How long should an emergency lighting battery last at the end of its operational life in the US?

This question was answered by Jonathan Bell, commercial director of Liteplan Limited.

European emergency lighting standards are a version of the global standards published by the International Electrotechnical Commission (IEC), of which the US is a member. 

The emergency lighting control gear standard BS EN 61347-2-7 (lamp control gear part 2-7: particular requirements for battery-supplied electronic control gear for emergency lighting (self-contained)), and the emergency lighting luminaire standard BS EN 60598-2-22 (part 2-22: particular requirements – luminaires for emergency lighting) are European versions of the similarly numbered IEC standards. 

The two standards both refer to annex A of the emergency lighting luminaire standard (IEC 60598-2-22:2014) and call for emergency lighting batteries to have a rated life of four years of normal operation. They recommend a maximum discharge rate of 60 per cent of the rated battery capacity for one-hour duration, and 25 per cent of rated battery capacity for three-hour duration. Ninety-minute capacity can be extrapolated to 40 per cent of rated battery capacity.

This means that a 4.5Ah battery should discharge at 1125mA for three-hour duration (3.375Ah), and 1800mA for 90-minute duration (2.7Ah).

Since battery capacity decays with age, batteries normally have more than nominal capacity when new, which means that a new good-quality battery will give well in excess of the rated duration at that point.

If an emergency light fitting is only providing three-hour duration (or 90 minutes in the US) when new, then its capacity will be expected to drop in the subsequent years, and it may not achieve the rated duration after four years.

To put this into context, when a battery from a reputable manufacturer is brand new, it will last well in excess of its required duration. For a battery that is required to last 90 minutes, it may last well in excess of 120 minutes when new. As mentioned above, these batteries are meant to have an operational life of at least four years. As the battery degrades over its lifetime, its capacity will reduce proportionately. In order to comply with standards, the battery should therefore last its rated duration at the end of that time.

If it fails to last its required 90 minutes (or three hours, depending on specification) before the end of its four-year operational life, then it is considered to be a failure. There are many factors, including over-testing and high ambient temperature, that can reduce a battery’s capacity over its life, but if treated properly, emergency lighting batteries should last their full duration well beyond the four years required by the standards.

There is one other point. Emergency control gear is required to control the current at which the load is driven during emergency operation from a battery. The lumen level from the light source should be constant, from the beginning of the discharge to the end of the required period, and also from the beginning of its life to the end of its life. A battery is considered to have failed when it no longer lasts its rated duration – and there should be no difference in light output at any point during operation within the stated life.

To read other questions answered by Jonathan Bell at Liteplan Limited click here.

If you’d like to ask a question about lighting, write to our Application Editor,
John Bullock: askjohn@luxreview.com

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Comments 1

Two separate questions are found here: How long must adequate lighting levels be maintained in a power failure, and how long does the standby service life of the related battery package need to be? The service life question deserves a good deal of consideration because replacing the battery is a fairly costly mandatory action. Best battery standby life requires a slightly more expensive charge-maintenance system that is often provided. The best system will have a closely regulated trickle charge that is current limited to a recommended value. How long does the adequate light level need to be held is a different question. If the need is strictly to enable a safe and orderly evacuation in the event of a fire, most applications would find 30 minutes totally adequate. Clearly the standards are intended to provide for far more that this.

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