Why a battery needs a dry room
A lithium ion battery cell is assembled and filled with a liquid called an electrolyte, which carries the charged particles that move between the two ends of a cell as it charges and discharges. Nearly every lithium ion battery uses an electrolyte containing a salt called lithium hexafluorophosphate, which reacts with water to form a corrosive acid called hydrofluoric acid. Because ordinary indoor air always carries some moisture, factories assemble and fill each cell inside a dry room, a sealed room with air far drier than any home or office. One peer reviewed study measured what happens without that protection. A sample of the electrolyte exposed to ordinary room air for a short time had its pH, a scale that measures how acidic a liquid is, crash from about 6.5 to under 1, corrosive enough to etch the glass holding it.
How dry the air actually runs
Dryness inside a dry room is measured by dew point, the temperature the air would have to cool to before water starts to condense out of it. A colder dew point means drier air. One dry room studied directly held a dew point of minus 45 degrees Celsius during the working day, rising to minus 25 degrees Celsius overnight, when no cells were being filled. A laboratory line at a battery research center in Germany ran its dry room at minus 70 degrees Celsius. Munters, a company that builds dry room equipment, says most of the battery factories it works with hold relative humidity under 1%, and describes a typical industry dew point target of minus 40 to minus 70 degrees Celsius.
What holding that air dry costs
Keeping a room that dry takes a lot of power, because the air has to be pulled out, stripped of its moisture, and pushed back in, 24 hours a day. A peer reviewed review of an industrial cell production line at a Johnson Controls factory found the dry room was the second largest energy user among the 10 steps that make a battery, using 29.37% of the total energy at the plant. Only the ovens that dry the electrode coating and recover its solvent used more, at 46.84%. A newer review aimed at full scale battery factories backs up that scale, stating that drying and the dry room together use more than 75% of total manufacturing energy.
Figures are from a peer reviewed review of an industrial pilot scale line at a Johnson Controls facility. The remaining 4 of the 10 named steps each used under 3% of total energy and are left off to keep the chart readable.
Show the numbers
| Drying and solvent recovery | 46.84 |
| Dry room | 29.37 |
| Stacking | 5.80 |
| Slitting | 5.35 |
| Enclosing | 5.20 |
| Calendering | 2.86 |
A far smaller line, a far bigger share
Not every dry room runs at that scale. A separate study measured energy use step by step on a small laboratory line at the Karlsruhe Institute of Technology in Germany, built for research rather than mass production. There, the dry room used 91.2% of total energy, far more than the 29.37% on the industrial line above. The authors of that study say their line is oversized for how few cells it actually makes, 42 times the energy share found on the industrial line. That laboratory number describes one small research line, not what a real factory spends.
This line is a small laboratory line at the Karlsruhe Institute of Technology, not a full factory. Its own authors say its dry room share is oversized for how few cells it makes, 42 times the share measured on the industrial line in the chart above. The 29.37% figure on that industrial line, not this 91.2%, is what this article uses for what a real factory spends.
Show the numbers
| Dry room | 91.2 |
| Formation | 2.9 |
| Coating | 2.2 |
| Degassing | 1.4 |
| Calendering | 0.8 |
| Packing | 0.6 |
| Vacuum drying | 0.5 |
| Electrolyte filling | 0.4 |
Who builds the equipment
Paul Blackmore, a project manager at the Energy Innovation Centre at WMG, explained why his team picked a particular equipment maker for a dry room built to test and train people on lithium battery cells.
I selected Munters due to their historical experience in providing low dewpoint solutions within battery applications.
Paul Blackmore, project manager at the Energy Innovation Centre at WMG. Source 5.
Munters, the maker Blackmore chose, says its dehumidifiers can cut energy use by up to 30% against unnamed conventional systems, and cut the energy used to reactivate its drying material by up to 45%.