MarDep have introduced some rather vague regulations for Lithium Ion batteries in Pleasure Vessels. Attached, below.
Members purchasing Li-Ion batteries with a combined rating of above 600Wh should ensure that the batteries, whether they are for propulsion or house use, are rated as IEC 62619 or IEC 62620.
Li-Ion batteries MUST have a battery management system to disconnect the batteries if they reach over-temperature, high/low voltage and charging/ discharging current limits. Batteries should be located away from hot areas, be securely fastened down, be away from water and have a battery disconnect service fitted.
A fire detection system should be fitted when Li-Ion batteries are in an enclosed compartment. Batteries should not be in a habitable space unless they can be vented outside the space. More information can be found in ISO/TS 23625.
COP Class 4 - Chpt XI - Battery requirement.pdf