Detection, ventilation and explosion control

The previous lessons showed that battery failures develop over time: a lead-acid battery gasses before a room becomes flammable, and a lithium-ion cell usually vents before it burns. Detection systems aim to find these failures early enough to act. Ventilation and explosion control aim to keep the gases that are released from reaching dangerous concentrations, or to limit the damage if they ignite. Together with the BMS and with suppression, covered in Lesson 9, they form layers of protection. This lesson explains each layer, how the layers work together and what site staff need to know to keep them working.

Layers of protection

No single safeguard is enough for a lithium-ion battery room. The approach used by NFPA 855 and by most hazard mitigation analyses is to provide several independent layers, so that if one fails, another acts. A typical arrangement is shown below.

The inner layers aim to prevent failure or catch it early; the outer layers aim to limit consequences. Design and approval of these layers is engineering work. Site staff need to know which layers exist, what their alarms mean, how they are tested and what must not be done to them, such as covering detectors, wedging doors open against a pressure relief design or switching off exhaust fans during maintenance without compensating measures.

Battery monitoring as detection

The first layer of detection is the battery's own monitoring. For lithium-ion, the BMS described in Lesson 6 measures every cell's voltage and temperature and flags abnormal behavior. For lead-acid, battery monitoring systems measure string and block voltages, temperatures and sometimes internal resistance or ohmic values, and can flag weak or overheating blocks. Monitoring is the earliest possible warning, often weeks or months before a failure, but it depends on the failing cell showing a measurable change and on someone acting on the information. The 2022 Pangyo fire, in which the monitoring system reportedly showed no abnormality beforehand, is a reminder that monitoring cannot be the only layer.

Off-gas detection

Off-gas detectors are designed to detect the electrolyte vapor or gases released when a lithium-ion cell begins to vent, before it enters full thermal runaway. Several sensing technologies are used, some aimed at the volatile organic compounds of the electrolyte solvents and others at hydrogen or carbon monoxide. Tests by manufacturers and independent laboratories have shown that off-gas detection can alarm minutes before smoke detection in some cell venting scenarios, giving time to isolate the affected rack, increase ventilation and evacuate. Response time depends on the sensor type, its location relative to the venting cell and the airflow in the room or cabinet.

Off-gas detection is not a single product type, and claims vary. When an off-gas system is proposed, the questions to ask are what gas or vapor it detects, at what concentration, how quickly it responds in tests representative of the actual installation, where the sensors will be placed (in each cabinet or rack, or at room level) and what action an alarm triggers. Typical actions include an alarm to the operations center, a signal to the BMS or UPS to stop charging and open contactors, and starting or boosting exhaust ventilation.

Gas detection for hydrogen and carbon monoxide

Hydrogen detection, described in Lesson 5 for lead-acid rooms, is also used in lithium-ion rooms because hydrogen is a major component of vent gas and is the component that most affects the lower flammable limit of the mixture. Hydrogen detectors are typically placed high, where hydrogen collects. Carbon monoxide detection helps assess the toxic hazard of an off-gas event and can inform the decision on whether it is safe for anyone to approach. Some systems combine several gases in one detector head.

Smoke, heat and flame detection

Conventional fire detection remains essential. Very early warning aspirating smoke detection (VESDA and similar systems), which draws air through sampling pipes to a sensitive detector, is common in data centers and can detect the smoke or aerosols from overheating cells earlier than spot detectors. Spot smoke detectors and heat detectors are used for the fire alarm and to trigger suppression systems, often in a cross-zoned arrangement that requires two detectors to alarm before release. HSE-323 covers fire detection and suppression systems in depth; this lesson's concern is that smoke and heat detection respond later in a thermal runaway sequence than off-gas detection, as the figure in Lesson 7 showed.

Exhaust ventilation for explosion prevention

Ventilation that keeps flammable gas below its lower flammable limit is the main way to prevent an explosion. For lead-acid rooms, Lesson 5 described continuous ventilation sized for hydrogen. For lithium-ion, the challenge is different: the gas is released suddenly and in large volumes during a failure rather than continuously in small amounts. NFPA 69 describes explosion prevention systems, and NFPA 855 accepts mechanical exhaust ventilation designed to keep the concentration of flammable gas below 25 percent of the lower flammable limit of the mixture during a thermal runaway event, typically activated by gas detection, as one means of explosion control.

The design starts from data. UL 9540A cell testing reports the volume and composition of the gas released by a cell in thermal runaway. From the composition, an engineer calculates the lower flammable limit of the mixture; from the volume and the assumed number of cells failing, the amount of gas that could be released; and from the room or enclosure volume, the exhaust rate needed to keep the concentration below 25 percent of the LFL. The lower flammable limit of a mixture of flammable gases can be estimated with Le Chatelier's rule, which combines the limits of the individual components weighted by their fractions.

Le Chatelier's rule can be written as \(LFL_{mix} = \frac{1}{\sum_i x_i / LFL_i}\), where \(x_i\) is the fraction of each flammable component in the flammable part of the mixture and \(LFL_i\) is its lower flammable limit. Inert components such as carbon dioxide are excluded from the sum and then accounted for by scaling, since they dilute the mixture. The program below applies the rule to an illustrative vent gas composition and estimates how much gas a room can hold before reaching 25 percent of the LFL, and the exhaust needed to dilute a given release rate.

# Illustrative: lower flammable limit of a vent gas mixture by Le Chatelier's rule,
# and dilution needed. Composition and release rates are illustrative only.
LFL = {"H2": 4.0, "CO": 12.5, "CH4": 5.0, "C2H4": 2.7, "solvent": 1.5}   # percent by volume
comp = {"H2": 0.30, "CO2": 0.25, "CO": 0.20, "CH4": 0.07, "C2H4": 0.10, "solvent": 0.08}

flammable = {g: f for g, f in comp.items() if g in LFL}
f_total = sum(flammable.values())
lfl_flam = 1 / sum((f / f_total) / LFL[g] for g, f in flammable.items())
lfl_mix = lfl_flam / f_total          # account for the inert CO2 in the vent gas
print(f"LFL of flammable part about {lfl_flam:.2f}%, of whole vent gas about {lfl_mix:.2f}%")

room_m3 = 7.0 * 10.0 * 3.6
limit = 0.25 * lfl_mix / 100
print(f"Room holds about {room_m3 * limit:.1f} m3 of vent gas before reaching 25% of LFL")

for cells_per_min in (1, 4, 16):
    release_m3_min = cells_per_min * 0.06     # 60 L of gas per cell (illustrative)
    exhaust = release_m3_min / limit
    print(f"{cells_per_min:2d} cells venting per minute -> exhaust about {exhaust:6.1f} m3/min "
          f"({exhaust * 35.31:7.0f} cfm)")

LFL of flammable part about 3.84%, of whole vent gas about 5.12%
Room holds about 3.2 m3 of vent gas before reaching 25% of LFL
 1 cells venting per minute -> exhaust about    4.7 m3/min (    166 cfm)
 4 cells venting per minute -> exhaust about   18.8 m3/min (    662 cfm)
16 cells venting per minute -> exhaust about   75.0 m3/min (   2649 cfm)

The results show that even with a good exhaust system, the design depends on an assumption about how many cells fail and how fast. If propagation is limited by module design, the release rate stays within what a practical exhaust system can handle. If many cells fail together, the release can overwhelm any reasonable ventilation, which is why exhaust is combined with propagation limits and with deflagration venting or other explosion protection.

Exhaust systems for lithium-ion explosion prevention share several practical requirements with hydrogen ventilation: intakes at the high points where lighter gases collect, and sometimes low points for heavier components; discharge to a safe outdoor location; fans and motors suitable for the gas; activation by gas detection with supervision of fan operation; and power from a reliable source. They must not be disabled during maintenance without a plan for alternative protection.

Deflagration venting and pressure relief

If gas does accumulate and ignite, deflagration venting limits the damage. NFPA 68 describes deflagration vents: panels or doors designed to open at a low pressure and release the expanding combustion gases to a safe area, so that the pressure inside the room or enclosure stays below what its structure can withstand. Many containerized BESS units and some battery cabinets have deflagration vent panels in their roofs. For indoor rooms, deflagration venting requires an exterior wall or roof to vent through and a safe discharge area, which is often difficult in a data center building.

Deflagration venting protects the structure but not the people near the vent. A vent opening releases flame and hot gas, so the discharge area must be kept clear, and nobody should stand near a battery enclosure's vent panels during an event. Pressure relief also affects what responders should do: opening a door on an enclosure that has been accumulating gas may admit air and create the conditions for a deflagration, with the open door acting as an unplanned vent toward the people who opened it. This was a central lesson of the McMicken explosion.

Combining detection, ventilation and explosion control

In a well-designed battery room, the layers work in sequence. The BMS watches the cells and acts on abnormal readings. If a cell vents, off-gas or hydrogen detection alarms, the BMS or UPS opens the affected rack's contactors, and exhaust ventilation starts or boosts to dilute the gas. If the event grows, smoke detection raises the fire alarm and may trigger suppression. If gas accumulates despite ventilation, deflagration vents or the room's construction limit the pressure. Throughout, the emergency response plan described in Lesson 11 directs people away from the room and gives the fire service the information it needs.