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Impact of batteries on emergency lighting evolution

Posted: 06 Jul 2015     Print Version  Bookmark and Share

Keywords:emergency lighting  battery  NiCd  nickel cadmium  luminaire 

We barely give a thought to emergency lighting on a daily basis. The only time that may change is when we need to find an alternative source of illumination because the lights have gone out at home and we are rummaging around to find a torch (hopefully with a working battery), or the box of matches and candles.

Once that alternative light source has been located and powered up, a sense of calm prevails and this is true be it in a domestic or public building. Hopefully not many of us will experience a light failure in a large public building such as a theatre, cinema or department store, but it is exactly this scenario, of hundreds of people in an unfamiliar environment, taken unawares and wanting to reach safety, with which emergency lighting is designed to cope. However to be able to respond on that one occasion emergency lighting needs to be powered and ready to go 24/7 and this is where the role of the battery comes to the forefront.

Within any emergency light solution you need three elements; an energy source, a convertor and a light source. In relation to a candle these are represented by the wax, the wick and the flame. In an emergency light the battery is the vital energy source but it has a number of weaknesses the first of which is that it will discharge and degrade over time, secondly battery performance can be impeded by extremes of temperature and thirdly they self-discharge which shortens their shelf life.

As they have evolved, different types of battery have been developed for different applications. The battery in a power drill is designed to be charged and discharged regularly whilst for emergency lighting the ideal battery is one that can remain permanently on charge and which will only be discharged once a year. This difference in behaviour is primarily determined by the varying chemistry within the battery cells.

NiCd, nickel cadmium batteries, are able to operate in both scenarios and were first developed for emergency lighting over fifty years ago. Their robust nature that allows them to be continually charged at high temperatures has ensured their place as the 'go to' solution for designers of emergency lighting, despite them being not only a relatively large component of any luminaire, but also an expensive component. In the first batteries of this type six or even ten individual cells were combined but over time, as engineering techniques and efficiencies improved, the number of cells required to emit the same level of energy was reduced down to between three and six. At the same time luminaires evolved using new generations of flourescent lamps, providing a more efficient solution, so the first step in the reduction of size without a compromise on performance and safety was complete.

NiCd versus NiMH
Over recent years and especially with the introduction of LED lighting there has been a continuing drive to reduce the size of luminaires but in many cases this has been restricted due to the need to provide a suitable housing for the emergency lighting batteries. With each cell of a NiCd battery measuring approx 35 mm diameter and 70 mm length and with electronic lamp driver circuits reducing down to low profile 21mm heights, its easy to see why luminaire designers were looking for an alternative battery solution. The introduction of NiMH (nickel metalhydride) batteries which can provide a comparable level of energy with a smaller diameter of only 18mm – 22 mm was the catalyst for the next step in the evolution of emergency lighting.

However it has not been a straight forward transition as NiCd batteries utilise a more robust technology than NiMH, which requires more care and attention when being charged and discharged in order to maintain an acceptable and comparable life expectancy. The technology is particular vulnerable to damage if charged continually at a high temperature and many emergency luminaries are typically running with internal ambient temperatures of 40°- 50°C every day of the year. As a response to these arduous conditions emergency system manufacturers, such as Tridonic, have developed NiMH compatible drivers specifically designed to meet the needs of the emergency market and which can offer a life span of four years plus in a typical application.

Such products often utilise pulse or intermittent charging techniques that charge the batteries in short burst of current with rest periods in between. This method gives optimum performance from the NiMH technology but must never be used with NiCd batteries. On the converse side it is possible, with great care, to have a common constant current charger that will charge both technologies.

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