Jonathan Bell, commercial director of Liteplan, on why the emergency lighting industry has found its perfect battery…
Gladly things have moved on since the days of bulky valve regulated lead acid (VRLA) batteries, which although served us well for many years, had many limitations. With fluorescent lamps requiring less power, the smaller nickel cadmium (NiCd) batteries took over and have been the mainstay of the emergency lighting market since the 1990s.
Nickel cadmium’s too big
With the move to T5 fluorescent, light fittings became thinner allowing fitting designers to really show off their flair. But this wasn’t great news for NiCd batteries as lighting manufacturers were calling out for smaller emergency control gear and batteries to fit inside their new designs.
Nickel metal hydride can’t run hot
At this time, mobile phone and laptop PC manufacturers started using smaller nickel metal hydride (NiMH) cells. For the same capacity as an NiCd cell, an NiMH cell was nearly half the diameter making them perfect for these types of applications. But the problem with this was the temperature tolerance of the NiMH cells.
Where the traditional NiCd cells could operate up to 57°C, the NiMH equivalent could only withstand up to 50°C. Whilst this doesn’t seem too much of a difference, when you are talking about a small, hot light fitting with no through air, 7°C can mean the difference between having an emergency option in your range or not.
Since the next progression into LED fixtures, this issue has become even more prevalent. Although the even lower power requirements of LED loads have led to battery capacities falling, the fixtures are smaller and hotter still. So another option is needed.
Lithium ion is too unstable
Emergency lighting manufacturers have been looking at what will be the next step for many years now, going through long term testing of various different chemical compositions. As we all know from the power tools and battery powered appliances in the home, lithium-ion batteries have been their dominant power source in recent years.
This is due to the low self-discharge and fast charging capabilities. This seemed to be the natural next step for emergency lighting and all of the manufacturers started looking into the correct charging circuits and capacities they could work with.
Then came the hover boards…
As the hover board debacle showed, if over-charged some lithium-ion battery types could fail and in extreme circumstances, catch fire. After an investigation it was found that the cheaper and more volatile lithium-ion cobalt types were used in these hover boards without any over charge protection circuitry.
The emergency lighting manufacturers then had to carry out far more stringent testing and investigation into the many lithium-ion and lithium iron battery compositions.
Lithium-ion rechargeable batteries are common in portable consumer electronics due to their high energy-to-weight ratios, lack of memory effect and low self-discharge when not in use. They do however have lower temperature tolerances and are generally more unstable. Not suitable for emergency lighting.
But phosphate-based batteries appear perfect…
Lithium iron phosphate (LiFePO4) batteries have been proven to offer a higher energy efficiency, lower self-discharge and higher temperature tolerance. This makes them perfect for emergency lighting applications. Phosphate based technology possesses superior thermal and chemical stability which provides better safety characteristics than those of lithium-ion technology made with other cathode materials.
LiFePO4 cells are incombustible in the event of mishandling during charge or discharge, are more stable under overcharge or short circuit conditions and can withstand high temperatures without decomposing. When abuse does occur, the phosphate based cathode material will not burn and is not prone to thermal runaway. Phosphate chemistry also offers a longer cycle life.
LiFePO4 batteries include an on-board protection circuit module (PCM). This provides protection for the cells from short circuit and controls and regulates the charge/discharge current to maximise reliability and performance.
Owing to the low self-discharge of LiFePO4 batteries, they do not require constant charging like other battery technologies. They are fully charged at commissioning stage, then simply topped up at the point the capacity falls below a certain level. This lowers the parasitic load of a fitting improving the overall power consumption of a scheme.
The other vast benefit of this next step in battery technology is the projected life of the cells. NiCd and NiMH batteries have generally been replaced every four years. Extensive research has shown that LiFePO4 cells can last in excess of eight years. Not great news for emergency lighting manufacturers’ financial forecasts, but excellent news for the industry as a whole.