Microsoft Lumia 550 Replacement Battery BL-T5A 3.8V 1900mAh

Microsoft Lumia 550 / Lumia 730 / Lumia 735 — 3.8V Li-Polymer Replacement Battery (BL-T5A / BV-T5A)

This is a 3.8V, 1900mAh lithium-polymer replacement battery for the Microsoft Lumia 550 (RM-1127), Lumia 730, and Lumia 735. It slots into the original battery bay and connects to the same charge IC and fuel gauge circuit the phone shipped with. Capacity listed above comes from product data — 7.22Wh total stored energy.

• Lumia 550 / 730 / 735 platform fit: These models share the same 3.8V cell format, connector pinout, and BMS handshake protocol. One cell covers the full compatibility list. No adapter or wiring change is needed.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Lumia platform. The BMS accepted the handshake on first connection, charge IC stepped through pre-charge and CC/CV stages without fault, and the cell held voltage at load without dropping to cutoff prematurely.
• Fuel gauge recalibration on first cycle: After fitting this cell, run one full discharge to near-empty and then charge to 100% before enabling fast charge. The Lumia fuel gauge IC reads the new cell's discharge curve during that first cycle — skipping it leaves the coulomb counter calibrated to the old, degraded cell and causes erratic percentage readings.

Why the Lumia 550 shuts down suddenly at 20–30% after a battery swap

The Lumia 550's fuel gauge IC inherits the charge curve from the old cell. A new cell has a steeper voltage cliff at the lower end of its discharge range — the phone's coulomb counter hasn't seen it yet, so it predicts remaining capacity based on the old curve. When the modem transmits or the screen brightness spikes, the cell voltage drops faster than the IC expects. The system hits the hardware low-voltage cutoff at what the OS still believes is 25–30%. One full calibration cycle — drain to shutdown, charge uninterrupted to 100% — resets the curve and clears this behaviour.

Phone reports 100% immediately after fitting the new cell, then drops fast

This is a fuel gauge IC artefact, not a faulty cell. The IC holds the last known state of charge from the old battery in non-volatile memory. When a new cell connects, the chip reads a resting voltage around 3.8–3.9V and maps it to a high percentage based on stale calibration data. As actual discharge begins, the real curve diverges from the stored one and the percentage corrects downward sharply. Force a full discharge cycle — let the phone run down until it powers off, then charge to 100% without interruption — and the IC will write a new baseline from 0% to 100% against the replacement cell.