NEC DB4000 Replacement Battery 3.7V 900mAh Li-ion

NEC DB4000 Series — 3.7V Li-ion Replacement Battery

This is a 3.7V 900mAh Li-ion cell that fits the NEC DB4000, DB4100, and DB3800 flip phones. These early-2000s compact handsets share the same battery bay and connector across the three models. Capacity is 900mAh (3.33Wh), matching the original specification.

• DB4000, DB4100, and DB3800 compatibility: All three models share the same physical form factor, connector orientation, and 3.7V nominal rail. The BMS handshake across this family is identical, so one cell fits without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the DB4000 platform. The BMS accepted the cell on first connection, charge termination triggered correctly at 4.2V, and the protection circuit tripped as expected under a simulated over-discharge condition.
• Fuel gauge recalibration after install: On first use, run the phone down to auto-shutdown, then charge to 100% without interruption. The NEC fuel gauge IC on these models was calibrated to the original cell's discharge curve. One full uninterrupted cycle resets the coulomb counter against the new cell and stops erratic percentage readings.

Sudden shutdown at 20–25% on the DB4000 after a cell swap

The DB4000's fuel gauge IC holds a discharge curve from the old, degraded cell. When the replacement cell hits a voltage point the IC associates with near-empty — even though actual charge remains — the phone cuts power to protect the modem and display rail. This is not a fault in the new cell. The IC simply has not yet mapped the new cell's actual voltage-to-capacity relationship. One full discharge-to-shutdown followed by an uninterrupted charge to 100% forces the coulomb counter to rebuild its curve against the new cell. After that cycle, shutdowns at 20–25% should stop.

DB4000 won't power on after sitting in storage with a flat battery

Li-ion cells in storage self-discharge slowly. If the cell drops below roughly 2.5V, the BMS enters lockout mode and blocks all current flow to prevent cell damage. The phone shows nothing when you press the power button — no logo, no charging indicator. Connect the phone to a charger and leave it untouched for 15–20 minutes. Most chargers push a small trickle current that nudges the cell voltage back above the BMS recovery threshold, typically around 2.9V, at which point normal charging resumes.