Oppo Realme C2 BLP721 Replacement Battery 3.85V 4000mAh

Oppo Realme C2 / C2s — 3.85V Li-Polymer Replacement Battery (BLP721)

The BLP721 is a 3.85V, 4000mAh Li-Polymer cell that fits the Realme C2, Realme C2s, R1941, and RMX1945. It replaces the original battery in phones that no longer hold charge, shut down unexpectedly, or fail to power on. Capacity figure comes from the product specification at 15.4Wh.

• C2 and C2s platform compatibility: Both the C2 and C2s share the same battery bay dimensions (86.00 × 62.30 × 4.20mm), the same 3.85V nominal rail, and the same BLP721 part number — which is why one cell covers both variants without connector modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the C2 platform. The BMS accepted charge-in handshake without fault flags, and the protection circuit tripped correctly at the expected low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging and run one full discharge-to-charge cycle at standard rate. The Realme C2's fuel gauge IC was calibrated to the old cell's discharge curve — one slow cycle lets it relearn the new cell before high-current charging begins.

Sudden shutdown at 20–30% on the Realme C2 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 misreads as critically low, it triggers a protective shutdown — even though real charge remains. The phone's coulomb counter needs one full uninterrupted discharge cycle to remap to the new cell's voltage curve. Charge to 100%, then let the phone discharge to auto-shutdown without interruption — after that cycle, the gauge tracks accurately.

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

Li-Polymer cells self-discharge in storage, and if the BLP721 dropped below approximately 2.5V per cell before installation, the BMS enters lockout mode to prevent damage — the phone shows no response to the power button. Connect the phone to a wall charger (not a PC port) and leave it for 20–30 minutes without pressing anything. The charge IC trickle-charges the cell out of lockout; once voltage recovers above the BMS re-enable threshold, the phone will boot and charge normally.