Swissvoice SV29 Replacement Battery 3.7V 1050mAh Li-ion

Swissvoice SV29 — 3.7V Li-ion Replacement Battery (20405928)

This 3.7V, 1050mAh lithium-ion cell replaces the original battery in the Swissvoice SV29 mobile phone. The SV29 is a compact rugged handset built for basic calling and messaging. Dimensions are 53.16 × 33.77 × 5.34 mm — verify these against your original cell before fitting.

• SV29 platform fit: The SV29 runs a single-cell 3.7V rail. This replacement matches that rail voltage and the physical connector pinout. The BMS on the SV29 motherboard reads cell voltage directly — no proprietary handshake is required, so the swap does not trigger a compatibility lockout.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the SV29 platform. The onboard charge IC accepted the new cell without fault flags. BMS cutoff triggered correctly at the low-voltage threshold, and the cell held stable voltage through a full discharge cycle.
• Fuel gauge recalibration on first use: On first use after installation, disable any fast-charge mode and run one complete discharge-to-charge cycle. This lets the fuel gauge IC build a new discharge curve against the replacement cell before it starts making capacity estimates — skipping this step causes the percentage readout to drift or jump early on.

Why the SV29 reports wrong battery percentage after a cell swap

The SV29 fuel gauge IC stores a learned discharge curve from the original cell. When you install a replacement, that stored curve no longer matches the new cell's actual voltage profile. The IC keeps reading against the old data, so the percentage shown on screen can be off by 15–25%. One full discharge from 100% down to automatic shutdown, followed by a full charge, resets the coulomb counter and retrains the gauge against the new cell.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops below the SV29 modem's minimum operating voltage while the screen or radio spikes current draw. The fuel gauge may still show 25% remaining, but the cell cannot sustain the voltage under that instantaneous load — the phone cuts out to protect the hardware. It is most common in the first few cycles before the fuel gauge has recalibrated. Run two full discharge-charge cycles and check whether the cutoff point shifts; if the phone still shuts down above 15%, measure resting cell voltage — it should read above 3.6V at that charge level.