Dell Latitude 5420 Replacement Battery 11.4V 3350mAh WY9DX

Dell Latitude 5420 / 5520 — 11.4V Li-Polymer Replacement Battery (WY9DX)

This is an 11.4V, 3350mAh (38.19Wh) lithium-polymer cell that replaces the original WY9DX battery in the Dell Latitude 5420 and Latitude 5520. Both are business-class ultrabooks sharing the same battery bay geometry, connector pinout, and BMS handshake protocol. Install this cell and the laptop powers on from the battery tray — no adapter required for initial boot.

• Latitude 5420 and 5520 shared platform: Both models run on Dell's same 11th-gen ultrabook chassis, using an identical three-cell 11.4V rail, matching connector, and the same EC firmware communication path. One cell fits both without modification.
• Bench tested on actual hardware: We cycled this cell through charge, full discharge to hibernate-cutoff, and a second full charge on a Latitude 5420 unit. The BMS negotiated correctly, charge termination triggered at 100%, and no fault codes appeared in the Dell Power Manager log.
• Post-install BIOS learn cycle: After fitting this cell, run one complete discharge until the laptop hibernates, then charge uninterrupted to 100% without using the laptop. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on Dell Latitude hardware.

BIOS reporting battery health as poor immediately after replacement

The Latitude 5420 stores battery health data in EEPROM on the old cell. When a new cell is installed, the BIOS reads no matching EEPROM history and flags the battery as degraded or unknown — even though the cell is new. This is not a fault with the replacement battery. Run one full discharge to hibernate-cutoff followed by an uninterrupted charge to 100%, and the BIOS learn cycle will write fresh data to the new cell's EEPROM. After that cycle, the health status in Dell Power Manager should update to Normal.

Laptop shutting down at 20–30% charge shown on screen

This happens when the fuel gauge IC has not yet calibrated against the new cell's actual charge curve. The percentage reading is based on old calibration data, so the displayed figure and the real remaining charge drift apart — the cell hits its voltage floor before the OS expects it to. Under combined CPU and display load, the voltage cliff arrives faster, triggering an abrupt shutdown. Run two full discharge-to-hibernate and charge-to-100% cycles without interruption; by the third cycle, the fuel gauge IC recalibrates and the displayed percentage tracks the actual cell voltage accurately.