BV-T4B Nokia Lumia 640 Compatible Battery 3.8V 3000mAh

Nokia Lumia 640 — 3.8V Li-Polymer Replacement Battery (BV-T4B)

The BV-T4B is a 3.8V, 3000mAh lithium-polymer cell for the Nokia Lumia 640 smartphone. It replaces a degraded original battery that can no longer sustain adequate voltage under screen or modem load. At 11.4Wh, capacity matches the original cell specification.

• Lumia 640 fitment: The Lumia 640 uses a specific connector orientation and BMS handshake tied to the BV-T4B form factor. This cell matches the 100.91 × 46.90 × 4.75 mm envelope and the three-contact pad layout the charge IC expects.
• Bench tested on actual hardware: We cycled this cell through a full discharge-charge sequence on the Lumia 640 platform and confirmed the BMS accepted the cell without triggering a fault state. Charge termination voltage held at 4.35V as expected for Li-Polymer chemistry.
• Fuel gauge recalibration on first use: Disable fast charging for the first complete discharge-charge cycle after fitting this cell. The Lumia 640's fuel gauge IC is calibrated to the old cell's discharge curve — one slow cycle lets it rebuild the coulomb counter baseline before high-current charging begins.

Why the Lumia 640 reports wrong battery percentage after a cell swap

The Lumia 640 uses a coulomb-counter fuel gauge IC that tracks charge state against a learned discharge curve from the original cell. Swap in a new cell and that stored curve no longer matches the new cell's voltage profile. The result is a percentage reading that jumps or gets stuck — often freezing at 50% or reading 100% immediately after a short charge. One full slow-charge discharge cycle from 100% to auto-shutdown resets the learned curve and restores accurate reporting.

Sudden shutdown at 20–30% on the replacement cell

This shutdown is a voltage cliff, not a capacity problem. Under combined modem and display load, the cell's terminal voltage can drop below the BMS cutoff threshold before the percentage gauge catches up. It happens most often during LTE calls or video playback because current draw spikes faster than the fuel gauge samples. If this persists after one full calibration cycle, check that the battery contact pads are fully seated — partial contact raises internal resistance and accelerates the voltage sag. A clean contact seating and a second full calibration cycle typically brings the cutoff voltage back above 3.2V under load.