W540BAT-6 Clevo Nexoc B509II Compatible Battery 11.1V 4400mAh

Clevo Nexoc B509II — 11.1V Li-ion Replacement Battery (W540BAT-6)

This is an 11.1V 4400mAh (48.84Wh) Li-ion battery for the Clevo Nexoc B509II and compatible notebooks including the Aquado M1519, W550SU1, and W550TU. It replaces OEM part numbers W540BAT-6, 6-87-W540S-427, and related variants. The connector, cell count, and BMS pin-out match the original specification.

• W550-platform compatibility: The B509II, Aquado M1519, W550SU1, and W550TU all share the same 6-cell battery bay geometry, 11.1V voltage rail, and SMBus BMS handshake — which is why one part number covers all of them. The EEPROM data on this cell is written to match that communication protocol.
• Bench tested on actual hardware: We cycled this battery through full charge and controlled discharge on a W550-platform machine. The BMS reported cell status correctly over SMBus, charge current tapered normally at the top of the range, and the protection circuit tripped cleanly at the low-voltage threshold.
• Post-install calibration cycle: After fitting this battery, run the laptop down to hibernate cutoff under normal use, then charge uninterrupted to 100% without interruption. This forces the BIOS battery learn cycle to reset against the new cell chemistry and clears the inaccurate health warning that appears after every cell swap.

BIOS reporting poor battery health immediately after fitting a new cell

The Clevo W550 platform stores battery health history in the BIOS independently of the cell itself. When a new battery is installed, the BIOS compares incoming SMBus data against its stored wear profile from the old cell — and flags a mismatch as poor health. This is a BIOS-side EEPROM artefact, not a fault with the replacement cell. Running one full discharge-to-hibernate followed by an uninterrupted charge to 100% triggers the learn cycle and rewrites the stored baseline against the new cell's actual capacity.

Laptop shuts down abruptly at 20–30% charge shown on screen

This happens when a worn cell can no longer sustain voltage under simultaneous CPU and display load — the cell voltage drops below the BMS cutoff threshold before the fuel gauge reaches 0%. A new cell eliminates that voltage cliff because its internal resistance is lower and it can deliver the required current without collapsing. If the same shutdown behaviour appears with a new cell, the fuel gauge IC may need additional calibration cycles to map the new cell's discharge curve accurately. Run two to three full discharge-and-charge cycles and verify the shutdown point moves below 10%.