Nokia BL-5J Replacement Battery 3.7V 1200mAh Li-ion

Nokia 5800 XpressMusic Series — 3.7V Li-ion Replacement Battery (BL-5J)

The BL-5J is a 3.7V Li-ion cell rated at 1200mAh (4.44Wh), direct replacement for the original Nokia battery. It fits the Nokia 5800, 5800T, 5800 XpressMusic, N900, and 19 additional Nokia models sharing the same form factor and connector. Physical dimensions are 60.12 × 38.12 × 5.75mm — measure your bay before ordering if you are unsure.

• 5800 series and N900 compatibility: These models share the BL-5J footprint, contact pitch, and 3.7V nominal rail. The BMS handshake on each device expects the same identification resistor value, so the replacement cell is accepted without triggering a battery-warning dialogue on boot.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a Nokia 5800 XpressMusic unit. The BMS accepted the cell on first connection, charge termination triggered correctly at 4.2V, and the protection circuit tripped as expected on an over-discharge test below 2.75V.
• Fuel gauge recalibration on first use: After fitting this cell, disable any fast-charge mode and run one full discharge-to-charge cycle before normal use. The Nokia fuel gauge IC carries its learned curve from the old cell — one complete cycle lets it recalibrate the coulomb counter against the new cell's actual discharge profile before you rely on the percentage readout.

Why the Nokia 5800 reports wrong battery percentage after a cell swap

The 5800 uses a coulomb-counter fuel gauge IC that builds a discharge curve over repeated cycles. When you swap the physical cell, the IC still references the capacity map it learned from the degraded original battery. This mismatch means the phone may show 50% remaining on a cell that is actually near cutoff voltage, or it may display 100% and then drop suddenly. One full discharge down to auto-shutdown followed by a complete charge to 4.2V resets the learned curve. After that single cycle, percentage readouts stabilise and track the new cell accurately.

Sudden shutdown at 20–30% remaining on the replacement cell

This is a voltage-cliff failure, not a capacity problem. Under heavy load — active call, GPS, or screen at full brightness — the cell voltage sags faster than the fuel gauge predicts, hitting the 3.0V under-load cutoff while the reported percentage still shows significant charge. It happens most often in the first few cycles before the fuel gauge IC has mapped the new cell's internal resistance curve. Run two full charge-discharge cycles under normal use, and the shutdowns should stop. If they persist past cycle three, check that the contact tabs on the battery are clean and making full contact — high contact resistance amplifies voltage sag at the same load.