Sony CMD-Z7 Replacement Battery QN-Z7BPS 3.7V 600mAh

Sony CMD-Z7 — 3.7V Li-ion Replacement Battery (QN-Z7BPS)

This is a 3.7V 600mAh Li-ion replacement for the QN-Z7BPS battery that powers the Sony CMD-Z7 smartphone. The CMD-Z7 is an early 2000s handset — original batteries from this era are long out of production and degrade significantly over two decades. This cell restores the device to functional operating capacity.

• CMD-Z7 fit: The CMD-Z7 uses a single-cell Li-ion pack with a dedicated connector and BMS handshake tied to Sony's charge management circuit. The QN-Z7BPS is the only part number that satisfies this handshake — substitute cells without the correct ID resistor will be rejected by the charge IC.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a CMD-Z7 board. The BMS accepted the cell on first contact, charge termination triggered correctly at 4.2V, and the protection circuit tripped as expected under an overcurrent event.
• Fuel gauge recalibration on first use: After installation, run one complete discharge down to automatic shutoff, then charge uninterrupted to 100% before daily use. The CMD-Z7's fuel gauge IC was calibrated to the original cell's discharge curve — skipping this cycle leaves the coulomb counter working off stale data, which causes erratic percentage readings from day one.

Why the CMD-Z7 reports wrong battery percentage after a cell swap

The CMD-Z7 uses a fuel gauge IC that tracks charge state by counting coulombs against a stored discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual cell chemistry and internal resistance. The result is percentage readings that bear little relation to real charge state — the phone may show 60% and shut down, or show 10% and keep running. One full discharge-to-shutoff followed by a complete uninterrupted charge resets the coulomb counter and lets the IC relearn the new curve.

CMD-Z7 shutting down suddenly at 20–30% remaining

This is a voltage cliff issue, not a capacity problem. Aged or freshly installed cells with slightly higher internal resistance drop below the BMS cutoff threshold when the processor or display pulls a load spike — even though the fuel gauge still reads 20–30%. The cell voltage collapses faster than the coulomb counter can update. To confirm, charge to 100%, let the phone sit idle until it shuts off, then check the resting cell voltage immediately — a reading below 3.2V under no load confirms the cell is not holding voltage under load and the replacement cycle should be repeated with a verified cell.