YotaPhone C9660 Compatible Battery 3.8V 1800mAh YT0125081

YOTAPHONE C9660 — 3.8V Li-ion Replacement Battery (YT0125081)

This is a 3.8V, 1800mAh Li-ion cell built to fit the YOTAPHONE C9660 smartphone. The C9660 runs dual displays — an LCD on the front and an e-ink panel on the rear — and both draw from this single cell. Part number YT0125081 matches the original unit's connector and BMS handshake requirements.

• Dual-display power rail: The C9660 routes power to both the LCD and e-ink controller from one battery circuit. The e-ink display refreshes intermittently but the LCD draws continuous current during active use. A cell that cannot hold its voltage under combined load will trigger the BMS before the gauge reads empty.
• Bench tested on actual hardware: We cycled this cell through full discharge and charge sequences, monitoring BMS handshake confirmation and voltage response under simultaneous dual-display load. The protection circuit tripped at the correct undervoltage threshold and recovered cleanly on reconnection.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard current. This lets the fuel gauge IC map its coulomb counter to the new cell's actual discharge curve before high-current charging begins on an uncalibrated cell.

Sudden shutdown at 20–30% on the C9660 after a cell swap

The C9660's fuel gauge IC retains the discharge curve from the old cell after a swap. When the modem transmits or both displays are active together, current draw spikes. If the gauge IC has not yet recalibrated to the new cell's voltage curve, it misreads state-of-charge — the cell hits its actual voltage floor before the OS expects it, and the BMS cuts power immediately. Running one full discharge down to automatic shutdown, then charging uninterrupted to 100%, rewrites the coulomb counter reference. After that cycle, shutdowns at falsely high percentages stop.

Device not powering on after the replacement cell sat in storage

Li-ion cells in storage self-discharge slowly. If the cell drops below approximately 2.5V per cell before installation, the BMS enters lockout mode and blocks charge current to prevent damage to a deeply discharged cell. The phone will show no response — no charge indicator, no boot. Connect the phone to a wall charger rated at least 5V/1A and leave it undisturbed for 20–30 minutes; the BMS recovery circuit trickle-charges the cell at low current until voltage climbs above the lockout threshold, after which normal charging resumes.