Buyer guide
Battery dry room design: dew point, dehumidification, envelope
A battery dry room controls moisture, so its target is a dew point and not only a particle class. Industry sources cite −30 to −40 °C dew point, and −40 °C works out to 0.079 g of water per kg of dry air by calculation, but the cell maker's process sets the real target.
By ZY Cleanroom · Updated · Sources at the end of the page
Dew point targets
Cleanroom Technology (8 September 2023) says dry room air is typically held at or below −30 °C dew point. A sponsored article in Charged EVs (3 November 2021) calls −40 °C, under 1% RH, the generally accepted level for lithium battery production. It adds that moisture-sensitive chemistries may need drier air.
ISO 14644-1 classifies air by particle concentration, so it sets no humidity target. Specify the particle class and the dew point separately. The table is calculated at 101,325 Pa on a frost-point basis, using the Murphy and Koop (2005) vapour pressure formulas. At −40 °C it matches the 0.08 g/kg in the Charged EVs article.
| Dew point | Vapour pressure, Pa | Water, g per kg dry air | Water, ppmv | RH at 20 °C |
|---|---|---|---|---|
| −30 °C | 38.0 | 0.233 | 375 | 1.6% |
| −40 °C | 12.8 | 0.079 | 127 | 0.55% |
| −50 °C | 3.9 | 0.024 | 39 | 0.17% |
| −60 °C | 1.1 | 0.007 | 11 | 0.05% |
Say which dew point you mean
If a sensor reports dew point over supercooled water, −40 °C means 18.9 Pa and 0.81% RH at 20 °C, about 1.5 times wetter than the table. Write the basis into the user requirements, with the sensor position.
Moisture load and dehumidification
People and airlock openings are the main internal loads (Charged EVs, 2021). A desiccant dehumidifier supplies air drier than the room, and the difference carries the load away. Some process steps use glove boxes, which can reach −65 °C dew point (Process Sensing).
Outside air carries far more water. Air at 25 °C and 60% RH holds 11.9 g/kg, about 150 times the −40 °C level. Each 10 m³/h of that air leaking in brings about 140 g of water per hour. Both figures are calculated.
Dehumidifier airflow example
Dry air flow equals the moisture load divided by the room humidity ratio minus the supply humidity ratio. The table assumes a load of 1 kg/h of water, a room at −40 °C dew point, and air at 1.2 kg/m³. All values are calculated. A −60 °C supply saves only about 24 percent of airflow compared with −50 °C.
| Supply dew point | Supply water, g/kg | Dry air for a 1 kg/h load |
|---|---|---|
| −50 °C | 0.024 | About 15,200 m³/h |
| −60 °C | 0.007 | About 11,600 m³/h |
Envelope tightness
Steel facings block vapour. Leaks occur at joints, door frames, cut-outs and penetrations. Size airlocks for people and carts per hour, because every opening moves wet air in.
Specify a test, not a word. ISO 9972:2015 measures the air permeability of a building or part of one by fan pressurisation. It does not cover single components. Set the test pressure and the limit in the user requirements.
Treat the dry room AHU casing the same way. HTM 03-01 Table 8 uses BS EN 1886 class L2 at +700 Pa and −400 Pa for hospital AHUs, a useful model for casing leakage.
Why the electrolyte step is dry
Process Sensing says electrolyte filling must be dry because the lithium salt reacts with water vapour and forms hydrogen fluoride.
Who approves the room
The cell maker validates the room against its own process. A safety or building regulator approves the owner's facility, not the contractor. To have a room list reviewed, send it to ZY Cleanroom, a cleanroom design-and-build contractor with its own panel factory in Tianjin, China.
Questions
Common questions
What dew point does a lithium-ion dry room need?
Industry sources cite −30 °C or drier, and −40 °C as generally accepted. The process and chemistry set the value.
Is −40 °C dew point the same as 1% RH?
No. At 20 °C it is about 0.55% RH on a frost-point basis and 0.81% over supercooled water (calculated).
What causes most of the moisture load?
People working in the room and airlock openings (Charged EVs, 2021).
Does a particle class cover a dry room?
No. ISO 14644-1 classifies particles only. Add the dew point and leakage test to the user requirements.
Related guides
- How to audit a cleanroom panel factory, on site or by video
- How to Import a Cleanroom from China
- How long does it take to build a cleanroom?
- How to choose a cleanroom contractor: questions and evidence
- Turnkey cleanroom contractor
- Room list builder
Sources
- Phil Laking, Designing a dry room for lithium battery manufacturing, Cleanroom Technology (8 September 2023; checked 31 October 2026)
- Charged EVs, How to choose the right dry room dehumidification solution for lithium-ion battery production (sponsored content, 3 November 2021; checked 31 October 2026)
- Murphy and Koop (2005), Review of the vapour pressures of ice and supercooled water for atmospheric applications, Quarterly Journal of the Royal Meteorological Society 131, 1539-1565 (formulas used for the calculated values; checked 31 October 2026)
- ISO 14644-1:2015, Cleanrooms and associated controlled environments, Part 1: Classification of air cleanliness by particle concentration (published 9 December 2015; checked 31 October 2026)
- Process Sensing, Why is moisture measurement critical in lithium-ion battery manufacture? (undated web page; checked 31 October 2026)
- ISO 9972:2015, Thermal performance of buildings, Determination of air permeability of buildings, Fan pressurization method (published August 2015; checked 31 October 2026)
- NHS England, Health Technical Memorandum 03-01, Specialised ventilation for healthcare premises, Part A (published 22 June 2021; checked 31 October 2026)
Project enquiry
Send what you have
A room list, a floor plan or only the floor area is enough to start. After you submit, send drawings by email or WhatsApp.
We reply by email within one working day. We can sign an NDA before you send drawings.
WhatsApp +44 7962 561184 is our UK number for export enquiries. The factory is in Tianjin, China.
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