BTY26166 Elson EL550 Replacement Battery 3.7V 550mAh

Elson EL550 — 3.7V Li-ion Replacement Battery (BTY26166)

This is a 3.7V, 550mAh Li-ion battery for the Elson EL550 mobile phone. It replaces OEM part BTY26166 when the original cell degrades or fails to hold charge through normal daily use. Dimensions are 39.40 × 38.90 × 4.36mm — confirm against your existing cell before fitting.

• EL550 platform fit: The EL550 uses a compact single-cell Li-ion pack at 3.7V nominal. The BTY26166 connector and PCB tab layout are specific to this form factor — not interchangeable with other Elson models in the same era without physical modification.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the bench. The BMS held cutoff correctly at both ends, and charge termination triggered cleanly without voltage overshoot.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging if your charger supports it and run one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before drawing any high-current load against an uncalibrated baseline.

Why the EL550 reports wrong battery percentage after a cell swap

The EL550's fuel gauge IC stores a learned discharge curve from the original cell. When you fit a new cell, that stored curve no longer matches the actual chemistry of the replacement. The phone reads estimated state-of-charge against the old model, so the percentage shown can be 15–25% off in either direction. One full discharge down to automatic shutoff, followed by an uninterrupted charge to 100%, forces the coulomb counter to reset against the new cell's actual capacity.

Sudden shutdown at 20–30% on the replacement cell

A 550mAh cell has limited headroom under combined modem, screen, and processor load. When the EL550 hits peak current draw — a call on a weak signal, for instance — the cell voltage sags sharply below the BMS cutoff threshold even though the reported percentage looks safe. The fuel gauge is reading average state-of-charge, not instantaneous voltage under load. If this happens consistently around 25–30%, run the recalibration cycle described above; the cutoff point typically stabilises once the coulomb counter is tracking the correct discharge curve. After calibration, repeated shutoffs at the same level point to a cell impedance issue — check resting voltage with a meter; it should read above 3.6V after a full charge.