NGM Boris BL-40 Compatible Battery 3.7V 900mAh Li-ion

NGM Boris — 3.7V Li-ion Replacement Battery (BL-40)

This is a 3.7V, 900mAh Li-ion cell built to replace the original BL-40 battery in the NGM Boris smartphone. It fits the Boris directly and restores the phone to full working condition when the factory cell has degraded or failed. Dimensions are 53.00 × 34.00 × 4.80mm — confirm these match your existing cell before fitting.

• NGM Boris compatibility: The Boris uses a straightforward removable Li-ion pack with no encrypted BMS handshake. The BL-40 connector and cell footprint are shared across OEM part numbers BL-40, BL-OS4, and BL-VA — all three reference the same physical pack for this model.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench, confirmed the BMS protection triggers correctly at low voltage, and verified the charge IC accepts the cell without fault flags on the first cycle.
• Fuel gauge recalibration on first use: On the first use after installation, run the Boris from 100% down to automatic shutdown without interruption, then charge fully in one go. This gives the fuel gauge IC a complete discharge curve to work from — skipping this step is the most common cause of erratic percentage readings after a cell swap.

Why the NGM Boris reports wrong battery percentage after a cell swap

The Boris uses a fuel gauge IC that builds its capacity model from accumulated charge and discharge data. When you swap in a new cell, the IC still holds the old cell's discharge curve in memory. It will map voltage readings against that stale curve, which produces percentage readings that no longer match the actual state of charge. One full uninterrupted discharge-to-shutdown followed by a full charge resets the coulomb counter and forces the IC to relearn the curve against the new cell. After that single cycle, percentage accuracy stabilises.

Sudden shutdown at 20–30% on the NGM Boris after replacement

This happens when the cell voltage drops below the load threshold faster than the fuel gauge expects — typically during a burst of modem activity or screen-on demand. The phone shuts off not because the gauge reads zero, but because the cell cannot sustain voltage under that instantaneous load. It is most common in the first few cycles before the fuel gauge IC has a calibrated curve for the new cell. Run two full discharge-charge cycles and check whether shutdowns persist; if the phone still cuts out, measure resting voltage at the point of shutdown — it should read above 3.5V on recovery.