LG Watch Urbane BL-S3 Replacement Battery 3.8V 400mAh

LG Watch Urbane / W150 — 3.8V Li-Polymer Replacement Battery (BL-S3)

This is a 3.8V, 400mAh Li-Polymer cell that replaces the original BL-S3 in the LG Watch Urbane (W150). It fits the cavity dimensions of that watch exactly — 30.90 × 27.50 × 6.50mm — and connects to the same flex connector on the main board. Swap it in when the original cell no longer holds charge through a normal day of use.

• Watch Urbane / W150 fitment: Both model designations use the same chassis, the same flex connector pinout, and the same BMS handshake — one cell covers both. The BL-S3 part number is the identifier that matters; LG used it across the W150 lineup regardless of regional naming.
• Bench tested on actual hardware: We cycled this cell on a W150 unit and confirmed the BMS accepted the new cell without fault flags. The fuel gauge IC re-initialised cleanly on the first full charge cycle after swap.
• Post-swap charge protocol for the Urbane: After fitting this cell, place the watch on its magnetic charger immediately and run a full charge to 100% before turning it on. The MAX17048 fuel gauge IC on the W150 loses its state-of-charge reference when power is removed — it cannot recalibrate without a complete charge cycle starting from near-zero. Skip this step and the percentage readout will be inaccurate for days.

Why the W150 fuel gauge reads 0% immediately after a cell swap

The LG Watch Urbane uses a Maxim MAX17048 coulomb-counter IC to track state of charge. When you disconnect the original cell, the IC loses its learned voltage-to-capacity curve entirely. The new cell may sit at 3.75V — physically around 50% charge — but the IC has no reference and defaults to 0% or a random low value. A full charge cycle from the moment of installation resets the IC's endpoints and restores accurate readings. Do not trust the percentage until that first full charge completes.

Always-on display and continuous heart rate monitoring cutting charge life to under a day

The W150's AMOLED panel in always-on mode draws roughly 15–20mA continuously — a significant fraction of a 400mAh cell on its own. Stack continuous heart rate polling on top and the combined draw can exceed 30mA sustained. A new cell at full 400mAh capacity will still drain faster than expected under this load profile. Disable always-on display and set heart rate monitoring to periodic rather than continuous in the LG Watch Manager app — that alone recovers meaningful capacity per charge cycle.