This question was asked by a reader in the US, where standards differ slightly from the UK and Europe (see boxout below).
The US standards for emergency lighting are set by the International Electrotechnical Commission (IEC), of which the US is a member. Annex A of the emergency lighting luminaire standard (IEC 60598-2-22:2014) calls for emergency lighting batteries to have a rated life of four years of normal operation. It recommends a maximum discharge rate of 40 per cent of rated battery capacity for a 90-minute duration. This means that a 4.5Ah battery should discharge at 1800mA for 90 minutes of use (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. However, if an emergency light fitting is only providing 90-minute duration 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 be lasting 90 minutes at the end of that time.
If it fails to last its required 90 minutes 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.
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