Siemens SX56 35H10008-80 Li-Polymer Replacement Battery 3.7V

Siemens SX56 — 3.7V Li-Polymer Replacement Battery (35H10008-80)

This 3.7V 1700mAh Li-Polymer battery replaces the original cell in the Siemens SX56 smartphone. It matches the OEM part number 35H10008-80 and fits the SX56 candybar-style handset directly. Voltage, capacity, and connector position align with the factory specification.

• SX56 platform fit: The SX56 uses a slim Li-Polymer cell with a low-profile connector tied to the phone's charge IC and fuel gauge. This replacement matches that connector pinout and the BMS handshake the charge IC expects. Any mismatch in cell impedance or connector wiring causes the phone to reject the battery on boot.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge passes, monitoring BMS cutoff at the low-voltage threshold and confirming the charge IC accepted the cell without triggering a fault flag. Capacity read-back settled within expected tolerance after two full cycles.
• Fuel gauge recalibration on first use: After fitting this cell, disable any fast-charge mode and run one complete discharge-charge cycle before normal use. The SX56's fuel gauge IC was calibrated against the old cell's discharge curve. One full cycle lets it map the new cell's voltage profile, so percentage readings reflect actual charge state.

Why the SX56 reports wrong battery percentage after a cell swap

The SX56 uses a coulomb-counter fuel gauge IC that learns a cell's discharge curve over time. When you fit a new cell, that stored curve no longer matches the new cell's actual voltage behaviour. The gauge reads percentage against stale data, so it can show 40% when the cell is near depletion. One full discharge to auto-off, followed by a full charge to 100%, resets the reference curve and restores accurate reporting.

Sudden shutdown at 20–30% on the replacement cell

This happens because a fresh Li-Polymer cell has slightly different internal impedance compared to the aged cell the phone shipped with. Under modem transmit load or screen brightness spikes, the terminal voltage dips sharply — enough to trip the low-voltage cutoff even when the gauge still shows charge remaining. The BMS protects the cell by cutting output before the gauge catches the sag. After two full charge-discharge cycles, internal impedance stabilises and the fuel gauge curve aligns; the shutdowns typically stop once cell voltage under load stays above 3.4V.