Avantree SP850 Replacement Battery 3.7V 1200mAh Li-ion

Avantree SP850 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1200mAh Li-ion replacement battery for the Avantree SP850 portable Bluetooth speaker. It slots in when the original cell degrades and can no longer hold a useful charge. Dimensions are 53.00 × 33.90 × 5.70mm — confirm these against your existing cell before ordering.

• SP850 fit notes: The SP850 runs a single-cell 3.7V Li-ion configuration. This cell matches the original voltage rail and physical footprint. The BMS on the speaker board manages charge termination and low-voltage cutoff, so the replacement cell needs to match nominal voltage exactly — this one does.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS accepted the cell cleanly, reached full charge termination at 4.2V, and low-voltage cutoff triggered before the cell dropped below 3.0V under load.
• First-cycle recalibration on the SP850: After fitting this cell, run one full discharge down to auto-shutoff, then charge uninterrupted to 100% before use. The SP850 charge controller recalibrates its fuel gauge against the new cell's discharge curve during this first full cycle.

Why the SP850 shuts off while still showing charge remaining

This happens when the fuel gauge IC is still calibrated to the old cell's discharge curve. The new cell has a steeper voltage drop under load, so the controller sees a voltage cliff before the displayed percentage reaches zero. The BMS triggers a protection cutoff at that point to prevent cell damage. One full discharge-to-shutoff followed by a full uninterrupted charge resets the calibration.

SP850 not powering on after the replacement cell sat in storage

Li-ion cells self-discharge in storage. If the cell voltage dropped below roughly 2.5V, the BMS enters lockout mode and blocks normal charging to prevent charging a damaged cell. Connect the speaker to a known-good USB power source and leave it for 20–30 minutes without attempting to power it on — most BMS circuits include a trickle pre-charge path that recovers the cell up to a safe threshold before allowing full charge current. If the charge indicator eventually activates, let it complete the full charge cycle to 4.2V before use.