About this replacement
Nokia Lumia 820 / Lumia 825 — 3.7V Li-ion Replacement Battery (BP-5T)
This is a 3.7V, 1650mAh (6.11Wh) lithium-ion battery that replaces the original BP-5T cell in the Nokia Lumia 820, Lumia 820.2, Arrow, and Lumia 825. It slots into the same removable battery bay and connects via the same three-contact interface as the factory cell. Capacity matches the original Nokia specification — 1650mAh from the product data, not inflated figures.
- Lumia 820 / 820.2 / Arrow / 825 platform fit: These models share the same physical battery bay dimensions (76.00 × 35.50 × 5.10mm), identical three-pin contact layout, and the same BMS voltage thresholds — so one BP-5T cell covers all four variants without any hardware modification.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Lumia 820 unit. The BMS accepted the cell without triggering a protection cutoff, and the charge IC brought the cell to 4.2V termination voltage cleanly across multiple cycles.
- Fuel gauge recalibration on first use: On first installation, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100% before use. The Lumia 820's coulomb counter needs a full reference cycle against the new cell's actual discharge curve — skipping this is what causes early percentage jumps.
Why the Lumia 820 reports wrong battery percentage after a cell swap
The Lumia 820 uses a coulomb counter in its fuel gauge IC that builds a discharge model over time. When you swap in a new cell, that model is still calibrated to the old, degraded cell's curve — so the reported percentage diverges from actual remaining charge. This mismatch shows as erratic percentage jumps or the phone shutting off while still showing 15–25% remaining. One full discharge-to-shutdown cycle followed by an uninterrupted charge to 4.2V termination lets the IC re-anchor its model to the new cell. After that single reference cycle, percentage reporting stabilises.
Sudden shutdown at 20–30% on the replacement cell
This happens when the modem or display pulls a current spike the cell's internal resistance can't sustain — the terminal voltage collapses briefly below the BMS cutoff threshold, and the phone shuts off even though the fuel gauge still shows charge remaining. It's most common on the first few cycles before the cell's impedance settles. Let the phone cool for two minutes after shutdown, power it back on, and check the resting voltage — if it recovers above 3.6V immediately, the cell is fine and the BMS tripped on a transient dip, not a dead cell. The shutdowns typically stop after three to five full charge cycles as the cell's internal resistance stabilises.