Mobistel EL550 Replacement Battery BTY26166 3.7V 550mAh

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

This is a 3.7V Li-ion cell rated at 550mAh (2.04Wh), built to replace the original BTY26166 battery in the Mobistel EL550 smartphone. It fits the EL550 directly — same footprint at 39.40 × 38.90 × 4.36mm. Install it when the original cell no longer holds a charge or causes unexpected shutdowns.

• EL550 fitment: The EL550 uses a compact removable cell with a low-current BMS matched to its light-draw smartphone platform. This replacement carries the same OEM part reference BTY26166 and sits within the same connector and physical envelope — no modification needed.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS accepted charge input without fault flags, held voltage above the 3.2V sag threshold under screen and modem load, and terminated correctly at full capacity.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate against the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell — preventing percentage drift from the first day.

Sudden shutdown at 20–30% on the EL550 after a cell swap

This is a voltage cliff failure, not a capacity problem. Under combined modem and display load, cell voltage drops sharply below what the fuel gauge IC predicted — the phone shuts down before the percentage reaches zero. The old cell's discharge curve is still stored in the coulomb counter. One full discharge-to-shutdown followed by a complete charge cycle resets the IC against the new cell's actual curve. After that cycle, the percentage readout tracks real remaining capacity accurately.

EL550 showing erratic percentage jumps after replacement

The fuel gauge IC in the EL550 builds its charge estimate from a learned discharge curve. When a new cell goes in, the IC has no reference data for it and interpolates badly — percentage can jump 10–15% in either direction mid-use. This is not a faulty cell. Run two full discharge-charge cycles without interruption and without fast charging enabled. By the end of the second cycle the coulomb counter has enough data to track the new cell's voltage curve and the jumps stop.