Battery systems in data centres

Every critical facility depends on stored energy. When the utility fails, there is a gap of several seconds to a minute or more before standby generators start, synchronize and pick up the load. Uninterruptible power supplies bridge that gap with batteries, and in most data centres the batteries are the largest store of chemical energy in the building after the generator fuel. Unlike fuel in a tank, battery energy is stored as electricity ready to be released, and it cannot be switched off. A battery string disconnected from its UPS is still a source of hundreds of volts and thousands of amperes; a damaged lithium-ion cell can release its stored energy as heat, flammable gas and fire even when nothing is connected to it at all.

This course is the third in the data centre sites track. HSE-320, Data Centre Safety Essentials, introduced battery rooms as one of the special spaces in a data centre, and HSE-321, Electrical Safety in Critical Facilities, treated the battery as a DC source to be identified and isolated when working on a UPS. HSE-322 goes into the battery itself: the electrical, chemical, thermal and explosion hazards of the battery systems used in data centres, the codes and standards that govern them, how they are detected and controlled, and how to respond when something goes wrong. HSE-323, Fire Suppression and Liquid Cooling Hazards, follows and covers the clean agent and sprinkler systems that protect battery rooms in more detail.

The engineering of battery systems, including sizing, chemistry selection and UPS integration, is covered in DCE-340, Backup Power Systems, especially Lesson 3 on batteries and Lesson 6 on battery energy storage. Electrochemistry itself belongs to the chemistry courses. This course takes the view of the people who work in, near and around battery systems: facility technicians, electricians, battery service contractors, security officers, safety professionals, emergency responders and the managers who make decisions about them.

Why batteries are a special hazard

Most hazards in a data centre can be removed before work begins. Electrical equipment can be de-energized and locked out, moving machinery can be stopped, and a hot aisle can be cooled. Batteries are different in four ways that run through this whole course.

• First, a battery cannot be fully de-energized. Opening a battery disconnect isolates the battery from the UPS, but every cell remains charged. The terminals of each tier, shelf and module stay live. Work on a battery is always work near energized parts unless the battery has been discharged and taken apart, which is rarely practical.
• Second, a battery combines several hazards in one place. It is an electrical source, a container of corrosive or flammable chemicals, a possible source of flammable and toxic gas, and, for lithium-ion, a possible source of self-sustaining fire.
• Third, a battery failure can escalate without any outside help. A single lithium-ion cell that enters thermal runaway can heat its neighbours until they fail too, and a lead-acid battery that is overcharged can produce hydrogen until a room becomes explosive.
• Fourth, the consequences of a battery incident reach well beyond the battery room. Data centre battery fires have taken down national government services and popular consumer platforms for days, and explosions at battery energy storage sites have injured firefighters who arrived to help.

Where batteries are found in a data centre

Batteries in a data centre come in several sizes and forms, and each has its own hazards. The largest and most important are the central UPS batteries, but they are not the only ones.

Central UPS batteries are the focus of most of this course. They are installed either in dedicated battery rooms, separated from other spaces by fire-rated construction, or in cabinets placed next to the UPS modules in the UPS room. Battery rooms are common for large lead-acid installations because of their weight, floor area and hydrogen ventilation needs. Lithium-ion batteries take up far less space and are usually supplied in factory-built cabinets, which has made cabinet installations more common. Each arrangement has advantages and drawbacks, discussed below.

Rack-level battery backup units have become common in hyperscale and AI facilities, where some designs place lithium-ion battery shelves inside each rack instead of using a large central UPS. HSE-321 Lesson 1 noted the move to higher-voltage DC distribution in AI racks; distributed batteries follow the same trend. They spread the energy across the data hall, which reduces the size of any one battery but means thousands of cells sit among the servers, where a failure affects the space people work in every day.

Battery chemistries at a glance

Lesson 4 and Lesson 6 describe the chemistries in more detail. For now, three families matter.

Lead-acid batteries have been the standard UPS battery for decades. Each cell has lead and lead dioxide plates in sulphuric acid electrolyte and a nominal voltage of 2 V. Flooded (vented) lead-acid batteries have free liquid electrolyte, need topping up with water and release hydrogen and oxygen during charging, especially at the end of charge. Valve-regulated lead-acid (VRLA) batteries hold the electrolyte in an absorbent glass mat or gel and recombine most of the gas internally, releasing it through a valve only under abnormal conditions. VRLA batteries are cheaper and easier to install, but they are sensitive to heat and typically need replacement every four to six years in UPS service.

Lithium-ion batteries store energy by moving lithium ions between a graphite anode and a metal oxide or phosphate cathode through a flammable organic electrolyte. The two chemistries most common in data centres are lithium iron phosphate (LFP), with a nominal cell voltage of about 3.2 V, and lithium nickel manganese cobalt oxide (NMC), with about 3.6 to 3.7 V. Lithium-ion batteries are lighter, smaller and longer-lived than lead-acid, tolerate higher temperatures and can be monitored cell by cell. Their chief safety concern is thermal runaway, a self-sustaining reaction that releases heat, flammable and toxic gas and sometimes fire. LFP is generally more resistant to thermal runaway than NMC, but neither is immune.

Nickel-cadmium batteries are found in some older sites and in generator starting and switchgear control systems. Sodium-ion batteries, which avoid lithium and use a non-flammable or less flammable chemistry in some designs, are emerging as an option for data centre backup; as of 2026 a few suppliers are marketing them for UPS and rack use, but installed numbers remain small compared with lead-acid and lithium-ion.

Battery rooms and battery cabinets

A battery room is a dedicated enclosed space for batteries, usually with fire-rated walls, a controlled door, dedicated ventilation, spill containment for flooded batteries, an eyewash station, and signs warning of the hazards. Racks hold rows of cells or monoblocs (multi-cell blocks), connected in series into strings by intercell connectors and cables. The room may serve one UPS or several.

A battery cabinet is a steel enclosure, often the same size as a UPS module, holding shelves or trays of monoblocs or lithium-ion modules, with an integral DC breaker or disconnect. Cabinets are usually placed in the UPS room, beside the modules they serve, which shortens the DC cables.

Neither option is always better. What matters for the people who work there is that each space is designed, signed and managed for the battery hazards it actually contains, and that workers know which hazards to expect before they open the door.

Strings, cells and the DC bus

A UPS needs a DC voltage of several hundred volts, so cells are connected in series into a string. The string voltage is the number of cells times the voltage per cell, and it changes with state of charge: a lead-acid cell floats at about 2.25 to 2.27 V when fully charged and falls to about 1.75 V at the end of a discharge. Several strings may be connected in parallel to provide more runtime or redundancy, each with its own disconnect. HSE-321 Lesson 7 explained how the battery connects to the UPS DC bus and why opening the battery breaker does not make the battery safe.

The energy stored in a string is roughly the string voltage times its capacity in ampere-hours. A string of 240 lead-acid cells (40 monoblocs of 12 V) with a capacity of 200 Ah stores about \(480 \times 200 = 96{,}000\) Wh, or 96 kWh, at nominal voltage. Only part of this can be used at the high discharge rates of UPS service, but all of it is present as a hazard. The short program below compares the strings at Cedar Ridge.

# Illustrative battery strings at the invented Cedar Ridge Colocation site.
# Nominal cell voltages: lead-acid 2.0 V, LFP 3.2 V, NMC 3.65 V (typical values).
strings = [
    ("Hall 1 VRLA string", 240, 2.0, 2.27, 200),
    ("Hall 2 LFP rack", 160, 3.2, 3.45, 100),
    ("Comparison NMC rack", 136, 3.65, 4.1, 100),
]
print(f"{'String':22s} {'cells':>5s} {'nominal V':>9s} {'float V':>8s} {'kWh':>6s}")
for name, cells, v_nom, v_float, ah in strings:
    nominal = cells * v_nom
    floating = cells * v_float
    kwh = nominal * ah / 1000
    print(f"{name:22s} {cells:5d} {nominal:9.0f} {floating:8.0f} {kwh:6.1f}")

hall1_strings = 4
print(f"Hall 1 has {hall1_strings} parallel strings: "
      f"{hall1_strings * 240 * 2.0 * 200 / 1000:.0f} kWh stored in one room")

String                 cells nominal V  float V    kWh
Hall 1 VRLA string       240       480      545   96.0
Hall 2 LFP rack          160       512      552   51.2
Comparison NMC rack      136       496      558   49.6
Hall 1 has 4 parallel strings: 384 kWh stored in one room

The numbers show two things. String voltages for every chemistry fall in the same range, so the shock and DC arc flash hazards are similar whatever the chemistry. And a single battery room can store hundreds of kilowatt-hours, more than enough to injure workers and destroy the room if released suddenly.

Who works on batteries

Several groups of people come into contact with data centre batteries. Facility technicians perform daily and weekly inspections, record readings and respond to alarms. Battery service contractors, often from the battery or UPS manufacturer, perform quarterly and annual maintenance, impedance testing and replacements. Electricians install and remove battery systems and their DC cabling. Security officers patrol battery rooms and are often first to notice a smell, a sound or an alarm. Fire services respond to battery incidents and need accurate information quickly. Each group needs training matched to its tasks, as Lesson 2 and Lesson 3 explain.

Battery energy storage systems on site

A growing number of data centres install battery energy storage systems (BESS) outside the building, in containers or prefabricated enclosures, to take part in grid services, shave peak demand, support on-site renewable generation or reduce generator runtime. DCE-340 Lesson 6 covers the engineering case. A BESS can store tens or hundreds of megawatt-hours, far more than a UPS battery, and usually operates at higher DC voltages. It is governed by the same core standards as indoor batteries, particularly NFPA 855 and UL 9540, and the large-scale incidents studied in Lesson 12 were mostly BESS events. Where a data centre has a BESS, the emergency plan, the fire service pre-incident plan and the training of site staff must include it.

A course roadmap

Lesson 2 maps the law, codes and standards that govern battery systems in your country. Lesson 3 covers electrical hazards, Lesson 4 chemical hazards and Lesson 5 hydrogen and ventilation in lead-acid rooms. Lessons 6 and 7 cover lithium-ion batteries, their management systems and thermal runaway. Lessons 8 and 9 cover detection, ventilation, explosion control, suppression and separation. Lesson 10 covers maintenance, handling, transport and disposal. Lesson 11 covers emergency response, and Lesson 12 brings everything together with published case studies and a battery room risk review at Cedar Ridge.