Navon T400 Replacement Battery 3.7V 1200mAh Li-ion

Navon T400 — 3.7V Li-ion Replacement Battery (T400)

This is a 3.7V 1200mAh Li-ion cell for the Navon T400 smartphone. It replaces the original battery when the cell has degraded, swells, or no longer holds a usable charge. Dimensions are 60.70 × 50.10 × 4.24mm — confirm these against your existing cell before installing.

• Navon T400 fitment: The T400 uses a compact removable Li-ion cell with a low 3.7V nominal rail. The connector pinout and BMS communication lines match the original cell, so the phone's charge IC recognises the battery on first boot.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a controlled rig. The BMS engaged protection at the expected cutoff voltage and resumed charging cleanly after a rest period. No anomalous heat or voltage drop was recorded.
• Fuel gauge recalibration after swap: On first use after installation, run one full discharge to auto-off then charge uninterrupted to 100% before using the phone normally. This gives the fuel gauge IC a complete reference curve against the new cell — skipping it causes percentage jumps and early shutdowns.

Why the T400 reports wrong battery percentage after a cell swap

The T400's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The chip reads voltage and maps it to a percentage using old data, so it shows numbers that are off by 10–20%. One complete discharge-charge cycle overwrites the reference and the gauge reads accurately from that point forward.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under load — typically during modem activity, screen-on bursts, or app CPU spikes. The phone's protection circuit triggers a shutdown before the displayed percentage reaches zero because the cell cannot sustain voltage above the 3.0V cutoff under that load. It is not a faulty battery — it is an uncalibrated fuel gauge reading a fresh cell. Run one full discharge cycle to 0% auto-off, then charge to 100%, and the shutdowns will stop.