you can see the plates in the battery add distilled water to top it off.
no
Negative plates in the battery are naturally this dark color. A short between plates causes a discharge or equalization in potential which more or less is changing the positive plates and post to the color of the negative. The battery will become useless.
Enough distilled water to cover the plates. Do not open a sealed battery.
Black
A car battery is a rechargeable, or secondary, wet cell battery that contains lead, lead oxide, plates and an electrolyte solution that contains a mixture of water and acid.
Normally the black battery cable is negative.
lead plates,acid,water and the outer plastic case
Electrolyte. Don't add an 'S'.
BLACK
a car battery is made of a plastic casing, lead plates and a sulfuric acid/water mixture