Mobistel Cynus F3 Replacement Battery BTY26181 3.7V 1600mAh

Mobistel Cynus F3 — 3.7V Li-ion Replacement Battery (BTY26181)

This is a 3.7V, 1600mAh Li-ion replacement cell for the Mobistel Cynus F3 smartphone, also sold as the MT-7511, MT-7511W, and MT-7511S. It slots directly into the original battery bay and connects to the same BMS-side contacts. Capacity is 1600mAh (5.92Wh), matching the OEM spec.

• Cynus F3 / MT-7511 series fit: The MT-7511, MT-7511W, and MT-7511S all share the same 3.7V battery bay, connector pinout, and BMS handshake — one cell covers the full model range.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Cynus F3 platform. The BMS accepted the cell on first connection, thermal cutoff triggered correctly under load, and protection circuitry held within spec throughout.
• Fuel gauge recalibration on first use: Disable fast charging if your device supports it, then run one full discharge-to-charge cycle before normal use. This allows the fuel gauge IC to map the new cell's discharge curve before higher charge currents are applied to an uncalibrated cell.

Sudden shutdown at 20–30% on the Cynus F3 after a cell swap

This happens because the fuel gauge IC still holds the discharge curve of the old, degraded cell. When the new cell hits a voltage point the IC maps to 20–30%, it may actually be near the cell's actual low-voltage cutoff under modem or screen load. The phone shuts down because real cell voltage drops below 3.2V under that load even though the percentage display hasn't caught up. One full discharge-charge cycle resets the coulomb counter and resolves this.

Phone won't power on after the replacement cell sat in storage

Li-ion cells self-discharge during storage. If the cell voltage dropped below 2.5V, the BMS enters lockout mode to prevent unsafe charging of a deeply discharged cell. The phone will show no response — not even a charge indicator — when you first plug it in. Connect the phone to a wall charger (not USB from a PC) and leave it for 15–20 minutes; the charge IC will trickle-charge the cell back above 2.8V, at which point the BMS releases lockout and normal charging resumes.