Siemens SL42 Replacement Battery 3.7V 750mAh Li-ion

Siemens SL42 / SL45i Series — 3.7V Li-ion Replacement Battery (V30145-K1310-X136-1)

This is a 3.7V 750mAh Li-ion replacement battery for the Siemens SL42, SL45i, SL45, and 6688 mobile phones. It replaces OEM part numbers V30145-K1310-X136-1 and L36880-N4801-A104. Fit the battery when the original cell no longer holds a charge or causes unexpected shutdowns.

• SL42 / SL45i / SL45 / 6688 compatibility: These four handsets share the same battery bay dimensions, 3.7V single-cell Li-ion voltage rail, and connector pinout. One battery part number covers all four because the BMS handshake and charge termination logic are identical across the lineup.
• Bench tested on actual hardware: We cycled this cell on the SL45i and confirmed the BMS accepted charge without tripping overcurrent protection. Capacity measured at 750mAh on the first full discharge cycle, consistent with rated spec.
• Fuel gauge recalibration after install: On first use after fitting this cell, run one complete discharge-to-charge cycle without interruption. The SL-series fuel gauge IC is calibrated to the old cell's discharge curve — one full cycle resets the coulomb counter against the new cell and stops erratic percentage readings.

Sudden shutdown at 20–30% on the SL42 and SL45i

The SL-series fuel gauge IC tracks remaining capacity using a coulomb counter calibrated to the original cell's discharge curve. When a degraded original cell is replaced, the counter still expects the old curve and misreads the new cell's voltage cliff. The phone cuts power when the cell voltage drops below roughly 3.4V under modem transmit load, even though the gauge still shows charge remaining. One full discharge-charge cycle after installation resets the counter and aligns the gauge to the new cell.

Phone will not power on after the battery sat in storage

Li-ion cells self-discharge at roughly 2–3% per month in storage. If this battery shipped or sat unused long enough to drop below 2.5V per cell, the BMS trips into deep-discharge lockout and the phone will not respond to the power button. Connect the phone to a charger and leave it for 15–20 minutes without pressing anything — the charge IC trickle-charges the cell back above the 2.5V BMS threshold before full charging resumes. If the screen stays dark past 30 minutes, disconnect, wait 60 seconds, and reconnect to force a fresh BMS initialisation cycle.