Siemens SL42 Replacement Battery 3.7V 950mAh Li-ion

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

This is a 3.7V, 950mAh Li-ion replacement battery for the Siemens SL42, SL45, SL45i, and 6688 mobile phones. These are early-2000s GSM candybar handsets that share a common battery bay and connector format. If your phone no longer powers on or drops off charge quickly, this cell replaces the original unit directly.

• SL42, SL45, SL45i, and 6688 compatibility: These four handsets run the same 3.7V single-cell Li-ion rail and use the same physical connector and bay geometry. The BMS handshake is passive on this platform — no proprietary authentication chip — so the replacement cell is recognised on power-up without a pairing step.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the SL45i platform. The charge IC accepted the cell on the first connection, BMS protection tripped correctly at the low-voltage cutoff, and the phone reported charge state without errors after one full cycle.
• Fuel gauge recalibration on first use: After fitting this cell, run the phone from 100% down to automatic shutdown, then charge uninterrupted to full. The SL-series fuel gauge IC calibrates its reference curve on that first complete cycle. Skipping this step causes the percentage display to read high and drop suddenly in the lower third.

Sudden shutdown at 20–30% on the Siemens SL42 and SL45

This happens when the fuel gauge IC is still calibrated to the old cell's discharge curve. The new cell has a steeper voltage cliff in the lower charge band, and the gauge underestimates how close it is to the cutoff. Under GSM transmit load — which pulls a short high-current burst — the cell voltage sags below the BMS cutoff threshold before the displayed percentage reaches zero. One full discharge-recharge cycle resets the reference curve and eliminates the early shutdown.

Phone warm near the battery during the first charge after fitting

A new Li-ion cell starts with slightly higher internal impedance than a broken-in cell. The charge IC on the SL-series pushes constant current into that higher-impedance cell, which produces more heat at the battery contact area during the first one or two cycles. This is normal and resolves as the cell breaks in. If the back of the handset is hot to the touch — rather than just warm — remove the battery and check that the contacts are seated flat with no debris causing a high-resistance connection.