Lithium Battery for Aviation Ground Power Units
The first battery ground power unit I commissioned sat on a Gulf stand where the tarmac measured 68°C at
two in the afternoon, replacing a diesel unit that had been shutting down on coolant temperature twice a week.
The handler did not care about the chemistry inside the cabinet. He cared about one thing: can this machine hold
400 Hz inside limits for a 35-minute turnaround while the crew runs two ovens and the IFE system boots 180
seats. Everything below comes back to that question.
I am Karl Huang, a senior lithium battery engineer, and I have sized packs for mobile and fixed ground power
units, 28 VDC start carts and gate electrification projects. This is the engineering I wish procurement teams
read before they send the one-line enquiry that says “90 kVA GPU, lithium, quote please”. A lithium battery for
an aviation ground power unit is not a larger solar battery: the load is set by power quality, transient
response and the apron environment, not by energy.
What a Ground Power Unit Does on the Ramp
A ground power unit exists to replace the aircraft auxiliary power unit (APU). Parked aircraft need 115/200 V
three-phase at 400 Hz plus 28 VDC for certain types and for starting; the alternative is burning jet fuel and
APU hours on the stand. Two output paths matter, with different consequences: a 400 Hz AC
inverter feeding a six-pin receptacle with interlock pins carries the turnaround energy, while the
28 VDC path is sized by current rather than energy. Aircraft-side characteristics are defined
in MIL-STD-704F and ISO 1540; the supply is covered by ISO 6858, which requires voltage regulation at the
aircraft plug rather than at the cabinet terminals, so you need remote sense at the connector or firmware
compensation for cable drop.
Reading a Turnaround as a Load Profile
The most common sizing mistake is quoting a GPU by its kVA plate. A 90 kVA unit rarely delivers 90 kVA. What we
measure: 15 to 35 kW average on a narrow-body (A320 / B737 class) with packs off and galley, lighting, IFE and
avionics running; 40 to 60 kW for 60 to 180 seconds at door open; 40 to 80 kW average on a wide-body with peaks
near 120 kW during cabin preparation. Turns run 25 to 45 minutes for narrow-body and 60 to 120 minutes for
wide-body.
That is 8 to 15 kWh per narrow-body turn and 40 to 70 kWh per wide-body turn, and a mobile unit does 8 to 14
turns per shift, plus 3 to 6 kWh per hour of driving if it is self-propelled. The 28 VDC path is the opposite
case: an APU start pulls 800 to 1600 A for 20 to 40 seconds, 22 to 45 kW of peak but only 0.15 to 0.5 kWh of
energy. Size from the energy number and the DC path collapses on the first start attempt.
Sizing the lithium battery pack: The Math I Use
The formula is simple; the discipline is in the derate terms: nominal capacity = deliverable energy /
(DoD × inverter efficiency × end-of-life factor).
Worked example, narrow-body apron unit, 14 turns at 12 kWh plus 4 kWh of driving: 172 kWh at the AC output,
divided by 0.94 for the inverter and DC bus, then by 0.90 for the depth-of-discharge floor, then by 0.80 because
warranty and dispatch rules are written against end-of-life capacity. That lands at 254 kWh, so we quote 250 to
270 kWh. Rate check: 90 kVA at 0.9 power factor is 81 kW, or 0.31C, which is comfortable. We also program a hard
floor equal to one full turnaround plus drive-back, about 18 to 20 kWh here, and block dispatch below it. An
aircraft that loses ground power mid-turn is an operational event, not a battery event.
400 Hz Power Quality: Where Inverter Design Decides Everything
This is the part that separates a real aviation GPU from a repurposed industrial inverter.
Windows. Output must stay inside 393 to 407 Hz and 108 to 118 V phase-to-neutral through all
load steps, with unbalance under 3% and total harmonic distortion under 5%. A 400 Hz transformer is smaller than
its 50 Hz equivalent, which is why aircraft use the frequency, but skin effect and core losses rise, so the
magnetics need thin laminations and litz wire in the filters.
Step load recovery. When galley and cabin lighting come on together, the DC-link capacitance
carries the first few milliseconds before the control loop responds. For a 100 kW step over 5 ms with a 10%
permitted dip on an 800 V bus, C = I × t / ΔV = 125 A × 0.005 s / 80 V, about 7.8 mF; we design to
10 to 12 mF because capacitors lose 10 to 20% of their capacitance over a decade. Skimp here and the symptom is
not a shutdown, it is an avionics reset, and the handler will blame the GPU forever.
Cable drop. Reactance scales with frequency, so at 400 Hz a cable has eight times the
reactance it has at 50 Hz and the aircraft cable becomes a real part of the voltage budget. Keep runs under about
30 m, size for 3 × 35 mm² plus neutral on a 90 kVA unit, and compensate at the plug.
Neutral and earthing. Triplen harmonics from IFE seat boxes, LED drivers and galley
controllers add in the neutral instead of cancelling, so we size it at 150 to 200% of phase cross-section.
Aircraft systems are ungrounded with ground fault detection, which a standard EMC filter with chassis Y
capacitors destroys, so use balanced filters and an insulation monitoring device to IEC 61557-8 that alarms
without tripping a live turn.
28 VDC Starting: The Peak Power Problem
At 1500 A everything is a resistance problem. A 15 m round trip of 120 mm² (4/0) copper is about
8.6 mΩ of loop resistance, a 13 V drop at 1500 A, and the starter simply hangs. The rules we apply: keep the
28 V run under 8 m with two parallel 120 mm² conductors per polarity, which brings the loop to roughly
2 mΩ and the drop to about 3 V under cranking; specify a DC contactor rated for at least 2000 A making and
breaking; and log peak current, minimum voltage and duration for every start, because the trend in minimum
voltage flags a degrading cable or lug months before it fails. At 300 connect cycles per day a contactor sees
over 100,000 operations a year, so it is a scheduled wear item rather than a fit-and-forget part.
Choosing the Cell Chemistry for Apron Duty
For a ground power unit the default is lithium iron phosphate, and the reason is thermal, not energy density.
- LFP: 150 to 180 Wh/kg, 350 to 400 Wh/L, 4000 to 6000 cycles at 25°C, self-heating onset
near 250°C. Heavier cabinet, which matters when towing and not at all in a fixed installation. - NMC 811: 240 to 280 Wh/kg, 2000 to 3000 cycles, onset 110 to 140°C. Halves cabinet mass,
and I still will not put it where the cabinet sits at 55°C unattended between turns. - LTO: 50 to 70 Wh/kg, 10,000 to 20,000 cycles, charge acceptance to -30°C. Right for
start carts in cold hubs. - Sodium-ion: 100 to 160 Wh/kg, 85 to 90% retention at -20°C, chargeable when cold, with a
cabinet 25 to 40% larger. Fine for fixed units, painful for towable ones.
I quote LFP for about nine out of ten GPU projects: this machine lives outdoors in the sun next to jet fuel
and de-icing fluid, maintained by whoever is on shift. Robustness beats specific energy.
Thermal Design on a 55°C Apron and at -30°C
Cycle life is brutally temperature sensitive. The same LFP cell that gives 6000 cycles at 25°C gives
3500 to 4000 at 35°C and under 2000 at 45°C. Calendar fade follows the same curve: 1.5 to 2.5% per year
at 25°C and 50% state of charge, 3 to 4% at 35°C, 6 to 8% at 45°C.
The battery is not the only heat source: a 400 Hz inverter at 96% efficiency rejects 3 to 4 kW at 81 kW output.
Cells want 20 to 35°C and power electronics tolerate 70°C, so the clean answer is one chiller with two
loops, 25°C to the cold plates and 40 to 45°C to the inverter. Air cooling works below about 80 kWh in
temperate climates and fails invisibly: the intake filter loads with apron dust and de-icing residue and the
pack derates on the hottest day of the year.
Cold side: charging below 0°C is prohibited because lithium plates on the anode and the damage is
permanent. Below 5°C we allow discharge but hold charge to 0.05C until heaters bring the cells above
5°C. Heating a 260 kWh cabinet from -20°C costs 4 to 8% of pack energy per cold night, which is why we
insist on shore power at the depot rather than self-heating from the pack.
Mechanical, Ingress and Jet-Blast Realities
An apron is a hostile mechanical environment. Our standard specification: random vibration to IEC 60068-2-64
plus 5 to 15 g shock with modules restrained so cell terminals carry no inertial load; IP54 minimum and IP65 or
IP66 where the unit is washed or sits in de-icing spray, with anodised enclosures, 316 fasteners and tin or
nickel plated busbars because glycol attacks bare aluminium; conformal coating to IPC-CC-830 plus an ePTFE
breather vent; captive fasteners and positive cable stowage, because foreign object debris is the fastest way to
lose ramp access; and a jet-blast exclusion distance with chocks or tie-downs at exposed stands.
Safety, Compliance and Airport Acceptance
The paperwork list is longer than the electrical one. Expect UN38.3 with batteries shipped at or below 30%
state of charge; IEC 62133-2 for cells and batteries; IEC 62619 for the system; UL 1973, or UL 9540 with UL
9540A data plus NFPA 855 spacing where the unit sits inside a building; EMC to IEC 61000-6-2 and IEC 61000-6-4
verified with the inverter running, not at idle; and an airport safety case covering CO and H2 gas detection,
vent direction, emergency stops at the connector and cabinet, and a fire response note from the local
authority.
The connector interlock deserves a call-out: the six-pin 400 Hz receptacle has control pins that open before
the power pins separate, and hot disconnect under load both destroys contacts and removes the signal the aircraft
uses to manage its own transfer.
Operations: Charging Windows, Dispatch Rules and Health Monitoring
Mobile units charge overnight: 260 kWh at 0.25C needs a 65 kW charger and about 4.5 hours including taper,
which suits a 16-hour shift and is why we do not design for high-rate opportunity charging on a ramp where there
is rarely a grid connection. Three operational rules matter more than hardware: never dispatch below 30% state of
charge or below the one-turn reserve, whichever is higher; replace contactors on count rather than on failure;
and estimate state of health from coulomb counting plus 1 kHz impedance, since a 25 to 30% impedance rise
precedes the capacity knee by 300 to 500 cycles. Telemetry belongs on the operations desk: state of charge, cell
temperature spread, pack delta voltage, insulation resistance, cumulative amp-hours and connect-cycle counts.
Economics: Battery GPU vs Diesel GPU vs APU Run
The comparison that closes deals is not battery versus diesel. It is battery versus leaving the APU running. A
narrow-body APU burns 60 to 120 kg/h of Jet A-1, roughly 75 to 150 L/h, so a 30-minute turn costs 40 to 75 litres
and, at 3.15 kg of CO2 per kg of fuel, 95 to 190 kg of CO2, plus a maintenance reserve of 100 to 300 USD per APU
hour. A diesel GPU uses 8 to 15 L/h under load, 4 to 8 litres per turn, with an overhaul every 6000 to 10,000
hours and noise of 70 to 80 dB(A) at 7 m. The battery unit draws about 13 kWh from the grid per turn, 2 to 4 USD
at industrial tariff, and runs under 60 dB(A).
Capital runs the other way: a diesel 90 kVA GPU is 60,000 to 120,000 USD against 180,000 to 300,000 USD for a
90 kVA / 260 kWh lithium unit. At 2000 to 3000 turns per year the fuel and maintenance delta pays back in 3 to
5 years, faster wherever carbon or air-quality rules penalise combustion on the apron. Against fixed gate power
the mobile unit wins on flexibility: one machine serves several stands, while fixed 400 Hz and pre-conditioned
air at every gate is a 150,000 to 400,000 USD per-gate civil project.
Commissioning and Acceptance Tests
Before a unit enters service we run six checks and encourage buyers to witness them:
- Insulation resistance at 1000 V: above 100 MΩ to energise, under 10 MΩ is a stop.
- Full charge, two-hour rest, cell delta voltage under 30 mV.
- Constant-power discharge at rated kW: at least 95% of nameplate energy.
- Step load with a scope on the 400 Hz output: dip under 10%, recovery under 50 ms, frequency inside 393 to
407 Hz. - Thirty minutes at rated load then an infrared scan; any joint more than 15 K above its neighbours gets
re-torqued. - Insulation monitoring device test with a 30 to 50 kΩ injected ground fault: alarm present, output not
tripped.
Frequently Asked Questions
How many aircraft turnarounds can a battery ground power unit serve on one charge?
A 260 kWh unit with a usable window of about 230 kWh delivers roughly 15 to 20 narrow-body turnarounds at
12 kWh each, or three to four wide-body turns at 50 to 60 kWh. We program a reserve equal to one full turnaround
plus drive-back, so the operator-visible number is a couple of turns lower than the raw arithmetic.
Can a lithium battery ground power unit start a jet engine?
It can start an APU, and it can supply the 28 VDC for engine start on aircraft that use electric starting,
provided the DC path is designed for the current. The requirement is 800 to 1600 A for 20 to 40 seconds with the
plug held above about 22 V, which means parallel 120 mm² conductors, a short cable run, and a contactor
rated for the making current. Energy is trivial; conductor and contactor design is not.
What size lithium battery does a 90 kVA ground power unit need?
For 14 turns a day at 12 kWh plus driving, about 170 kWh of AC energy, the calculation is
170 / 0.94 / 0.90 / 0.80, which lands near 250 kWh. We would quote 250 to 270 kWh. Units that only cover
four or five turns a shift can run 100 to 120 kWh, provided the peak power step is still inside the pack’s
continuous rating.
How long does the lithium battery last in airport duty?
LFP at 25°C gives 4000 to 6000 cycles to 80% of nameplate. In real apron service with an average cell
temperature of 30 to 35°C, plan on 3000 to 4000 cycles, which at 2500 turns per year is roughly 8 to
12 years. The dominant variable is cell temperature, not cycle count, so cooling and siting are life-extension
measures, not comfort features.
Is it safe to operate and charge a ground power unit battery on the apron?
Yes, with conditions. The pack needs IEC 62619 and UN38.3 certification, an IP54 or better enclosure, gas
detection with an alarm, and a documented vent direction. Charging happens at the depot on shore power, not on
the stand, and charging below 0°C is blocked by the BMS unless a heater has brought the cells above
5°C. Most airports will also want a short safety case and a fire response note before granting ramp access.
What is different about 400 Hz compared with 50 or 60 Hz output?
Frequency and voltage windows are tight (393 to 407 Hz, 108 to 118 V phase-to-neutral), transformers and
filters are physically smaller, and cable reactance is eight times higher than at 50 Hz, which makes cable length
a real part of the voltage budget. Neutral conductors must be oversized because triplen harmonics from cabin
switch-mode loads add in the neutral. The output also has to stay floating, so standard EMC filters with chassis
Y capacitors cannot be used.
What certifications does a lithium aviation ground power unit need?
UN38.3 for transport, IEC 62133-2 for the cells and batteries, IEC 62619 for the battery system, UL 1973 or
UL 9540 with UL 9540A depending on where it is installed, EMC to IEC 61000-6-2 and IEC 61000-6-4, and supply
quality to ISO 6858 with aircraft compatibility to MIL-STD-704F or ISO 1540. Local fire and electrical codes add
NFPA 855 in the United States and the relevant national installation rules elsewhere.
How cold can a battery ground power unit operate?
Discharge is fine below zero, with LFP capacity at 75 to 85% of room-temperature value at -10°C, but
charging below 0°C is prohibited. In cold hubs we specify a shore-power heater that holds the pack at 10 to
15°C overnight, or move to LTO or sodium-ion where cold charging is required and volume allows.
What I Tell Buyers Before They Write the Specification
Write the specification around the duty cycle, not the kVA plate. Give me the aircraft mix, turns per shift,
turn duration, worst-case ambient, whether the unit is towed or self-propelled, and whether you need 28 VDC
starting. Chemistry, pack size and cooling architecture then follow directly, and the machine that arrives will
hold 400 Hz inside limits on the hottest day of the year. That, not the datasheet, is what the ramp remembers.
Further Reading
References
