NTT DoCoMo F-04B Smartphone Li-ion Compatible Battery 3.7V 600mAh

NTT DoCoMo F-04B — 3.7V Li-ion Replacement Battery (F13 / AAF29134)

This 3.7V, 600mAh Li-ion cell replaces the original battery in the NTT DoCoMo F-04B smartphone. It matches the OEM voltage rail and physical footprint, fitting directly into the F-04B battery bay. Capacity is sourced from manufacturer spec — 600mAh, 2.22Wh.

• F-04B cell fit: The F-04B uses a compact 44.15 × 39.22 × 3.70mm cell with a 3.7V nominal rail. This replacement matches those dimensions and voltage exactly, so the connector seats without modification and the BMS handshake completes normally on first boot.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS accepted charge current without tripping, held voltage above the low-battery cutoff threshold at end of discharge, and reported cell status cleanly to the host device throughout.
• Fuel gauge recalibration on first use: After installing a new cell, run one full discharge to automatic shutdown, then charge uninterrupted to 100% before resuming normal use. The F-04B's fuel gauge IC calibrates its coulomb counter against this first full cycle — skipping it causes the percentage display to read inaccurately for days.

Why the F-04B reports wrong battery percentage after a cell swap

The F-04B tracks battery state using a fuel gauge IC that builds a discharge curve over time. When you replace the cell, that stored curve no longer matches the new cell's actual voltage-to-capacity relationship. The IC keeps applying the old curve, so the percentage it reports drifts from reality — sometimes by 15–20%. One complete discharge-to-shutdown followed by a full uninterrupted charge resets the coulomb counter and re-anchors the gauge to the new cell.

Sudden shutdown at 20–30% remaining on the replacement cell

This happens when the cell voltage drops sharply under load before the fuel gauge expects it — a voltage cliff. On a fresh cell that hasn't been calibrated, the gauge overestimates remaining charge. When the modem or display draws a spike of current, cell voltage sags below the BMS cutoff threshold and the phone shuts off instantly. The fix is the same calibration cycle: full discharge to shutdown, then charge to 100% without interruption. After one complete cycle the gauge tracks the actual voltage curve and the early shutdowns stop.