Battery Acid is Cloudy
When Flooded Lead Acid (FLA) batteries are new the acid electrolyte will be clear of any debris or discolouration.
Over time, though, battery acid may become cloudy light grey. Don’t worry; this is OK! It’s related to sulfation being removed from the battery plates, a normal part of the charge cycle. It will be most common after equalizing the batteries.
However if the electrolyte is murky dark grey or black or contains black debris it may be a sign that the batteries have been overcharged and/or improperly maintained. If the batteries have sustained damage, it’s probably permanent and will result in a reduced capacity and longevity. Your best course of action is to ensure your charging and maintenance regime are correct for your battery type, size, and age going forward. This will maximize whatever life and performance remains in the battery bank.
Note that an aged bank needs a different charging regime than a new bank. Consult your battery owners manual for details and consider a professional inspection and review.