Battery dry room design splits one room into zones, the driest held at minus 40 degrees Celsius, and Fraunhofer says that dry air can cost 60% of a factory total energy

Battery dry room design follows a ladder, not one number

Battery dry room design does not aim for a single dryness level, it follows a ladder that runs colder as the cell chemistry gets more sensitive to water. A dry room is a sealed room built to hold air far drier than any home or office, measured by dew point, the temperature air must cool to before water condenses out of it. Canrud, a battery equipment supplier, names minus 40 degrees Celsius, under 1% relative humidity, as the baseline for most lithium ion cell assembly. Tighter processes, including silicon and lithium metal work, run at minus 50 to minus 60 degrees. Emerging chemistries push to minus 60 to minus 80 degrees.

A real 55,000 square foot room shows the ladder in steel and concrete

In 2021, Bry Air built a 55,000 square foot dry room for a prismatic cell pilot line, split into 2 zones at 2 different dew points. The fill room, used for electrolyte filling, the step that reacts worst with moisture, was held at minus 40 degrees Fahrenheit, the same temperature as minus 40 degrees Celsius. A second, less dry assembly room ran at minus 22 degrees Fahrenheit, about minus 30 degrees Celsius by conversion, a figure Bry Air itself never states in Celsius. Keith Hoge, director of the battery business segment at Bry Air, explained the choice behind the split.

The drier room is reserved exclusively for highly moisture sensitive operations like electrolyte fill, since that is the more valuable real estate.

Keith Hoge, director of the battery business segment at Bry Air. Source 4.

An average person releases 1,500 to 2,000 grains of moisture an hour through breathing and sweat, Bry Air states, which is why the moisture load inside a dry room scales with crew size.

What holding that air dry costs at a real pilot factory

The Fraunhofer Research Institution for Battery Cell Production, a German public research institute, runs its own pilot battery factory, the FFB PreFab, in Munster, Germany. Its clean and dry rooms run on 8 separate dehumidification systems and hold a dew point of minus 60 degrees Celsius, dried with electric heating rather than gas, through a 3 rotor system. Fraunhofer states its clean and dry rooms use between 25% and 60% of the plant total energy, and that the range comes from which cell chemistry the factory is running.

The colder the target, the more air has to move

AFRY, an independent engineering firm with no dry room equipment to sell, names the mechanism behind that cost. Holding a room at minus 40 to minus 50 degrees Celsius takes 30 to 60 air changes an hour, complete replacements of all the air in the room. Holding the same room, same crew, at minus 60 degrees takes 180 air changes an hour, 3 to 6 times as many, a ratio calculated here from the 2 AFRY figures above. Moving that much air needs equipment large enough that AFRY says the air handling gear for the tightest class of clean room can occupy the entire floor area of the room it serves.

The design lesson

The lesson holds at any scale. Reserve the coldest, most expensive air for the 1 step that cannot tolerate anything less, and let every other zone run warmer. A separate MAOWCE article, battery dry room, covers the condensation risk this same design work is built to avoid.