National Electrical Code (NEC) Article 480 and Section 690.71 address battery installation and containment, and should be referred to prior to designing or building your battery enclosure. In most instances, residential battery systems are limited to 50 VDC nominal. (Requirements for battery packs operating at greater than 50 VDC nominal are not addressed in this article.)
Regardless of battery type (sealed or flooded), adequate ventilation is required to “prevent the accumulation of an explosive mixture.” While ventilation specifics are not clearly outlined in the NEC, some important considerations are identified. In the NEC Handbook, an explanation is given for Section 480.9 (A), stating that “hydrogen disperses rapidly and requires little air movement to prevent accumulation. Unrestricted natural air movement in the vicinity of the battery, together with normal air changes for occupied spaces or heat removal, normally is sufficient. If the space is confined, mechanical ventilation may be required in the vicinity of the battery.”
Because hydrogen is lighter than air and will tend to concentrate at ceiling level, the NEC Handbook states that “some form of ventilation should be provided at the upper portion of the structure. Ventilation can be a fan, roof ridge vent, or louvered area.” A common approach used to meet these requirements, especially when flooded batteries are used, is the inclusion of one or more air intake vents installed low on the battery enclosure, used in conjunction with a pipe-connected exhaust vent that routes gases to the outdoors.
All live parts of battery systems, including terminals and cable lugs, are required to be guarded, or covered, to protect against the possibility of an electrical short if a tool or other metal object is inadvertently dropped across the batteries. In addition, access to the battery bank should be limited, either by locking the battery room or enclosure, or restricting access with some other permanent means (Article 110.27).
The battery enclosure cover or doors should allow adequate and convenient access to the battery bank for qualified people, and adequate working clearances should be provided (Article 110.26).
Finally, the NEC Handbook includes the following reference to flooded versus sealed batteries: “Although valve-regulated batteries are often referred to as ‘sealed,’ they actually emit very small quantities of hydrogen gas under normal operation, and are capable of liberating large quantities of explosive gases if overcharged. These batteries therefore require the same amount of ventilation as their vented counterparts” (Article 480.9).