HSTNN-CB72 Compaq Presario CQ40 Replacement Battery 10.8V 4400mAh

Compaq Presario CQ40 Series — 10.8V Li-ion Replacement Battery (HSTNN-CB72)

This 10.8V, 4400mAh (47.52Wh) lithium-ion battery replaces the original cell in the Compaq Presario CQ40 series notebooks. It fits a wide range of CQ40 variants, including the CQ40-305AU and CQ40-313AX, using OEM part numbers HSTNN-CB72, HSTNN-DB72, HSTNN-IB72, and several others. The connector, BMS handshake protocol, and voltage rail all match the original HP/Compaq charging controller.

• CQ40 series compatibility: The CQ40 line shares a common 10.8V three-cell voltage rail and a standardised six-pin connector across all sub-models. The BMS in this replacement communicates the same EEPROM identifiers the charging IC expects, so the system does not flag an unrecognised cell at POST.
• Bench tested on actual hardware: We ran this battery through full charge and discharge cycles on a CQ40-series board. The BMS held cutoff correctly at both the high-voltage ceiling and the low-voltage floor, and the charging controller accepted the cell without throwing a firmware fault.
• Post-install calibration on the CQ40: After fitting this battery, run one full discharge until the laptop hibernates at its low-voltage cutoff, then charge uninterrupted to 100% without interruption. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on CQ40-series machines.

BIOS reporting battery health as poor immediately after replacement

The CQ40 BIOS reads health data from EEPROM registers stored on the battery controller — not from live cell voltage. When you swap in a new cell, those registers still reflect the degraded state of the old battery. The BIOS will display "poor health" or a low wear-level percentage until the learn cycle overwrites the cached data. Run one full discharge-to-hibernate followed by a complete uninterrupted charge. After that cycle, the BIOS should read the new cell's actual state and the health warning clears.

CQ40 shutting down abruptly at 20–30% charge shown on screen

This happens when the fuel gauge IC loses calibration against the new cell's actual capacity curve. The OS reads a percentage based on voltage thresholds mapped to the old cell — those thresholds no longer match. Under full CPU and display load, the cell voltage drops faster than the uncalibrated gauge expects, and the laptop hits hardware cutoff while the screen still shows 20–30%. Two to three full discharge-and-recharge cycles recalibrate the fuel gauge IC. After calibration, the shutdown point should align with the gauge reading near 5%.