Orbik's Technical Manager Simon Coates explains the changes in ermergency lighting and battery technology.
There are a number of battery technologies now being used within the emergency lighting industry to provide the power to the light source upon failure of the normal lighting. The three most commonly-used technologies are
Nickel Metal Hydride (Ni-MH),
Nickel Cadmium (Ni-Cd) and
Lithium-ion. There are other battery technologies that can also be used, such as Lead Acid, but we’ll concentrate on the three types commonly found in ‘self contained’ luminaires. These are the luminaires where the battery forms an integral part of the luminaire. We’ll look at three major factors that need to be taken into account when specifying this type of equipment:
Operation: the chief differences between each of the battery types, including charging, working environment and energy capacity
Reliability & Safety: the long-term reliability of the battery type and the ease of serviceability along with some of the risks of the use of the particular battery technologies.
Environment: The possible impact/advantages on the environment of the use of particular battery technologies.
THE OVERALL SITUATION
Emergency lighting has recently undergone a revolution, with the widespread use of LEDs as a light source. This has brought with it both advantages and disadvantages over the ‘traditional’ emergency light sources – filament lamps and small fluorescent tubes. The widespread use of LEDs has resulted in changes to the battery requirements, primarily as a consequence of reduced electrical load requirements from the LED source.
Traditional light sources typically required a battery capacity of 4000mA and voltages in the region of 6V up to 12V, depending upon the required light output of the luminaire. Comparable LEDs have reduced this requirement to battery capacities in the region of 1500mA, and with values as low as 800mA and typical voltage of 3.6V now being considered. LEDs have also brought a reduction in the size of luminaire used for emergency lighting. This, in turn, has also created a demand for physically smaller batteries, without any loss of available capacity.
The most popular battery type to satisfy these new conditions is
Lithium-Ion.
There are currently six common Lithium-Ion chemistries in use, not all of which are used in emergency lighting applications. The general consensus within the emergency lighting industry is for the adoption of
Lithium Iron Phosphate (chemical term: LiFePO
4 ) for use in self-contained luminaires. The remaining five available Lithium-Ion chemistries have their place, but not necessarily within the emergency lighting industry and will be ignored for our purpose here.
OPERATION
Of the three battery types under consideration LiFePO
4 has the best power density. In other words, you need a much smaller physical-sized battery for the load you wish to supply. This is referred to as specific energy or energy density,
| Battery type |
Specific energy |
Example size/capacity |
| LiFePO4 |
90 -120Wh/Kg |
Ø22mm x L 65mm – 3.2V/3.2Ah |
| Ni-Mh |
60-120Wh/Kg |
Ø18mm x L 215mm – 3.6V/4Ah |
| Ni-Cd |
40 – 60Wh/Kg |
Ø32mm x L 176mm – 3.6V/4Ah |
OPERATING TEMPERATURES
When it comes to the operating environment, all battery types have their limitations.
Nickel Cadmium (Ni-Cd) will offer good performance over 0 – 54°C. It will endure short term exposure to 70°C and -20°C during discharge.
Nickel Metal Hydride (Ni-MH) performs well over a similar range of temperature, with some manufacturers quoting ranges of -35°C to 75°C for charge and -45°C to 85°C for discharge.
Lithium Phosphate (LiFePO4) has quoted operating temperature ranges of 0 – 45°C and discharge -20°C to 60°C, but experience is suggesting that much greater care needs to be taken when operating these batteries at their extremities.
NOTE: Good advice would be to check with the manufacturer first as extreme temperatures will have an effect upon the life, available capacity and the ability to charge.
CHARGING REGIMES
Nickel Cadmium (Ni-Cd) is the most forgiving with charge. Whilst all three types require correctly controlled charge regimes, Ni-Cd is arguably the simplest. Ni-Cd batteries are current-charged and as long as the charge-current is kept low (typically 0.05 – 0.1C of the batteries rated capacity) then the battery will forgive no-charge control, and overcharge has little effect upon the battery life. However if higher charge rates are required then some form of control is necessary, to prevent damage to the battery.
Nickel Metal Hydride (Ni-MH) batteries require a constant current charge. But additional care is required to ensure reliable operation. Overcharge in Ni-MH can result in venting (release of gas if the cell is over-heated) and long term reduction in of battery performance. Ni-MH chargers are often referred to as 2-stage chargers; this in effect means, typically, an initial 0.1C of the battery capacity followed by a ‘trickle’ charge of 0.025% of battery capacity. There are additional methods which can be adopted to increase the life of the battery such as temperature monitoring to determine the charge rate (dT/dt) as well as monitoring the battery voltage to determine the charge state of the battery.
NOTE: You can use a Ni-MH charger with Ni-Cd battery as they both function with this type of charge but not visa–versa. Again, always check with both the control gear manufacturer and the battery manufacturer.
Lithium Phosphate (LiFePO
4) requires a controlled battery charge regime, typically consisting of an initial current charge up to 60% of final charge followed by a final stage, constant, voltage to the manufacturers recommend cell voltage. LiFePO
4 batteries do have some tolerance to overcharge but prolonged exposure to overcharge creates carbon dioxide resulting in a risk of venting and the addition risk of fire-venting.
NOTE: Before we all panic, remember this occurs during a failure of the charger and not during normal operation.
Additional precautions are required by battery manufacturers to include a thermal protection circuit with each battery supplied and it is only under tight control that ‘raw’ cells can be supplied. Under normal control equipment standards (BSEN 61347) and the battery standard (IEC 61951 and 62625 Nickel based, BSEN62620 Lithium-Ion) this additional temperature monitoring must be included as a failsafe.
HOW MUCH IS ALL THIS GOING TO COST?
A very good question. In terms of energy density the LiFePO
4 is the most cost effective, least effective being the Ni-Cd. But in terms of unit costs, the thing that most people consider, the situation is reversed, with N-Cd the cheapest and LiFePO
4 the most expensive.
NOTE: There are a vast range of battery voltages and capacities currently in use within the industry which makes the ‘purchase cost only’ choice complicated. On-going servicing also needs to be taken into account when looking at the more relevant whole-life costing.
RELIABILITY/SAFETY
Do these battery technologies offer long-term reliable operation? The short answer is yes, but we must pay close attention to the technical specification. The answer is to select quality products and pay attention to the working environment, as these will all have an impact upon system reliability.
Ni-Cd and Ni-MH have a long history of providing continued reliable operation when these factors are taken into consideration. The emergency lighting industry takes great pride in designing compliant luminaires to provide a four-year design life of the battery employed. There is no reason why emerging technologies such as LiFePO
4 should not also have the same reliability designed into products, ensuring extended periods of reliability. Ni-CD have typical duty cycle quotes of 500, Ni-Mh of 500+ (some quoting as high as 5000) and LiFePO
4 1500+ (again with numbers of 10000 being quoted).
A word of caution, the emergency lighting standards for luminaires and batteries have tests for accelerated aging, often more onerous then the battery manufacturers own tests, so this figure will tend to be on the high side. However, this should not have an impact upon the declared 4-year life if life testing is carried out correctly.
COMBUSTABILITY
Lithium Ion batteries have been surrounded by media coverage, concerning potential fire risks. There have been high profile cases involving mobile phones, laptops, children’s toys and, most recently, head torches. However, in all of these cases the fault lay with either a mismatch of charger to battery, or the general quality of battery and/or charger. Where the battery itself is found to be the culprit the fault often lies with inferior products, including some counterfeits.
The primary cause of these risks lies with the build quality of the battery, with failure of the separators used in the construction of the battery. Reputable companies go to great lengths to minimise fire risk by using venting methods, thermal fuses and shut down separators. Often the press coverage has ignored the actual percentages of failure in the use of this technology. Poor quality or unsuitable chargers will directly contribute (and may be the primary factor) of the failure of the battery.
Sometimes failures are related to external factors, rather than the equipment itself; such failure is often associated with incorrect electrical installations. There are recorded instances of damages to luminaires directly associated with shorts on external cabling, and of no direct result of battery failure.
MITIGATING RISK
Nevertheless a risk is a risk and we are obliged to take precautions to reduce risk whenever we introduce any type of battery into a building. Whilst considering the suitability of batteries and chargers lets also think about the physical precautions that could also be taken.
We are all familiar with the ‘fire-rated downlighter’ and the new focus on the ‘fireproof’ compliance of plastic materials (TPa and TPb), so let’s take guidance from this and apply it to the battery technology in question. These batteries are either directly housed inside a luminaire or in close proximity. When enclosed in the luminaire the standards (BSEN60598-2-22) require consideration be given to the flammability of the luminaire. It’s worth noting that arguments are currently circulating within the industry regarding the suitability of this standard to address the issue of luminaire flammability and compliance to building construction, regardless of the battery technology employed. Despite this, sourcing a luminaire that is compliant with, and tested to, these standards represent the basis for good practice.
Be aware, though, that there are a great number of luminaire enclosures that incorporate intumescent materials, and they are considered fit for this purpose. So what should we do about direct contact to such a fire risk from an external influence? Both Ni-Cd and Ni-MH batteries will emit toxic fumes such as cadmium and chloride due to PVC sleeving, as well as flammable gases such as hydrogen – more hazards, then.
With LiFePO
4 itis not necessarily the gases released from the battery but the ease at which a battery can become unstable and result in a fire risk. A LiFePO
4 battery becomes unstable at temperatures as low as 130 -150°C, which can result in thermal runaway. Under a fire condition the protection offered by the control gear and the battery itself will be ineffective. As the battery becomes heated it increases the risk of venting and ‘flame’ venting. Burning lithium is a ‘metal fire’ with temperatures in excess of 1000°C. The danger here is the spread of fire due to the failure of fire compartments at these temperatures.
NOTE: Little is known about the effect upon the construction of fire compartments and the risk of their failure due to lithium fire. The recommendations here would be to consider the use of additional physical precautions, but we wait with interest on the lighting industry and Building Regulations for recommendations on this.
ENVIRONMENTAL IMPACT
Great care needs to be taken in the disposal of Ni-Cd batteries. NiCd batteries contain between 6-10% cadmium. Nickel and Cadmium are both highly dangerous elements. Nickel has reported carcinogenic effects upon humans and exposure to Cadmium, a heavy metal, can result in damage to the respiratory tract, liver and kidneys. This is the reason for an EU-wide ban on NiCd, with few exemptions – one of these being emergency lighting.
Ni-MH contains no such heavy metals and in the USA is considered non-toxic and safe to dispose in landfill.
LiFePO
4 contains no toxic metals and is considered the least toxic of the three types. However, when you consider the environmental impact on production then LiFePO
4 has its critics. The mining of lithium is concentrated in an area known as the ‘lithium triangle’ the deserts of Chile, Argentina, and Bolivia, followed by Australia as second choice.
As the demand for Lithium increases the effect upon the environment of these areas has come under increased scrutiny. Of the three types Ni-Cdhas the least greenhouse gas emissions during manufacture, LiFePO
4 and Ni-Mh produce the most greenhouse gases and they require most energy in their production.
This means that Ni-Cd batteries are cheap to produce and have the least environmental impact during production, but are on the flip side the most toxic of the three technologies discussed. This presents a problem with disposal at end of life. The advice for the disposal of any battery should be to use a registered recycling centre or an approved battery treatment operator (ABTO).
WEEE RESPONSIBILITIES
Under the Waste Electrical and Electronic Equipment Directive (WEEE Directive) there is a responsibility for the supplier to take back electrical equipment including batteries. Note, this service is not free but the supplier should declare the costs and be a registered member of a WEEE compliance scheme.
The CIRCULAR ECONOMY
There are other factors we can consider when looking at environmental impacts. There is a move within Europe to promote a Circular Economy. The EU has a dedicated Circular Economy Plan and is discussing with the lighting industry the proposal for the mandatory removability of lamps and control gear. What this means is the industry should now begin to consider the ease of repair and serviceability of products, shifting away from our historical attitude favouring equipment disposal.
IN SUMMARY
When specifying products involving suitable control gear and battery combinations, always contact the manufacturer for their recommendations. Check the compliance of the control gear (BSEN 61347); select a battery from a reputable supplier and check their products compliance (IEC 61951 and 62625 Nickel based, BSEN62620 Lithium-Ion). The same is true of luminaires (BSEN60598-2-22).