NGM BIRDY Replacement Battery 3.8V 2100mAh Li-ion

NGM Birdy — 3.8V Li-ion Replacement Battery

This is a 3.8V, 2100mAh Li-ion cell for the NGM Birdy smartphone. It slots in as a direct cell swap when the original battery can no longer hold voltage under screen, modem, or app load. Capacity figure comes from the product data: 2100mAh (7.98Wh).

• NGM Birdy compatibility: The Birdy runs a single-cell 3.8V nominal rail. This cell matches that rail and the physical footprint — 64.31 × 55.88 × 4.80mm — so the connector seats without flex pressure on the cable.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS accepted charge from the Birdy's charge IC without tripping cutoff, and voltage held stable across the full discharge curve down to the 3.0V low-voltage threshold.
• First-cycle fuel gauge reset: After fitting this cell, disable fast charging and run one complete discharge-to-charge cycle. The fuel gauge IC on the Birdy calibrates its coulomb counter against the new cell's discharge curve. Skipping this step is why the percentage reading jumps or reads wrong in the first few days.

Why the Birdy reports wrong battery percentage after a cell swap

The Birdy's fuel gauge IC builds its capacity model from hundreds of charge and discharge data points collected on the original cell. When you fit a new cell, that learned model no longer matches the real discharge curve. The IC keeps calculating state-of-charge against its old data, so it shows 40% when the actual cell voltage is closer to 3.5V. One full discharge cycle — from 100% down to automatic shutdown, then a full charge with fast charging off — forces the coulomb counter to rebuild its curve against the new cell. After that cycle, percentage reporting tightens up to within a few percent of actual.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff, not a capacity problem. Under high-draw moments — mobile data handoff, screen brightness spike, or a background sync — cell voltage drops sharply below the BMS cutoff threshold even though the fuel gauge still shows charge remaining. It happens most often in the first few cycles before the gauge IC has recalibrated to the new cell's internal resistance profile. Run two full discharge-charge cycles with mobile data active and screen at normal brightness. If the phone still shuts off abruptly, check that cell voltage at shutdown reads above 3.2V using a battery diagnostic app — anything below that confirms a BMS trip, not a faulty cell.