Oppo BLPA83 Replacement Battery 3.91V 4850mAh Li-Polymer

Oppo RMX3939 / Realme C61 C63 C65s — 3.91V Li-Polymer Replacement Battery (BLPA83)

This is a 4850mAh 3.91V lithium-polymer replacement cell for the Oppo RMX3939 and the Realme C61, C63, and C65s smartphones. It matches the OEM part number BLPA83 and fits the same flex-cable connector and frame recess as the factory cell. Total energy is 18.96Wh.

• RMX3939 and Realme C-series compatibility: These four models share the same mainboard voltage rail, BMS handshake protocol, and physical cell envelope. The BLPA83 part number covers all of them from the factory, so one cell fits across the group.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the RMX3939 platform. The BMS accepted the cell without error flags, and the charge IC completed a full CC-CV cycle to 4.35V without thermal or current cutoff events.
• 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 build a new discharge curve against the replacement cell before high-current charging pushes current into an uncalibrated coulomb counter.

Sudden shutdown at 20–30% on the Realme C61 and C63 after a cell swap

This is a voltage-cliff issue, not a capacity fault. When the fuel gauge IC is still calibrated to the old cell's discharge curve, it reports 20–30% remaining at a point where the new cell's actual voltage has already dropped below what the modem and display demand under load. The processor sees an undervoltage event and forces a shutdown before the percentage hits zero. One full discharge-to-shutdown followed by an uninterrupted charge to 100% resets the coulomb counter and corrects the reading.

USB-PD or VOOC fast charge not activating after replacement

Some replacement cells present slightly higher impedance on the first cycle than a worn OEM cell, which causes the charge IC to fall back to standard 5V charging as a protective measure. This is not a fault with the cell or the port. Run one full slow charge from flat to 100% with fast charging disabled in settings. After that cycle, re-enable fast charging — the charge IC will re-negotiate the VOOC or USB-PD contract against the now-primed cell and should resume at full current.