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Optimising your choice of emergency lighting batteries Choosing the right batteries for emergency lighting can deliver energy savings says Peter Adams, Technical Manager at NVC Lighting, an ECA Commercial Associate.

PETER ADAMS Technical Manager at NVC Lighting

E

mergency lighting kicks in when there is a mains supply failure, to help ensure on-site safety and help with building evacuation in the event of fire. Under the Regulatory Reform (Fire Safety) Order 2005, businesses must install suitable emergency lighting in their premises.

Two constituent components for selfcontained emergency lighting are the control gear (module) and the battery. For emergency lighting systems, the battery must be rechargeable and the choice of battery can affect energy use, due to the way it is charged, and have other impacts on the environment.

Battery types:

Traditional batteries in self-contained emergency lighting are Nickel-Cadmium (Ni-Cd) and Nickel-Metal Hydride (Ni-MH).

Each type brings its own benefits and limitations: Ni-Cd: A mainstay for emergency lighting applications. Ni-Cd has been used for many years due to its robust nature and ability to operate even in harsh environments. A drawback, however, is that the battery has a relatively low energy density, making it unsuitable for fitting into newer, more discreet luminaires. A further disadvantage is that cadmium is a hazardous chemical with end of life recovery challenges.

Ni-MH: Introduced as a ‘greener’ alternative to Ni-Cd, Ni-MH has no heavy metal content making it easier to recycle.

Ni-MH also has a higher energy density, making it more suitable for luminaire conversions. One disadvantage, however, is that it is not as robust as Ni-CD and

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Autumn 2021 | Issue 47

more susceptible to degradation from high temperature exposure.

Battery charging and energy consumption

‘Parasitic’ energy demand is the energy used when an appliance is off but still on ‘standby’ power. For most of the time emergency lighting is in standby mode, and its function only becomes apparent if there is a power failure. Naturally, the battery for emergency lighting must remain fully charged, ready to provide illumination when needed. However, the way the battery is charged influences the amount of parasitic energy used.

Ni-Cd batteries prefer constant current charging which results in almost all the delivered load being consumed by the charge duty. This also results in energy being lost as heat as current is continually passed through the battery. Using Ni-Cd, a typical 3W non-maintained luminaire consumes around 2.5W of power per hour.

Ni-MH batteries on the other hand, prefer an intermittent or ‘pulse’ charge cycle (they will degrade if charged by a constant current). This type of charge cycle works by delivering a constant current charge to the battery for an initial 24hrs, after which it relaxes to a ‘pulse’ cycle which applies short ‘top up’ charges interspersed with a longer ‘rest’ period, typically around 5-10 minutes rest to 1 minute top up. Not only does this cool the battery, but it results in less energy being consumed by the charge duty. With Ni-MH, a 3W non-maintained luminaire will consume around 1.3W of power per hour.

Enter Lithium based batteries

More recently, the self-contained emergency lighting sector has seen a

HEALTH TECHNICAL & SAFETY

transformation with parasitic energy consumption with the advent of Lithiumbased battery technologies, and in particular, Lithium Iron-Phosphate (LiFePO4).

Lithium Iron-Phosphate batteries have very high energy density, and thier cell chemistry means LiFePO4 can provide a perfect set up for self-contained emergency lighting. These batteries have a hightemperature tolerance (higher than Ni-Cd or Ni-MH), making them extremely versatile for luminaire conversions, and the lack of heavy metals and hazardous chemicals make them easier to recycle.

NVC’s dedicated PRO emergency luminaire range already uses LiFePO4 and by the end of 2021 the company aims to replace all its Ni-Cd offerings with Lithium-based battery technology.

As with Ni-MH, LiFePO4 does not accept constant current charging so it should be intermittently charged. The application of intermittent charge brings a greater charge efficiency which, coupled with a low selfdischarge rate (3-5% per month compared to 10-20% per month for Ni-Cd), means that with LiFePO4, a typical 3W non-maintained luminaire will use around 0.7W of charging power.

As we know, electricity prices vary, but assuming an average price of £0.145/ kWh and parasitic usage of 24hrs/day, savings can be calculated on an annual basis as follows:

NVC SENECA

NVC SENECA PRO

using Ni-Cd – Charging power: 2.3W

using LiFePO4 – Charging power: 0.7W

(2.3 x 24) = 55.2 watt-hours/day/1,000 = 0.0552kWh/day (x 0.145) = 0.08pence

(0.7 x 24) = 16.8 watt-hours/day/1,000 = 0.0168kWh/day (x 0.145) = 0.0025pence

0.08 x 365 = circa £2.90/annum

0.0025 x 365 = circa 0.90pence/annum

Average saving £2.00 per luminaire per annum

NVC’s dedicated PRO emergency luminaire range already uses LiFePO4 and by the end of 2021 we aim to replace all our Ni-Cd offerings with Lithium-based battery technology. This offers a robust and compliant emergency lighting system that can provide significant savings on parasitic energy usage, with less environmental impact.

Our PRO range is also complemented by an automatic self-test facility which tests in line with BS EN 50172:2004 and BS EN 60234:2012, removing the need for costly and time-consuming manual testing and maintenance.

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