Boat Batteries and the BSS: Securing, Venting and Isolating
Checked against its sources on 20 Oct 2026.
Batteries that are not properly secured come first in the Boat Safety Scheme (BSS) list of its ten most common failures. Here is what the examiner checks, and what you can do before the day.
For charging, care and replacement, follow your battery maker’s instructions.
Keep them secure
Batteries must not be able to move more than 10mm in any direction. Boxes, cradles and frames must not move when pushed lightly.
Batteries need to be held down as well as side to side. A cradle or frame that reaches at least half the battery’s height can be enough.
Take extra care when you put batteries back after removing them. Check the straps or restraints afterwards.
Cover the terminals
Every metal part of the battery terminals and connections must be covered or insulated. Covers must be made of insulating material and be in good condition.
A deck board or locker lid can count as a cover only if it is made from or lined with insulating material and is not removed for anything except getting to the batteries.
Ventilate open-vented batteries
Unsealed or open-vented batteries give off hydrogen. They must be kept in a ventilated space, with a ventilation path to the outside.
If they are in a dedicated battery box or space, the ventilation must be at the top or the highest point of the sides, and any ducting must run level or upwards.
If no maker’s figure is available, the BSS gives this guide for minimum ventilation:
Ventilation (mm²) = number of cells x capacity in Ah x 1.935
If your charging source can deliver more than 2kW (about 150A at 13.8V), the BSS strongly recommends fan-assisted ducted ventilation.
Sealed batteries can be kept in an unventilated space only if the maker allows it. Have the maker’s documents ready for the examiner.
Keep batteries away from heat sources.
Keep them away from gas and petrol parts
The top of each battery must be at least 300mm below any metal gas or petrol parts above it. If it is closer, those parts must be enclosed in a shield or conduit made of insulating material.
Battery isolators
- Fit a suitable isolator to each battery or bank of batteries, as close to it as practical. Being easy to reach matters more than being close.
- The isolator, or the means to work it, must be in view or its location clearly marked. A permanent label is the simplest fix.
- Most circuits must pass through the isolator. Equipment that needs power all the time, such as an automatic bilge pump or alarm, can bypass it, but must have a suitable fuse or circuit breaker.
Unmarked isolators are one of the ten most common BSS failures.
Chargers
Battery charging equipment must be ventilated, complete and in good condition.
Lithium-ion batteries
The BSS warns that fire services are seeing more fast-spreading fires from damaged or failing lithium-ion batteries. This includes installed battery banks as well as e-bikes, laptops and other devices charged on board. Read the BSS lithium-ion advice before fitting or charging lithium batteries on your boat.
Sources
All read 20 Oct 2026.