AEG QX9-1-40 Cordless Vacuum Replacement Battery 21.6V 3000mAh

AEG QX9-1-40 Series — 21.6V Li-ion Replacement Battery (405 54 78-29/3)

This is a 21.6V, 3000mAh Li-ion battery pack for the AEG QX9-1-40, QX9-1-40GG, and QX9-1-P4GG cordless stick vacuums. It replaces OEM part 405 54 78-29/3 (also listed as 4055478293). The pack slots into the same battery bay and communicates with the vacuum's BMS over the same contact rail as the original.

• QX9-1-40 series fit: The QX9-1-40, QX9-1-40GG, and QX9-1-P4GG share the same 21.6V battery bay, contact arrangement, and BMS handshake protocol. All three models draw from the same voltage rail, so a single pack covers the full series without any adapter or modification.
• Bench tested on actual hardware: We cycled this pack through charge and discharge on the QX9-1-40 platform and confirmed the BMS communicated correctly at both ends — no false low-battery cutoff during the draw phase, and no refusal to accept charge at the dock contact.
• Dock charging discipline for QX9 vacuums: Remove the battery from the charging dock once it reaches full charge. The QX9 dock does not interrupt current when the pack is full. Leaving the vacuum docked permanently causes trickle-charge heat buildup inside the cells and accelerates capacity loss over repeated cycles.

Suction dropping before the battery indicator reaches low

On the QX9-1-40, suction loss often appears to be a battery issue but is driven by voltage sag at the motor under load. When the filter is partially blocked, the motor works harder, drawing more current than the BMS expects at that state of charge. The BMS reads the voltage dip as a near-empty pack and begins throttling power to the motor. Clean or replace the filter first — if suction recovers and the indicator stops dropping early, the battery was not the root cause.

Motor cutting out mid-clean and then recovering after a few seconds

This is a BMS overcurrent trip, not a dead battery. When sustained suction restriction — from a dense carpet pile, a blockage, or a clogged filter — forces the motor to draw above the pack's rated discharge ceiling, the BMS trips and cuts output to protect the cells. Power returns once the BMS resets, typically within a few seconds. Clear any blockage, check the filter, and resume at a lower suction setting if the floor type allows — this stops the trip-and-recover cycle without replacing the battery.