Battery Box Design and Construction for a Residential Power System
© A. Sindelar

The purposes of the battery box are:

1.  To provide physical protection to the batteries, from tools, falls, dust and debris, etc.
2.  To meet the electrical safety requirements of the National Electrical Code (Sections 480 and 690-71).
3.  To provide a means of collecting and safely venting hydrogen gas to the outdoors.
4.  To maintain a consistent temperature among the cells.

When the battery approaches full charge voltage, the cells will begin to “gas”. This means simply that the plates within each cell are no longer able to absorb all of the energy being sent to them, and the excess energy is used to convert the water in the cells into hydrogen and oxygen gases. Gassing is an important part of the charging process: the process brings weaker cells up to the charge level of stronger cells, and the bubbling action destratifies the electrolyte. The amount of gassing should be controlled but not eliminated.

The hydrogen and oxygen gas mixture is explosive if allowed to collect to a hydrogen concentration exceeding about 4%. It also contains minute amounts of sulfuric acid, which is corrosive to electronics and most metals. As such, a well-built battery box is both sealed and vented, so that the gases may be carried to the outdoors to dissipate. Fortunately, hydrogen is our lightest element, so it easily rises in air. A vent at the top of the box will work. A second low vent allows ambient makeup air to get into the box.

Batteries lose their effective capacity as their temperature drops. The ideal temperature is 77°F. At 0°F they lose about 45% of their rated capacity (R. Perez, 1985). At 50°F they lose about 15%; this is a good design temperature for the interior of a mechanical room, power shed, or other unheated space, and for the batteries. If the building envelope isn’t sufficiently insulated, additional insulation should be placed between the box and outside walls. If it is on a cold slab, the floor of the box can be built on 2X4 sleepers 8-12” o.c. with rigid foam fit between them.

Plywood works fairly well for residential boxes. Most owner-builders and building crews can make a good box, and it is adaptable to specific sites and needs. The wood will quickly decay, however, if it comes in contact with battery electrolyte.

Box construction notes:

Plywood should be ½” thick at minimum; better is ¾”, or a combination of ¾” and ½” panels. The box may be called on to support a ton or more of batteries. “A-C” or “B-C” grade or equivalent is a recommended minimum grade. The best plywood material we have used is called “Baltic Birch”, available through regional wholesale lumber suppliers (or through us). It is ½” thick with nine plies, and comes in 60” by 60” sheets. Besides being visually attractive, it is very dimensionally stable, easy to work, and the 60” dimension reduces waste, as many common battery enclosures are longer than the 48” width of a standard plywood panel.

The inside of the box should be carefully caulked, and then painted or treated with a penetrating sealer, to resist minor spills or corrosive vapors. For best results, the bottom and lower sides may be lined with “pond liner” material, made either of sheet vinyl or EPDM rubber to contain any accidental spills. The liner should extend 6-12” up the sides of the box, with folded corners forming a leak proof container. The outside should be sealed if called for by good building practice. If the front of the box is made to be removable, the cells are easier to remove and replace, but this is generally only necessary with heavier industrial cells (over 150 pounds each).

The cover should be hinged at the top, to allow access to the battery for service. A piano hinge works well, as it prevents warpage and is easy to seal. The cover should be fully weatherstripped. Soft neoprene weatherstrip works very well (Teno or equivalent). Foam also is adequate but won’t last. The cover should have latches; hooks and eyes, especially if they have adjustable machine screw threads, make a good basic latch. A hasp and clip or padlock is wise if children may be around.

A well-designed box will have a base similar to a kitchen cabinet, in order to allow for a toe kick space and to raise the batteries for ease of maintenance. 2X4s on edge work well for this.

The vent should typically be made of 2” PVC water pipe or equivalent. Larger sizes of PVC pipe may be used, but excessive venting can cause too much heat to escape in winter. A good solution is to seal the box well, with a 2” exhaust (top) vent. Then control the airflow by using one or more 2” PVC inlet vents with removable reducing bushings (1½” or as needed) to control air flow. The inlet vent holes are typically 6-8” above the floor of the box. We often just add three to five 1” diameter holes, evenly spaced on front and sides. The exhaust vent should exit through the building wall or roof, with all laterals rising. An insect screen should cover the outside end, sufficient to protect from insects, birds, and rain and snow.

A commercial product called “Power Vent” (available from us) can be installed in a vertical section of the 2” PVC stack, and works well. We always recommend a Power Vent if the batteries are installed within the living space in order to maintain warmer winter temperatures. A small DC fan comes on before the battery state-of-charge reaches the gassing voltage, and a backflow flap prevents reverse flow of air into the room. In systems using an Outback charge controller or FX-series inverter, or a Trace SW-series inverter, the Power Vent operation may be automatically controlled by one of the auxiliary relays in these units.

We can refer you to a good woodworker who has built very attractive and high quality battery boxes and power cabinets for us and for our customers.