Energy in an emergency: How the evolution of the battery is having a knock on effect in emergency lighting
Emergency lighting is something we give little thought to 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 (see Fig 1) 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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Other benefits of NiMH is the absence of any heavy metal such as cadmium, which means they are not subject to recent European legislation which sought to ban the use of cadmium in all re-chargeable batteries. However the EU, recognising that emergency lighting was a special case has given a special dispensation, under the auspices of the Battery Directive, to this particular sector to allow the continued use of NiCd. But there is a caveat; with manufacturers being obliged to instigate a comprehensive recycling programme and declare their success or otherwise at meeting specified recycling targets. So whilst the NiCd batteries may be both cheaper to manufacture and are ideally suited to the emergency lighting application, the additional cost of the recycling programme is also forcing the industry to look for yet another alternative.
The current estimate from Tridonic is that 30% – 40% of emergency lighting luminaires are now utilising the smaller NiMH batteries, despite a significant increased cost over NiCd that would provide an equivalent performance.
Where are we now and a sign of the future
There is no doubt that the requirement for smaller LED luminaires will continue as designers continue to demand ever slimmer luminaires for the sleek interiors expected in any 21st century building. One attractive option is the Li-ion battery as it can be produced in a variety of finished forms. Tridonic is now leading the way in the development of such technologies and firmly believes that this could be the way forward. These new batteries are a highly attractive alternative to the NiMH models as they are about 60% of the equivalent size compared to NiMh and each cell provides 3.7 volts in comparison to the 1.2 volts of each cell in the NiMH battery. This means that a three cell battery can be replaced by a single cell battery that is actually cheaper, however it is never that simple. The big problem is that due to the volatile nature of Lithium ion, charging is made much more difficult. Managing the charging characteristics is key and each battery requires an electronic protection circuit that will monitor the condition of the battery and shut it down if it becomes unstable.
A further consideration is what happens when a battery needs replacing. All NiCd batteries designed for emergency lighting are interchangeable, making maintenance relatively easy. With Li-ion the replacement cell needs to be an exact replica and the emergency driver unit will conduct a ‘handshake’ check to confirm it is the right unit prior to charging recommencing. This additional built-in electronic capability obviously adds to the cost, bringing them back in line with NiMH prices. On the upside, once the Li-ion battery has been charged it will then retain its charge for a long period of time, far longer than the NiMH, so overall choosing Li-ion can deliver a significant energy saving when you consider the number of emergency luminaires likely to be found in any one building.
Both observers of the lighting industry and the key players have been surprised by how quickly the switch to LED has taken place, many being caught out by the market’s willingness to change. It is not going to be as quick or easy when it comes to emergency lighting power supplies. Tridonic believes that for some applications the Li-ion option will be the way forward in the next couple of years and that will coincide with the proposed 2016 European review of legislation. At this stage it is still unclear if the collection targets of 45% for NiCd will be met and just what the ramifications would accrue. But that is not the end, there are other energy sources that have yet to be fully explored including super capacitors and hydrogen fuel cells, but those are for discussion at a later date.
The old candle solution was actually very clever. Even if you leave it untouched in a drawer for 10 years, providing you have the correct convertor your emergency light source will perform. The challenge in the 21st century for luminaire manufacturers is to develop a battery with an equivalent level of reliability compared with the candle but which meets the safety and style requirements of a modern generation.
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