LG BSL-21G G7100 Compatible Battery 3.7V 850mAh

LG G7100 / Q800 / G7120 — 3.7V Li-ion Replacement Battery (BSL-21G)

This is a 3.7V 850mAh Li-ion replacement cell for the LG G7100, Q800, and G7120 smartphones. It replaces OEM part BSL-21G directly. Fit this when the original cell no longer holds charge or the phone shuts down unexpectedly under load.

• G7100, Q800, and G7120 compatibility: All three models share the same battery bay dimensions, 3.7V nominal voltage rail, and BSL-21G connector pinout — the BMS handshake is identical across the series, so one cell covers all three variants.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the G7100 platform. The BMS accepted charge without flagging an incompatibility error, and the protection circuit tripped correctly at the low-voltage cutoff threshold during the drain test.
• Fuel gauge recalibration on first use: On first installation, disable fast charging and run one full discharge-charge cycle at standard current. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated coulomb counter — otherwise the percentage readout will be unreliable from day one.

Sudden shutdown at 20–30% on the BSL-21G replacement cell

This happens because the phone's fuel gauge IC is still referencing the discharge curve of the old, degraded cell. The new cell has a steeper voltage cliff at lower state-of-charge, and the modem or screen draws enough current to pull cell voltage below the BMS cutoff threshold before the gauge reaches 0%. The fix is one full uninterrupted discharge to automatic shutdown followed by a full charge — this forces the coulomb counter to re-anchor its end-point calibration. After that cycle, shutdowns below 20% should stop.

Phone not powering on after the BSL-21G sat in storage

Li-ion cells stored at low charge self-discharge over time. If the cell drops below approximately 2.5V, the BMS enters lockout to prevent damage and the phone will show no response when the power button is pressed. Connect to a wall charger — not a computer USB port — and leave it for 20–30 minutes without attempting to power on. Most BMS circuits will accept a trickle from the charge IC at this voltage and exit lockout once the cell clears 3.0V, at which point the phone will boot normally.