Now, both class societies in the Maritime Battery Forum have rules for energy storage on ships. DNV GL has had energy storage in the rules for some time (access here) and ABS released theirs earlier this year.
The ABS Guide for Direct Current (DC) Power Distribution Systems for Marine and Offshore Applications can be found here.
The DNV GL rules for energy storage systems are found in DNV GL rules for classification: Ships (RU-SHIPS) Part 6 ch. 2, section 1.
In addition also BV has released rules for battery systems. The BV rules can be found in Pt F, Ch 11, Sec 22.