About this replacement
Sony Z5 Series — 3.7V Li-ion Replacement Battery (QN-Z5BPS)
This is a 3.7V, 650mAh Li-ion replacement cell for the Sony Ericsson Z5 and related compact clamshell models. It fits the Z5, Z15, Z18, Z28, and six additional variants that share the QN-Z5BPS footprint and connector. Capacity data is sourced from the product specification, not estimated from third-party listings.
- Z5 family cross-compatibility: The Z5, Z15, Z18, and Z28 share the same battery bay dimensions, 3.7V nominal rail, and connector pinout. The BMS handshake across these models expects the same cell impedance profile, so one cell covers the full group without modification.
- Bench tested on actual hardware: We cycled this cell through a full charge and discharge on compatible hardware and confirmed the BMS accepted the new cell without lockout, and charge termination triggered correctly at 4.2V.
- Fuel gauge recalibration on first use: On first installation, run one complete discharge to low battery warning, then charge uninterrupted to 100%. The Z5's fuel gauge IC is calibrated against the old cell's discharge curve — one full cycle resets that reference and stops percentage readings from jumping or reading high when the phone shuts down early.
Sudden shutdown at 20–30% on the Z5 after a cell swap
The Z5's fuel gauge IC holds a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-versus-capacity behaviour of the replacement. The phone reads 25% but the cell voltage has already dropped below the modem's minimum draw threshold, causing an abrupt shutdown. One full discharge-charge cycle rewrites the coulomb counter reference and aligns the percentage display to the new cell's actual curve.
Phone warm near the battery compartment on first charge after replacement
A new Li-ion cell typically has slightly higher internal impedance than a conditioned cell, and the charge IC compensates by pushing a constant-current phase that runs longer than usual. That extended CC phase generates more heat than you'd see on a fully cycled battery. This is normal for the first one or two charge cycles and resolves as the cell conditions. If the case is hot to the touch — not just warm — disconnect and check that the battery contacts are seated flat against the connector pins with no lateral pressure.