Battery Solution for Airfield Ground Lighting
I have spent most of the past decade building lithium packs that must work unwatched. Airfield ground lighting is the hardest version of that problem I have taken on. The lights are life-safety equipment, the load is a series constant-current circuit rather than a normal AC appliance, the changeover budget is measured in fractions of a second, and the hardware sits outdoors in whatever weather arrives. This is the sizing method my team uses when an airport asks for a battery solution for airfield ground lighting: what to count, what to specify, and the failures that catch people out.
Why Airfield Ground Lighting Backup Is Not a Normal UPS Job
This is not a data centre UPS with a different label. A data centre tolerates a five-second dip; a runway does not.
The changeover number drives the whole architecture
ICAO Annex 14, Volume I, sets the secondary power supply requirements for visual aids. For a precision approach runway category II or III, changeover must not exceed one second. For category I and for runways used for low-visibility take-off it is fifteen seconds, and non-instrument runways get a much longer window if the operator documents it. Those numbers are not interchangeable: a static UPS can bridge one second and a diesel generator essentially cannot. If your aerodrome needs the one-second case, stored energy sits online permanently and the generator becomes the long-duration layer behind it.
What “essential lighting” means in your layout
Not every circuit has to stay lit. Approach lighting, runway edge, threshold and PAPI are almost always on the standby bus; taxiway centreline and apron floodlighting usually are not. On one regional project we cut the battery from 60 kWh to 26 kWh purely by agreeing in writing which circuits were essential, at which step, for how long. That agreement is the highest-value document in the project.
Reading a Series Constant-Current Circuit as a Load
Airfield lighting circuits are constant current, typically 6.6 A and occasionally 20 A for high-intensity approach, driven by a constant current regulator. Everything in the load calculation follows from that.
Series current, not nameplate watts
In a series circuit current is fixed and voltage floats: total power equals circuit current multiplied by the sum of every fixture voltage drop, so adding fixtures adds volts, not amps. The regulator’s efficiency is typically 88 to 94 percent and its input power factor degrades at low brightness steps. Do not size the battery from a nameplate lamp count; measure the regulator input with a power analyser on site.
Five-step 6.6 A regulators step the output down through roughly 5.2, 4.1, 3.4 and 2.8 A, and since lamp and driver voltage also falls, input power at step two or three is often only 35 to 65 percent of step five. If the requirement is “runway edge at step three for two hours”, sizing for step five doubles the cost for nothing.
Quartz, LED and the number people get wrong
Quartz and halogen fixtures draw 45 to 200 W each on a 6.6 A circuit, and filament resistance rises as they heat, which is mildly self-stabilising. LED replacements drop far less voltage, often 3 to 5 V instead of about 6.6 V, at equal or better output. The circuit current stays 6.6 A, so total power falls and the regulator needs less compliance voltage, which is why an LED retrofit can free headroom to add fixtures. The mistake I see is assuming the battery shrinks by the same percentage as the lamp wattage; regulator losses and controls do not.
A worked count
Take a 3,000 m runway with edge lights every 60 m on both sides: about 102 fixtures. At 4 V each the circuit needs roughly 410 V at 6.6 A, about 2.7 kW at the lamps, and near 3 kW of input per circuit once a 90 percent regulator is included. A single-runway aerodrome with four to six essential circuits lands between 20 and 45 kW of standby load. That is smaller than most people expect, which is why generator-less designs are realistic at smaller fields.
Runtime: Bridging a Generator Versus Replacing It
There are two honest architectures here, sized with completely different targets.
Bridge and ride-through sizing
The bridge design assumes a generator exists. The battery covers generator start, voltage and frequency stabilisation, and the operator’s acceptance checks: 60 to 300 seconds at full load. At 30 kW that is 0.5 to 2.5 kWh delivered, plus margin for the next start attempt. The battery is small and rarely deep-cycled, so calendar life dominates. I still specify a lithium battery rather than lead-acid, because maintenance visits cost more than the cells and lead-acid capacity at minus 20 degrees Celsius is a fiction.
Generator-less runtime and the derating stack
The second design carries the load alone, typically two to eight hours. Regional aerodromes choose it when a diesel set runs a few hours a year and still needs monthly exercise, fuel polishing and an environmental permit. The arithmetic is unforgiving. Delivered kWh equals load kW multiplied by hours, divided by inverter efficiency of 0.92 to 0.95, by usable depth of discharge of 0.8 to 0.9, and by a temperature factor. At minus 20 degrees Celsius an unheated lithium iron phosphate pack delivers roughly 60 to 70 percent of rated capacity, so either accept a 1.4 to 1.7 factor or heat the compartment and pay the heater load instead. Add the heater, controls and monitoring gateway to the load, then size the cells.
Inverter and Regulator Compatibility
This is where most battery-backed airfield lighting projects fail their first site test, and rarely because of cell quality.
SCR notches and control-loop oscillation
Constant current regulators are phase-controlled thyristor or triac units. They chop the sine wave, producing commutation notches, a high crest factor and substantial current harmonics. A conventional inverter with meaningful output impedance, or a constant-power loop, will fight the regulator’s own current loop. The symptom is visible flicker at every step change and, occasionally, a nuisance trip. Specify an online double-conversion inverter with voltage distortion under 3 to 5 percent when feeding a six-pulse thyristor load, and low output impedance up to a couple of kilohertz. Then test the real regulator, not a resistor bank: a step sweep between steps one and five, and a stability run at the lowest step.
Earthing, ground-fault detection and isolation
Series lighting circuits are floating or corner-earthed, and carry their own insulation monitoring. An isolation transformer between the inverter and the regulator keeps the airfield ground-fault scheme from treating the battery and inverter as part of the lighting loop; the standby neutral must never be bonded so that fault detection sees the battery system. On cold start, LED drivers and legacy magnetic transformers both pull inrush current, so confirm the inverter holds voltage while several circuits ramp together.
Cold, Coastal and Airside Conditions
Enclosure and microclimate
Northern sites see minus 35 to minus 40 degrees Celsius. Lithium iron phosphate cannot accept charge below zero and discharges poorly below minus 20, so the answer is an insulated compartment held between 5 and 25 degrees Celsius, with 200 to 600 W of heating for a small vault. Sodium-ion cells tolerate cold far better, and a battery that simply works at minus 30 removes the heater from the critical path. De-icing fluid, jet fuel, salt fog, UV and blown grass all attack enclosures, so specify the coating and gasket system you would use for taxiway signage, keep equipment clear of the runway and taxiway safety areas, and give responders a documented way to isolate the DC side.
Commissioning, Telemetry and What the Tower Needs
A battery that works but is not monitored is a liability with a warranty.
Tests I will not sign off without
A capacity test at site ambient. A load-bank test through the real regulator. A changeover time measured at a fixture, not at the contactor, because cable and transformer behaviour belongs in the number. A step-change flicker check across all five steps. A cold-start test after at least twelve hours of soak. Cells and packs are qualified to UN 38.3 for transport and IEC 62133-2 or IEC 62619 as a pack; stationary installations follow UL 1973, with UL 9540 and UL 9540A where the local code asks. The lighting side sits under the IEC 61822 and 61823 family, with visual aid requirements drawn from ICAO Annex 14 and national equivalents.
The alarm the tower actually needs
Airfield lighting control and monitoring must report three things: on battery, estimated remaining runtime, and a hard backup-unavailable fault. That third one has to reach the control tower before a landing clearance is issued, over dry contacts plus a serial or network link, tested on a schedule and logged. I also recommend keeping one spare regulator circuit’s worth of capacity so a failed string does not force a runway closure while parts ship. A custom battery solution for this duty is judged on availability over ten to fifteen years, not on the datasheet numbers on the day it ships.
Frequently Asked Questions
How long should an airfield ground lighting battery backup run?
It depends on the runway category and what your aerodrome manual promises. With a generator, the battery only bridges 60 to 300 seconds. Without one, two to eight hours is the practical range, and the requirement is usually written as a specific brightness step rather than full intensity. Agree it in writing before sizing anything.
Can a battery replace the standby generator at a small airport?
Frequently yes, for one or two runways with four to six essential circuits. At 20 to 45 kW, two to eight hours is a modest amount of lithium battery, and removing the generator also removes monthly exercise runs, fuel polishing and permits. You gain maintenance simplicity; you take on a firmer replacement budget, because the cells now cycle and age.
Why does the lighting flicker when the backup supply takes over?
Almost always an interaction between the inverter and the phase-controlled regulator. The regulator chops current, creating notches and harmonics, and an inverter with high output impedance or an aggressive constant-power loop will oscillate against it. The fix is electrical, not mechanical: a double-conversion inverter with low output impedance, under 3 to 5 percent voltage distortion into a six-pulse load, tested against the real regulator rather than a resistive bank.
Which battery chemistry suits a cold-weather airfield?
Lithium iron phosphate with a heated, insulated compartment is the default, for cycle life and thermal stability near fuel. Sodium-ion suits sites where temperature is the dominant risk, since it discharges usefully around minus 30 degrees Celsius and takes heating off the critical path, at the cost of roughly 15 to 25 percent more volume. Semi-solid state cells only make sense when enclosure volume is the binding constraint.
How do I size a battery for a 6.6 A series lighting circuit?
Measure the regulator input with a power analyser at the step you will operate, then add your other essential circuits. Do not sum lamp nameplate watts, because the circuit is constant current and the regulator’s efficiency and power factor vary with the step. Once you have the kilowatts, divide by inverter efficiency and depth of discharge and apply your temperature factor.
Does switching to LED fixtures reduce the backup power I need?
Yes, but less than the wattage comparison suggests. LED fixtures drop far less voltage on the same 6.6 A circuit, so total power falls and the regulator needs less compliance voltage. However, regulator losses, controls, monitoring and heating do not fall with the lamps, and the inverter still faces the same inrush on cold start. Expect a real reduction with a smaller tail than the marketing implies.
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