About this replacement
Nokia Lumia 820 Series — 3.7V Li-ion Replacement Battery (BP-5T)
This is a 3.7V, 1200mAh Li-ion replacement battery for the Nokia Lumia 820, 820.2, Arrow, and Lumia 825. It slots into the removable battery bay and restores power to all core functions — calls, display, modem, and apps. Capacity matches the original BP-5T specification at 4.44Wh.
- Lumia 820 series fit: The 820, 820.2, Arrow, and 825 share the same BP-5T form factor and connector pinout. The battery bay dimensions are identical across these variants — 76.00 x 35.50 x 5.10mm — so the cell fits without modification.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on the Lumia 820 platform. The BMS handshake completed correctly, charge termination triggered at the expected voltage ceiling, and the protection circuit responded to over-discharge conditions without tripping prematurely.
- Fuel gauge recalibration on first cycle: After installing this cell, disable fast charging and run one complete discharge down to automatic shutdown, then charge uninterrupted to 100%. The Lumia's fuel gauge IC calibrates its coulomb counter against the new cell's discharge curve on that first cycle — skipping it causes erratic percentage readings for the first several days of use.
Sudden shutdown at 20–30% on the Lumia 820 after a cell swap
This is a voltage cliff issue, not a capacity fault. Under peak modem load — LTE handoff, active call, or screen at full brightness — the cell voltage drops sharply. If the fuel gauge IC still references the old cell's discharge curve, it misreads remaining capacity and the OS shuts down before the percentage reaches zero. The fix is one full uncalibrated cycle: discharge to auto-shutdown, then charge to 100% without interruption. After that cycle, the coulomb counter resets to the new cell's actual curve and the premature shutdowns stop.
Phone won't power on after the replacement cell sat in storage
Li-ion cells self-discharge in storage. If this battery has been sitting unused, its resting voltage may have dropped below 2.5V per cell — the threshold where the BMS locks out to prevent damage. The phone will not respond to the power button or show a charging indicator at first. Connect to a wall charger and leave it untouched for 15–20 minutes; the BMS needs a trickle current to recover enough voltage to re-initialise before normal charging resumes. If the charging indicator appears, the BMS has recovered — continue charging to full before powering on.