Panasonic CF-VZSU45 Toughbook CF-Y5 Replacement Battery 10.8V 5200mAh

Panasonic Toughbook CF-Y5 Series — 10.8V Li-ion Replacement Battery (CF-VZSU45)

This is a 10.8V, 5200mAh Li-ion replacement battery for the Panasonic Toughbook CF-Y5 rugged notebook. It fits the CF-Y5 series, including the CF-Y5KC2AXS, CF-Y5KW2AXS, and CF-Y5KW4AXS among others. It replaces OEM part numbers CF-VZSU45 and CF-VZSU45U.

• CF-Y5 series compatibility: Every model in this series runs the same 10.8V power rail and uses the same multi-pin connector with BMS handshake. The BIOS reads cell data over that bus, so the same replacement works across all CF-Y5 variants without adapter or modification.
• Bench tested on actual hardware: We ran this battery on a CF-Y5KW2AXS and monitored BMS communication over the SMBus connection. The cell accepted a full charge cycle, reported accurate voltage to the BIOS, and the protection circuit tripped correctly at low-cell threshold without a hard lockout.
• Post-install calibration on the CF-Y5: After fitting this battery, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. The CF-Y5 BIOS stores a battery learn cycle — skipping this step leaves the fuel gauge IC working from stale EEPROM data, which triggers a false "poor health" warning on first boot.

BIOS reporting battery health as poor immediately after replacement

The CF-Y5 BIOS reads battery health from EEPROM data stored on the BMS, not from real-time cell measurement. When a new cell goes in, the EEPROM still holds cycle count and degradation flags from the old pack. The BIOS sees that data and flags poor health before the new cell has run a single cycle. One complete discharge-to-hibernate followed by an uninterrupted charge to 100% rewrites the learn cycle data and clears the warning.

CF-Y5 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 discharge curve. The percentage shown is calculated from old capacity data, so the reading diverges sharply from real cell voltage under combined CPU and display load. When actual cell voltage hits the BMS cutoff threshold, the laptop shuts down — even though the gauge still shows charge remaining. Run two full discharge-to-hibernate and full recharge cycles to let the fuel gauge IC recalibrate against the new cell. After calibration, the gauge should track accurately down to the BMS cutoff voltage of approximately 9.0V.