Emergency Lighting Battery Chemistries & Cell Sizes Explained

Two batteries with the same voltage and capacity may not be interchangeable. The chemistry determines how the battery charges and performs under load. The cell size determines whether it will physically fit inside the luminaire. Getting either wrong means the emergency light may not operate correctly when it is needed.

The Early Years — Nickel-Cadmium (NiCd)

Nickel-Cadmium batteries became the standard for emergency lighting in the 1970s and 1980s and remain in widespread use today.

NiCd established itself for four practical reasons:

• Mechanical robustness. NiCd cells tolerate the charge-discharge cycling that emergency lighting demands without significant degradation.
• High-temperature performance. NiCd operates reliably at elevated temperatures inside sealed emergency light fittings where ambient temperatures can reach 40°C or higher during normal operation.
• Predictable cost. NiCd manufacturing scaled rapidly across multiple suppliers — GS Yuasa, Tridonic, Liteplan, Mackwell — creating a competitive market that kept costs stable for luminaire manufacturers.
• Clean voltage increments. The 1.2V nominal cell voltage means pack voltages build in clean 1.2V steps — 2.4V, 3.6V, 4.8V, 6V, 7.2V — which map well to low-voltage DC emergency circuits.

Common NiCd Cell Sizes

NiCd emergency lighting batteries are built from cylindrical cells. The cell size determines both the physical dimensions of the pack and the available capacity.

The D cell and Sub-C cell dominate UK emergency lighting. If an existing emergency pack uses D cells or Sub-C cells, the replacement will almost certainly use the same cell size — the physical fit is determined by the luminaire housing.

The Move Towards Smaller Batteries

From the 1990s onward, emergency luminaire design trended towards thinner, more compact fittings. Surface-mount and recessed LED emergency lights replaced the bulkier fluorescent battens that had dominated the market. The battery pack had to follow the same trajectory — smaller, shorter, narrower — without sacrificing capacity.

D cells, at 34mm diameter, became a physical constraint in these slimmer fittings. C cells (26mm) and Sub-C cells (23mm) offered a narrower profile while retaining sufficient capacity for the standard 3-hour emergency duration. The Sub-C cell became the default choice for 1.5Ah to 2.2Ah packs — a capacity range that covers a large proportion of modern maintained emergency luminaires.

Nickel-Metal Hydride (NiMH)

NiMH emerged as an alternative to NiCd in the late 1990s, driven initially by environmental legislation. The EU Battery Directive progressively restricted cadmium in portable batteries. Although industrial emergency lighting batteries received exemptions, many manufacturers began transitioning their product lines voluntarily.

NiMH offers several engineering advantages over NiCd:

• Higher energy density. A NiMH cell of the same physical size as a NiCd cell can store roughly 30–40% more energy. This allows either longer emergency duration from the same pack size, or the same duration from a smaller pack.
• No cadmium. NiMH cells eliminate the toxic heavy metal entirely, simplifying disposal requirements for end-of-life batteries.
• Reduced memory effect. NiCd cells can develop voltage depression if repeatedly partially discharged and recharged. NiMH cells are significantly less prone to this, reducing maintenance callouts for batteries that test poorly but are not actually faulty.

NiMH also has a narrower operating temperature range than NiCd. In high-temperature environments — such as sealed emergency fittings in unconditioned plant rooms or south-facing external bulkheads — NiCd may remain the better choice.

Common NiMH Cell Sizes

Lithium Iron Phosphate (LiFePO₄)

Lithium Iron Phosphate is the newest major chemistry in emergency lighting. It has become the preferred technology for many modern LED emergency luminaires designed within the last five years.

LiFePO₄ differs from NiCd and NiMH in several fundamental ways:

• Cell voltage is 3.2V, not 1.2V. A 4-cell LiFePO₄ pack produces 12.8V, compared to 4.8V from a 4-cell NiCd or NiMH pack. LiFePO₄ packs cannot be substituted for NiCd/NiMH packs without changing the emergency control gear — the voltage is fundamentally different.
• Longer cycle life. LiFePO₄ cells sustain several thousand charge-discharge cycles. Tridonic quotes a design life of up to 12 years for their LiFePO₄ range under appropriate operating conditions — maintained within their specified temperature range, charged with compatible control gear, and not subjected to deep discharge beyond the design specification.
• Higher energy density by weight. LiFePO₄ packs are lighter than equivalent NiCd or NiMH packs of the same capacity.
• Excellent thermal stability. LiFePO₄ is intrinsically safer than other lithium chemistries and does not experience thermal runaway under normal operating conditions.
• Flat discharge curve. LiFePO₄ maintains a stable voltage throughout most of its discharge cycle, which suits LED emergency lighting where the driver circuit requires consistent input voltage for the rated emergency duration.

The trade-off is cost. LiFePO₄ cells are more expensive to manufacture than NiCd or NiMH cells, and packs typically carry a higher unit price. The longer design life partly offsets this over time.

Common LiFePO₄ Cell Sizes

Choosing the Correct Replacement Battery

Match the Chemistry

The replacement battery should use the same chemistry as the original. The emergency control gear — the charger circuit inside the luminaire — is designed for a specific chemistry. NiCd chargers use a different charge termination method to NiMH chargers. LiFePO₄ chargers operate at a different voltage entirely.

If the original battery is NiCd, fit NiCd. If it is NiMH, fit NiMH. If it is LiFePO₄, fit LiFePO₄. Some modern emergency LED drivers are compatible with both NiCd and NiMH — but this must be confirmed from the luminaire or control gear documentation before substituting.

Match the Cell Size

The replacement battery must physically fit inside the luminaire housing. Emergency light fittings are designed around a specific cell size and configuration. Even if the voltage and capacity match, the physical dimensions will not.

Match the Configuration

Two batteries with identical voltage, capacity, chemistry, and cell size may still not be interchangeable if their physical configuration differs:

• A Stick (single row end-to-end) is longer and thinner
• A Side by Side (two parallel rows) is wider and shorter
• A Twin Stick (two separate groups connected by leads) allows placement in two different positions within the fitting

[Diagram: Side-by-side comparison of Stick vs Side by Side vs Twin Stick configurations for the same cell count.]

Match the Connector

Emergency lighting battery packs use a variety of connectors: AMP plugs, JST connectors, spade terminals (tags), bare flying leads, and proprietary connectors such as Tridonic’s R2A system. The correct replacement must use the same connector — or the installer must be prepared to re-terminate the connection, which should only be done by a competent person.

[Photograph: Labelled connector identification guide showing AMP, JST, Tags, and Flying Leads.]

Summary

Several hundred distinct emergency lighting batteries are in regular use across the UK. This guide has covered the most common chemistries, cell sizes, and the principles for identifying the correct replacement. Many specific battery models fall outside the most common types — specialist packs for specific luminaire models, bespoke configurations, and discontinued products that remain in service.

If you cannot identify your battery from the label, or if the label has deteriorated beyond legibility, BatteryVault can help. Provide the voltage (measured or printed), the number of cells, the physical layout, the cell dimensions, and a photograph of the connector — and we’ll identify the correct replacement from our catalogue. For batteries that are no longer in production, we can arrange bespoke pack manufacture to the original specification.

Related Guides