Olympia Joy II 3.7V 900mAh Replacement Battery Li-ion

Olympia Joy II — 3.7V Li-ion Replacement Battery (Joy II)

This 3.7V, 900mAh Li-ion cell replaces the original battery in the Olympia Joy II smartphone. It restores normal operation for calls, messaging, and apps when the factory cell has degraded or stopped holding charge. Dimensions are 53.00 × 34.00 × 4.30mm — confirm against your existing cell before fitting.

• Joy II platform fit: The Joy II uses a compact, low-draw 3.7V single-cell architecture. This replacement matches the original voltage rail and connector orientation, so the charge IC and fuel gauge IC see the same electrical interface as the factory cell.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Joy II platform. The BMS accepted the cell without rejection events, and charge termination triggered correctly at full capacity.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging if available and run one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before it reports percentage data to the OS — skipping this step is the most common cause of erratic percentage readings after a cell swap.

Sudden shutdown at 20–30% on the replacement cell

The Joy II's fuel gauge IC retains the discharge curve of the old, degraded cell. When a new cell is fitted, the IC's stored curve no longer matches actual cell voltage under load. The modem and screen draw current spikes that push the cell below the IC's cutoff threshold — even though real charge remains. One full discharge to auto-off, followed by an uninterrupted charge to 100%, resets the coulomb counter and clears this mismatch.

Device not powering on after the replacement cell sat in storage

Li-ion cells self-discharge during storage. If the replacement cell's voltage drops below approximately 2.5V, the BMS enters lockout mode and blocks normal charging to prevent damage. The Joy II may show no response at all — no boot, no charge indicator. Connect to a wall charger and leave it for 20–30 minutes without pressing the power button; the charge IC on most Joy II units will trickle-charge the cell back above the BMS re-enable threshold before handing off to normal CC/CV charging.