The battery in an emergency luminaire is a wear part: it loses capacity over the years and must be replaced before the luminaire can no longer reach its rated duration. Common chemistries are nickel-cadmium (NiCd), nickel-metal hydride (NiMH) and lithium batteries, above all lithium iron phosphate (LiFePO4). LiFePO4 is regarded as particularly long-lived, thermally stable and free of memory effect, while NiCd is being displaced because of its toxic cadmium content.
What does the battery in an emergency light do?
An emergency luminaire battery is a rechargeable energy store in a self-contained luminaire that powers the lamp for the rated duration when the normal supply fails. In normal operation the luminaire is connected to the mains and the charging electronics keep the battery full. If the mains fails, the luminaire switches to battery operation automatically.
This creates an unusual load profile: the battery is kept on permanent charge for years, often at elevated temperature inside the housing, and is rarely discharged – usually only during the annual duration test. How well a chemistry copes with these conditions determines service life and maintenance effort. In central or group battery systems the energy store sits in one place instead; see self-contained vs central battery for the comparison.
Which battery types are used?
| Property | NiCd | NiMH | LiFePO4 | Li-ion (e.g. NMC) |
|---|---|---|---|---|
| Typical life in emergency luminaires | medium | medium | long | medium to long |
| Behaviour in heat | robust | sensitive, ages faster | good | heat accelerates ageing |
| Behaviour in cold | good | moderate | charging below 0 °C restricted | charging below 0 °C restricted |
| Memory effect | can be pronounced | low (lazy battery effect) | practically none | practically none |
| Thermal safety | high | high | very high among lithium types | lower, protection electronics essential |
| Energy density | low | medium | medium | high |
| Environment | contains toxic cadmium | cadmium-free | free of cadmium and cobalt | cadmium-free, may contain cobalt and nickel |
These values are qualitative. Actual service life depends heavily on cell quality, charging electronics and ambient temperature; the figures in the luminaire's datasheet are what counts.
Nickel-cadmium (NiCd)
For decades NiCd was the standard in emergency luminaires: robust and tolerant of permanent charging and high temperatures. The drawback is cadmium, a toxic heavy metal. In the EU, cadmium in portable batteries is largely banned; emergency lighting was long covered by an exemption. Whether and for how long that exemption continues under the new EU Batteries Regulation (EU) 2023/1542 should be checked case by case. NiCd hardly plays a role in new installations.
Nickel-metal hydride (NiMH)
NiMH contains no cadmium and stores more energy in the same volume. In return it is more sensitive to heat and permanent charging. In warm mounting positions, a NiMH battery ages noticeably faster.
Lithium iron phosphate (LiFePO4)
LiFePO4 is a lithium-ion variant with iron phosphate as the cathode material. It is regarded as particularly thermally stable, tolerates many charge and discharge cycles, has practically no memory effect and low self-discharge. Like all lithium batteries it needs suitable charging and protection electronics (a battery management system). Charging is restricted in the cold, which matters for luminaires in unheated areas.
Other lithium-ion batteries
Conventional Li-ion cells, for example with an NMC cathode, offer the highest energy density and allow very compact designs, such as in emergency drivers. They place higher demands on protection electronics and are more sensitive to overcharging and heat than LiFePO4.
How long does an emergency light battery last?
There is no universal figure. Conventional NiCd and NiMH batteries in emergency luminaires are often designed for a service life of around four years, and in practice they are frequently replaced after four to five years. Lithium batteries, LiFePO4 in particular, achieve considerably longer service lives with suitable charging electronics and moderate temperatures.
Temperature has the greatest influence. A rule of thumb from battery engineering: any sustained increase in temperature accelerates chemical ageing. Luminaires under roofs, in plant rooms or in poorly ventilated ceiling recesses therefore lose capacity faster than those in air-conditioned corridors.
Why do batteries lose capacity – and why does it matter?
Every battery loses capacity over the years, even if it is hardly used. For emergency lighting this is critical, because the luminaire must not just light up somehow – it must do so for the full rated duration, for example one or three hours. Which duration applies is explained in emergency lighting duration.
A battery that managed three hours at commissioning may deliver only two after a few years. In daily use the luminaire still shows green and the monthly function test is passed – only the annual duration test reveals the problem. That is why EN 50172 calls for this test; see emergency lighting testing intervals.
Practical tip: Compare the results of the annual duration tests over the years. If luminaires installed in the same year only just reach their rated duration, plan to replace the batteries for the whole group rather than waiting for individual failures.
When and how is the battery replaced?
Replacement is due when the duration test is failed, the status LED reports a battery fault or the replacement interval specified by the manufacturer is reached. What a red indication means is covered in emergency light flashing red.
A few points matter when replacing:
- Use only approved replacement batteries: type, voltage, capacity and connector must match the manufacturer's specification. A different battery can overload the charging electronics or fail to reach the rated duration.
- Have the work done by a qualified electrician: the luminaire is part of a life-safety system.
- Record the date: note the replacement date in the logbook and ideally on a label inside the luminaire.
- Allow for charging time: after replacement the battery needs time to charge. Emergency luminaires are normally designed to reach their full duration again within 24 hours.
- Prove the function: once charged, carry out and record a function test.
NOTLUX luminaires use a replaceable LiFePO4 battery with a service life of at least five years. Batteries are wear parts and excluded from the product warranty; the NOTLUX maintenance contract includes replacing all batteries in year 5 to 6.
How are old emergency light batteries disposed of?
Batteries must not go into general waste. Across the EU, the Batteries Regulation (EU) 2023/1542 sets the framework for take-back and recycling, implemented through national law – in Germany, for example, the former Batteries Act (BattG) has been replaced by the Battery Law Implementation Act (BattDG). Producers and distributors are generally obliged to take batteries back; commercial quantities are often returned via the maintenance contractor or the manufacturer. Tape over the terminals of lithium batteries before transport to prevent short circuits. The exact obligations depend on quantity, battery type and country; if in doubt, ask your waste disposal contractor.
Frequently asked questions
How often does an emergency light battery need replacing?
That depends on the chemistry, temperature and the manufacturer's specification. NiCd and NiMH batteries are often replaced after about four to five years, while LiFePO4 batteries with suitable electronics usually last considerably longer. At the latest, replacement is needed when the annual duration test is no longer passed.
Which battery is best for emergency lights?
For new installations LiFePO4 has largely become established: long service life, high thermal stability, no memory effect and no cadmium. What really matters, though, is how battery, charging electronics and mounting conditions work together.
Can I replace a NiCd battery with a lithium battery?
Only if the luminaire manufacturer explicitly approves it. Lithium batteries need different charging electronics. A battery that does not match the electronics may be overcharged or fail to reach the rated duration.
How can I tell that the battery is weak?
Typical signs are a red or amber status LED, a failed duration test or a luminaire that switches off early during a test. On self-testing luminaires the fault also appears in the test report.
Does a new battery need to be registered or reset?
Many luminaires initially show a charging or fault indication after replacement until the battery is full. A function test should follow. Whether an additional reset is required is stated in the manufacturer's instructions.
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