Sony Ericsson U100 BST-43 Replacement Battery 3.7V 950mAh

Sony Ericsson U100 / YARI Series — 3.7V Li-ion Replacement Battery (BST-43)

This is a 3.7V, 950mAh Li-ion replacement battery for the Sony Ericsson U100, U100i, YARI, and Vulcan. It uses the BST-43 cell spec and fits all handsets in that lineup. Slide it in where the original sits — the connector and footprint match directly.

• U100 and YARI platform compatibility: These models share the same battery bay dimensions, 3.7V single-cell architecture, and BST-43 connector pinout. The BMS handshake protocol is identical across the U100i, YARI, and Vulcan variants, so one cell covers the full range without firmware or connector differences.
• Bench tested on actual hardware: We cycled this cell on the U100 platform and confirmed the BMS accepted charge without fault flags on first insertion. Charge termination triggered correctly at 4.2V, and the protection circuit tripped cleanly at the low-voltage cutoff threshold during full discharge testing.
• Fuel gauge recalibration on first use: After fitting this cell, run one full uninterrupted discharge down to auto-shutdown, then charge to 100% without interruption. The U100's fuel gauge IC maps its percentage readings against this first discharge curve — skipping it leaves the coulomb counter calibrated to the old cell and the percentage display will be inaccurate from day one.

Why the U100 reports wrong battery percentage after a BST-43 swap

The U100 uses a fuel gauge IC that tracks charge state by counting coulombs against a stored discharge curve from the previous cell. When you install a new BST-43, that stored curve no longer matches the new cell's actual voltage-to-capacity profile. The IC keeps using stale reference data until it sees a full uninterrupted cycle. One complete discharge-to-shutdown followed by a full charge resets the reference and brings the percentage display back into line.

Phone shuts off suddenly at 20–30% on the replacement cell

This happens because the new cell hits a voltage cliff under screen or radio load before the displayed percentage reaches zero. The U100's processor draws a short current spike when the modem transmits or the display backlights — if the cell voltage drops below roughly 3.4V under that load, the phone shuts down even though the resting voltage read by the fuel gauge looked fine a moment earlier. The fix is the same full recalibration cycle: drain completely to auto-shutdown, then charge uninterrupted to 100%. After one full cycle the gauge IC will track the actual voltage curve and the premature cutoff will stop.