Batteries and the BCL

The batteries are the last layer of the overview's defence-in-depth — below the three main generators, below the emergency generator, below the static inverter that wears their DC as AC. They are also the chapter's most counter-intuitive component. The overview already let two surprises slip: the batteries are "permanently connected to the two hot busses" yet, in the normal configuration, "disconnected most of the time". Both are true at once because a battery has two output paths with two different rules — a permanent, contactor-less path to its hot bus, and a conditional, BCL-gated path to the DC BAT BUS.

This article makes the whole battery layer explicit: three batteries and three Battery Charge Limiters (BCLs), every charge trigger and protection threshold, the four jobs the main battery does, the two "side-warnings" the BCL drives for components that are not even batteries, and what the BAT pushbutton can and cannot switch off. It also closes the 40 Ah / 37 Ah capacity question the overview flagged. The static inverter (the battery's AC avatar) is left to Static Inverter; the 2PC/3PC essential-feed channels to DC Network Transfer; the BAT FAULT / lithium-fire handling to TR and Battery Faults; and the full battery-only endurance picture to Battery-Only Flight.

1. Scope and boundaries

By the end of this article you should be able to answer five questions:

1. How is a battery built, how is it vented, and is the capacity 40 Ah or 37 Ah?
2. What are the five triggers that start a charge, the current threshold that ends it, and what is the "10 minutes after fully charged" window?
3. What are the protection layers (thermal runaway / deep discharge / hot-bus stabilisation) and their numbers?
4. What are the main battery's four jobs, and how does "refuel without energising" work?
5. What are the BCL's two side-warnings, and how is a BAT FAULT reset?

A boundary to fix now: this article owns the battery body, the entire BCL logic, the BAT pushbutton, the discharge horn, and refuelling on battery. It leaves the static inverter, the 2PC/3PC corridor, the lithium-fire procedure and the battery-only endurance numbers to the neighbouring articles named above.

2. Architecture — three batteries, three BCLs

Three identical batteries, each with its own BCL. Two main batteries (BAT 1, BAT 2) live in the avionics compartment; the APU battery sits aft, behind a cargo panel:

 Avionics compartment (zone 121)              Bulk cargo, behind panel 162RW
 ┌───────────┐   ┌───────────┐                ┌───────────┐
 │   BAT 1   │   │   BAT 2   │                │  APU BAT  │
 │   2PB1    │   │   2PB2    │                │   2PB3    │
 └──┬─────┬──┘   └──┬─────┬──┘                └──┬─────┬──┘
  perm│ 6PB1│    perm│ 6PB2│                  perm│  5PB│
    ▼     ▼        ▼     ▼                       ▼     ▼
 701PP    └─►  DC BAT  ◄─┘ 702PP            709PP    └─► 309PP
 HOT BUS 1     BUS 3PP     HOT BUS 2        APU HOT      (APU start chain)
    │             ▲           │                │
 BCL 1 (1PB1) ── closes ── BCL 2 (1PB2)     BCL APU (1PB3) ── closes 5PB
                 6PB1/6PB2

 Emergency direct feed (art. 11):  BAT 1/2 ──► 2PC / 3PC ──► DC ESS BUS
                                                         └─► static inverter ──► AC ESS

Note the two-path design read off the diagram: each battery feeds its HOT BUS permanently, with no contactor in the way (701PP / 702PP / 709PP), and feeds the DC BAT BUS only through a line contactor (6PB1 / 6PB2 for the mains, 5PB for the APU battery) that the BCL opens and closes. Per AMM 24-38-00, "the three BCLs are identical and interchangeable", and a pin programming determines each unit's role (1, 2 or APU) — so a spare BCL is the same part for any of the three positions.

Construction. Per AMM 24-38-00:

"In each battery there are 20 nickel cadmium accumulators installed in a titanium case. Each accumulator can be removed separately from the battery. The main features of the battery are: ‐ nominal voltage: 24 VDC, ‐ nominal capacity: 40 AH, ‐ high electrolyte reserve: 60 cm3, ‐ high instantaneous power, ‐ two ventilation ducts ‐ explosion proof."

Twenty nickel-cadmium cells in series in a titanium case, each cell individually replaceable; 24 VDC nominal, 40 Ah nominal capacity, a 60 cm³ electrolyte reserve, and an explosion-proof design. The three batteries are identical and interchangeable.

[!warning]- The capacity is 40 Ah — the "37 Ah" is an internal manual conflict

The overview flagged that the AMM general (24-00) section cites 37 Ah in one place, while the dedicated AMM 24-38 chapter states a "nominal capacity: 40 AH". FCOM agrees with the dedicated chapter — per FCOM DSC-24-10-20-20, "Two main batteries, each with a normal capacity of 40 Ah … An identical third battery (40 Ah) is dedicated to APU start." This is an AMM internal inconsistency (general overview vs dedicated chapter), not a configuration difference. With the dedicated chapter and FCOM both stating 40 Ah, 40 Ah is taken as the value; the lone 37 Ah is read as a stale/erroneous figure.

Ventilation. Nickel-cadmium cells gas off hydrogen on overcharge, and the battery vents it overboard with no electrical power at all. Per AMM 24-38-00:

"Each battery has two ventilation ducts for the evacuation of any hydrogen or oxygen emanation. One duct is not connected at one end, it is the avionics compartment venting duct. The other duct is connected to a venturi leading overboard. The differential pressure (Delta P) between the cabin and the outside is used to provide battery ventilation."

One duct draws cabin air in; the other is connected to a venturi leading overboard. The cabin-to-outside differential pressure (Delta P) drives the airflow — a passive extraction that consumes no power and runs continuously in flight.

3. What the main battery does — four jobs

Per AMM 24-38-00 the BCL controls a battery contactor "which: ‐ Keeps the battery charged. ‐ Gives battery protection from thermal runaway or short circuit. ‐ Supplies the equipment on the ground when no normal power is available. ‐ Prevents the battery from a full discharge. ‐ Supplies the equipment during transient fault configuration. ‐ Makes stable the related hot bus." The four pilot-relevant jobs follow from that list.

Job 1 — emergency transient supply. Per AMM 24-38-00:

"Power supply the DC essential and AC essential equipment particularly during transient phase in emergency configuration. The two batteries serve to power supply: ‐ the DC ESS BUS (4PP) and, ‐ the sub busbars 901XP and 931XP via the static inverter and through the contactors 2PC and 3PC in the following configuration: as long as the Constant Speed Motor/Generator (CSM/G) is not connected to the essential network (CSM/G starting phase), when the CSM/G is unavailable (at slat extension with Ram Air Turbine (RAT) extended)."

The batteries bridge the DC ESS BUS (4PP) and, via the static inverter, the AC essential sub-busbars (901XP / 931XP) through contactors 2PC/3PC — during the CSM/G's start-up seconds, and when the CSM/G is unavailable (slats extended, RAT out). That is the same 2PC/3PC corridor laid out in DC Network Transfer; here the battery is the source itself.

Job 2 — the "smoothing pad" for electrical transfers. Per AMM 24-38-00:

"During the electrical transfers, the hot busses, which are permanently power supplied by the batteries, serve as a back-up to the sensitive equipment."

During the brief interruption of any source switch (a failed NBPT, the AC ESS feed change-over), the sensitive equipment hanging on the permanently-connected hot buses never blinks. This is the everyday value of "permanently connected hot buses".

Job 3 — the sole source on the ground. With no other source, pressing the BAT pushbuttons closes 6PB1/6PB2 onto the DC BAT BUS (3PP) and feeds the essential network through 2PC/3PC and the static inverter. The ground/flight difference is decisive and is fixed verbatim in FCOM. On the ground, per FCOM DSC-24-10-30-30, with both batteries in AUTO they supply "DC ESS BUS, DC BAT BUS, DC LAND RECOVERY … And, through the static inverter, the: ‐ AC ESS BUS, and ‐ AC ESS GND ‐ AC LAND RECOVERY"; in flight on batteries only, "The AC ESS GND is lost." The ground case keeps AC ESS GND alive (needed for ground servicing/evacuation segments) where the flight case drops it — the single line that separates the two states.

Job 4 — refuelling without energising. The two refuel busbars hang on the hot buses, so the aircraft can be refuelled completely dark. Per AMM 24-67-00 the batteries supply "on the ground, to: the ESS FUEL BUS (501PP), the MAIN FUEL BUS (502PP), when AC power supply sources are not available." The three supply configurations of the refuel network:

The third row is "refuel a dark aircraft": each battery claims one refuel busbar through its hot bus, and the ground engineer throws one panel switch. In any other combination 501/502PP are not energised.

The APU battery has a single job. Per AMM 24-38-00, the APU battery (2PB3) supplies "permanently to the HOT BUS 709PP, to sub-busbar 309PP through the contactor 5PB, to the APU starter through the contactors 5PB and 5KA during the starting of the APU." On the ground with no source it can start the APU alone; with a source present it assists the APU TR.

4. The BCL charge cycle — five triggers, two end thresholds, a ten-minute window

Start of charge (any one of five, 6PB closes). Per AMM 24-38-00 the charge cycle begins:

"Two hours after the last charge cycle, when the voltage of the 28 VDC BAT BUS (3PP) is higher than 27 V and the voltage of battery 1(2) is lower than 26.5 V, or ‐ When battery 1(2) discharge goes into the 28 VDC HOT BUS 701PP (702PP) by more than 1 Ah and the voltage of battery 1(2) is lower than 26.5 V … or ‐ When a change from ground to flight occurs and the voltage of battery 1(2) is lower than 26.5 V, or ‐ When battery 1(2) supplied power to the network and the voltage of battery 1(2) is lower than 26.5 V, or ‐ When the voltage of battery 1(2) is lower than 25.5 V."

End of charge, and the 10-minute window. Per AMM 24-38-00:

"When battery 1(2) charge current is lower than 4 A and decreases for: 10 seconds on the ground … 15 minutes in flight (only during the first charge). Contactor 6PB1(2) opens." "BCL 1(2) will keep battery 1(2) contactor closed for 10 minutes when battery 1(2) is charged. During these 10 minutes, battery 1(2) can charge or discharge: When battery 1(2) voltage is higher than the DC BAT BUS voltage, the current goes from battery 1(2) to the DC BAT BUS and the amber arrow is shown. When the DC BAT BUS voltage is higher than battery 1(2) voltage, the current goes from the DC BAT BUS to battery 1(2) and the green arrow is shown."

The charge ends when current falls below 4 A and is decreasing — confirmed for 10 s on the ground, 15 min in flight (the latter only on the first charge) — and 6PB opens. The first quote is the full AMM version of the FCOM panel line "charging cycle ends when battery charge current goes below 4 A (for 10 s on ground, 15 min in flight)". The second quote defines the ECAM arrow semantics: for ten minutes after a full charge the contactor stays closed and the battery may swing either way — battery higher than the DC BAT BUS → amber arrow (discharging into the bus); bus higher → green arrow (charging). Seeing the arrows flicker just after a full charge is the normal observation window, not a BCL malfunction; once it ends, 6PB opens and the battery returns to its "disconnected most of the time" normal — charge then disconnect, never float-charge, the first discipline against nickel-cadmium thermal runaway.

APU BCL differences. The APU BCL uses different numbers. Per AMM 24-38-00 its charge ends when current is "lower than 8 A and decreases" (mains: 4 A); its thermal-runaway criterion adds a slope, "more than 8 A and increases with a slope more than 0.375 A/min"; and its deep-discharge confirmation is "lower than 23 V during more than 60 seconds" (mains: 16 s).

A teaching handle for the whole cycle: the BCL is a tight-fisted, conscientious finance director. The company account (the battery) is normally sealed (charge then disconnect, never float). Five situations open it (the five triggers — a periodic audit, an over-1-Ah expense claim, a return from a trip, a prior advance, a balance below the red line). After topping up it still reconciles for ten minutes (the window) before closing the books. Three risk gates follow in §5.

5. Protection — the layered thresholds

The BCL carries four protection layers for the mains (the APU BCL adds an APU-start layer). The verbatim thresholds, per AMM 24-38-00:

"Contactor 6PB1(2) opens if: ‐ Charge current is more than 8 A and increases. ‐ Charge current is more than the set threshold (150 A) during 90 seconds." (thermal runaway / short circuit) "On the ground, when BAT1 (2) voltage is lower than 23 V during 16 seconds, the battery power supply relay is de-energized and contactor 6PB1(2) opens automatically. In this condition, on the ground, the batteries cannot supply DC essential bus 4PP and AC essential bus 9XP (contactors 2PC and 3PC are open)." (deep-discharge protection) "a Software protection If there is DC BAT BUS (3PP) short circuit (I discharge > 100 A during 0.3 second), contactor 6PB1(2) opens. b Hardware protection If there is current discharge > 400 A, battery voltage < 13 V during five ms, the battery power supply relay is de-energized and contactor 6PB1(2) opens." (hot-bus stabilisation)

Per AMM 24-38-00 the APU-start protection reads: "During APU start, when the APU battery is lower than 9.5 V during 1.5 seconds or lower than 12 V during 15 seconds, the contactor opens automatically" — protecting the APU battery from being dragged flat by the start surge.

Two readings (synthesis, from the verbatim above). The software vs hardware split handles two timescales: the software layer catches the ordinary "bus short pulls the battery down" fault; the hardware layer catches the extreme "battery nearly shorted to ground" case (400 A / 13 V / 5 ms — too fast for software), with a relay-level cut as the final backstop. Thermal runaway is the classic nickel-cadmium failure (rising temperature → falling internal resistance → rising current → rising temperature, a positive feedback); the "more than 8 A and increasing" criterion catches that acceleration signature, not a simple absolute-current limit — which is exactly why the APU version adds an explicit slope term (0.375 A/min).

[!warning]- The ground deep-discharge cut also opens 2PC/3PC — the ground ESS is no longer battery-fed

When the ground deep-discharge protection fires (< 23 V for 16 s), it does not merely open 6PB; it also opens contactors 2PC and 3PC, so on the ground the batteries can no longer supply DC ESS BUS (4PP) or AC ESS BUS (9XP). This is the same 23 V / 16 s red line that the FCOM BAT pushbutton AUTO note states (§7), seen from the battery side. It exists to stop a parked aircraft from flattening its batteries completely.

6. The BCL's two side-warnings, and reset

Beyond charging, the BCL drives two FAULT legends for things that are not its own battery. Per AMM 24-38-00:

"8 EMER GEN FAULT control In emergency configuration, if the CSM/G does not operate, the FAULT legend on the EMER GEN pushbutton switch comes on. 9 BAT FAULT warning control If thermal runaway or short circuit, the FAULT legend of the BAT 1(2) pushbutton switch comes on."

Function 8 is the hidden one: the red EMER GEN FAULT legend of Emergency Generator is lit by the BCL. It is logical — in the emergency configuration the only intelligent box still alive and still aware that "the CSM/G should be running and isn't" is the battery-powered BCL.

Reset after a BAT FAULT. Per AMM 24-38-00:

"After a BAT FAULT, the BCL functions are not available. If there is no other power supply source, release the BAT 1(2) pushbutton switch, the OFF legend comes on. Then push the BAT 1(2) pushbutton switch, the OFF legend goes off."

After a BAT FAULT all BCL functions are lost; the reset is to release the BAT pushbutton (OFF legend on) then press it back in (OFF legend off). This re-arms the BCL but does not cure the fault — a recurring thermal runaway will simply trip again, and the procedure for that is in TR and Battery Faults. Reset once; do not keep trying — a thermal-runaway positive feedback does not heal itself.

7. The BAT pushbutton and the discharge horn

The FCOM panel description is the operator-facing twin of the AMM logic. The full AUTO/OFF logic, per FCOM DSC-24-20:

"Auto: The Battery Charge Limiter controls automatically the connection and the disconnection of the corresponding battery to the DC BAT BUS by closing and opening of the battery line contactor. ‐ The batteries are connected to the DC BAT BUS in the following cases: • Battery voltage below 26.5 V (battery charge). The charging cycle ends when battery charge current goes below 4 A (for 10 s on ground, 15 min in flight) • On the ground (with speed below 50 kt), when batteries only are supplying the aircraft • In flight DC generation lost (limited to 7 s). ‐ The batteries are connected to the DC ESS BUS when batteries only are supplying: • In flight, • On ground, speed above 50 kt, • On ground, speed below 50 kt, provided they are both selected auto. Note: 1. In normal configuration the batteries are disconnected most of the time. 2. A battery automatic cut off logic prevents batteries from discharging completely when the aircraft is on the ground (parking). Automatic battery contactors open when: ‐ The aircraft is on the ground ‐ The main power supply (external power plus all generators) is cut off ‐ The battery voltage is lower than 23 V for more than 16 s. The flight crew can reset the contactors by switching the BAT pb-sw to OFF then to AUTO. OFF: The Battery Charge Limiter is not operating, the DC ESS BUS is not connected to the battery (except in flight in emergency configuration)."

This single quote nails down several facts the chapter leans on: ① "In normal configuration the batteries are disconnected most of the time" is FCOM's own wording (the source of the intro's counter-intuitive point); ② the 50 kt speed gate for connecting to the DC BAT BUS (cross-confirmed in article 11); ③ the OFF position's in-flight emergency exception — even at OFF, in flight in the emergency configuration the DC ESS stays connected to the battery (a "survival overrides the switch" behaviour); ④ the 23 V / 16 s / on-ground three-part automatic cut-off (the §5 deep-discharge layer). Note also that OFF stops the BCL but the hot buses remain supplied — the battery is physically, permanently on its hot bus.

[!warning]- The BAT pushbutton switches off the BCL's management, not the battery's existence

Pressing BAT 1 + BAT 2 to OFF does not kill the batteries. The hot buses (701PP / 702PP) stay live because the battery is permanently wired to them with no contactor; the 470PP CIDS emergency functions (PA/EVAC) stay supplied (see article 11); and in flight in the emergency configuration the DC ESS stays connected even at OFF. BAT OFF removes the BCL's management authority, not the battery's survival role.

The discharge horn is the ground reminder that "the aircraft is eating its batteries". Per FCOM DSC-24-10-30-30:

"On ground, an external horn is activated, if the following conditions are met: ‐ External power contactor OFF (ground power unit disconnected, or EXT PWR pushbutton at OFF; ‐ Generators not running; ‐ BAT pushbutton switched at auto, and batteries connected to the aircraft network. If the batteries are not switched off, the horn automatically stops after 5 min."

External power off + generators not running + BAT in AUTO and connected → an external horn sounds, and stops automatically after 5 min if the batteries are not switched off. It is an alert for ground personnel, not the crew — and the horn going quiet does not mean the drain has stopped; the real backstop is the 23 V / 16 s automatic cut-off.

8. Reading the ECAM ELEC DC battery line, and flight-deck scenarios

On the ECAM ELEC DC page, per FCOM DSC-24-20 the battery line reads: voltage green (amber below 25 V or above 31 V); current green (amber when discharge > 5 A); arrow green-down = charging / amber-up = discharging / no arrow = BCL disconnected (the normal state); during an APU start the APU battery values can read abnormal yet stay green (the same exemption granted to the APU TR). With BAT OFF the line shows OFF.

Six scenarios:

1. Cruise scan shows a green arrow on BAT 1. The BCL is topping up (one of the five triggers — most often ① periodic or ② hot-bus accounting past 1 Ah). It should end within a few minutes to 15 minutes; only a sustained high charge current that will not end is abnormal (a thermal-runaway precursor — article 27).
2. Arrows flicker just after a full charge. Normal — the 10-minute observation window (§4); small charge/discharge swings are legal during it.
3. BAT 2 FAULT. The BCL judged thermal runaway / short and 6PB2 is already open. After the procedure, reset is one BAT OFF→AUTO cycle; if it trips again, stop trying (a thermal-runaway positive feedback does not self-heal).
4. Batteries left on overnight. The horn stops after 5 min, but the battery keeps draining; the real floor is the 23 V / 16 s automatic cut-off (it will not be flattened completely). Finding the battery auto-disconnected next morning, think "a protection did this" first, then check capacity once charging is restored.
5. Refuelling a dark aircraft at an out-station. Refuel door open + REFUEL ON BAT to BAT — BAT 1 feeds 501PP, BAT 2 feeds 502PP. Reset afterwards: this is directly consuming the emergency reserve.
6. After selecting BAT 1+BAT 2 OFF for evacuation. The hot buses are still live (battery permanently connected), the 470PP CIDS emergency functions still supplied (article 11), and in the in-flight emergency configuration DC ESS would not drop on OFF either — BAT OFF is not an "off switch" for the battery.

9. Dispatch view (MEL)

The dispatch picture mirrors the TRs in Transformer-Rectifiers: the survival floor is no-go, the APU-side channel is conditionally dispatchable. Per the operator MEL (MI-24-38):

The logic is clean: the survival floor (main batteries + main BCLs) is wholly no-go — the last layer of defence-in-depth tolerates no gap; the APU side (APU battery / APU BCL) is conditionally dispatchable because it only serves APU start, not flight. ELEC APU BAT OFF is a crew-selected state and so falls outside the MEL coordinate system. The full dispatch treatment is in MEL Dispatch View.

Self-test

[!note]- Q1. How is a battery built, and is the capacity 40 Ah or 37 Ah?

Twenty nickel-cadmium cells (each individually replaceable) in a titanium case; 24 VDC nominal, 40 Ah nominal capacity, 60 cm³ electrolyte reserve, explosion-proof; two ventilation ducts (one venting from the avionics compartment, one to an overboard venturi, driven by cabin/outside Delta P) evacuate hydrogen/oxygen. The capacity is 40 Ah — the dedicated AMM 24-38 chapter and FCOM both state 40 Ah; the lone "37 Ah" in the AMM general section is an internal manual inconsistency, not a configuration difference. The two main batteries are in the avionics compartment, the APU battery aft behind panel 162RW. Three batteries, identical and interchangeable.

[!note]- Q2. Walk the BCL charge cycle.

Five start triggers: ① 2 h periodic + 3PP > 27 V + battery < 26.5 V; ② discharge into the HOT BUS > 1 Ah and < 26.5 V; ③ ground-to-flight transition and < 26.5 V; ④ has supplied the network and < 26.5 V; ⑤ < 25.5 V unconditional. End: charge current below 4 A and decreasing (confirmed 10 s on ground / 15 min in flight, the latter first-charge only), then 6PB opens; the contactor is kept closed for a 10-minute window after a full charge (charge or discharge allowed — green-down / amber-up arrows). APU BCL: ends at 8 A, deep-discharge confirmation 60 s, thermal-runaway criterion adds a 0.375 A/min slope.

[!note]- Q3. List the protection layers and their numbers.

Thermal runaway / short: charge current > 8 A and increasing, or > 150 A for 90 s → contactor opens + BAT FAULT. Deep-discharge (ground): < 23 V for 16 s (APU 60 s) → automatic cut-off, **2PC/3PC open too**. Hot-bus stabilisation: software on a 3PP/309PP short (discharge > 100 A for 0.3 s), hardware at discharge > 400 A and voltage < 13 V for 5 ms (relay-level). APU-start protection (APU battery): < 9.5 V for 1.5 s, or < 12 V for 15 s → 5PB opens.

[!note]- Q4. What are the main battery's four jobs?

① Emergency transient supply (CSM/G start bridge and slats-out-with-RAT, via 2PC/3PC + static inverter to DC ESS and 901XP/931XP). ② A smoothing pad — the permanently-supplied hot buses back up sensitive equipment during electrical transfers. ③ The sole ground source (DC BAT BUS + ESS network; on the ground AC ESS GND is kept, in flight it is lost). ④ Refuel without energising (via the hot buses to 501PP/502PP, with REFUEL ON BAT set to BAT). The APU battery's single job is APU start (alone on the ground, or assisting the APU TR).

[!note]- Q5. What are the BCL's two side-warnings, and how is a BAT FAULT reset?

Side-warning ①: in the emergency configuration, if the CSM/G does not operate, the BCL lights the red EMER GEN FAULT legend. Side-warning ②: on thermal runaway / short, the BAT FAULT legend. After a BAT FAULT all BCL functions are lost; reset is BAT pushbutton released (OFF legend on) then pushed in (OFF legend off). Reset once only — a recurring thermal runaway will trip again and is handled by procedure.

Key takeaways

References

Per AMM 24-38-00 D/O (battery construction — 20 nickel-cadmium cells, titanium case, 40 AH, 60 cm³, explosion-proof, venturi ventilation; positions; BCL interchangeability and pin programming; the four main-battery functions and the APU-battery function; the full charge cycle — five start triggers, 4 A end, the 10-minute window and arrow semantics; thermal-runaway, deep-discharge, software/hardware hot-bus and APU-start protections; the EMER GEN FAULT / BAT FAULT side-warnings and the reset); AMM 24-67-00 (the three refuel-bus configurations, BAT 1→501PP / BAT 2→502PP, REFUEL ON BAT); FCOM DSC-24-20 (the BAT pushbutton AUTO/OFF logic, "disconnected most of the time", the OFF in-flight emergency exception, hot buses remain supplied; the ECAM ELEC DC battery-line thresholds 25–31 V / discharge > 5 A); FCOM DSC-24-10-20-20 (the 40 Ah figure and the BCL minimal definition); FCOM DSC-24-10-30-30 (battery-only DC distribution — AC ESS GND kept on the ground, lost in flight; the discharge horn three conditions and 5-minute auto-stop); the operator MEL MI-24-38 (main batteries / main BCLs no-go, APU battery / APU BCL conditional, APU BAT OFF outside the MEL). The "tight-fisted finance director" analogy, the software-vs-hardware-timescale reading, and the thermal-runaway positive-feedback note are integrative syntheses of the verbatim sources above and contain no facts from outside the library.

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, and QRH for operational use.