Vivo X30 Pro B-K5 Replacement Battery 3.87V 4200mAh

Vivo X30 Pro / V1938T — 3.87V Li-Polymer Replacement Battery (B-K5)

This is a 3.87V, 4200mAh Li-Polymer cell built to the B-K5 specification. It fits the Vivo X30 Pro, X30 Pro 5G, V1938T, and V1938. If your X30 Pro is shutting down unexpectedly, holding less charge than it used to, or not powering on at all, this cell replaces the original unit directly.

• X30 Pro and V1938 series fit: The X30 Pro, X30 Pro 5G, V1938T, and V1938 share the same battery bay dimensions and connector pinout. All use the B-K5 form factor — 73.70 × 62.64 × 5.00mm — with the same BMS communication line to Vivo's charge IC.
• Bench tested on actual hardware: We ran this cell through a full charge and discharge cycle on the X30 Pro. The BMS handshake completed without error, charge IC accepted the cell, and the fuel gauge IC began tracking state of charge correctly after one full cycle.
• First cycle after installation: Disable fast charging for your first complete discharge and charge cycle. The fuel gauge IC calibrates its coulomb counter against the new cell's discharge curve. Running fast charge into an uncalibrated cell pushes current before the IC has accurate reference points.

Sudden shutdown at 20–30% on the X30 Pro after a cell swap

The X30 Pro uses a coulomb counter–based fuel gauge IC that was calibrated to your original cell's discharge curve. A new cell has a different voltage-to-capacity curve, so the IC underestimates remaining charge. When the modem fires at full power or the screen brightness spikes, the cell voltage drops below the BMS cutoff threshold — typically around 3.2V per cell — faster than the IC predicted, triggering a hard shutdown. One complete discharge to auto-off followed by a full charge to 100% resets the IC's reference points and stops these premature cutoffs.

USB-PD or Vivo FlashCharge not activating on the first charge after replacement

On the first connection after a cell swap, the charge IC on the X30 Pro runs a BMS interrogation before it will negotiate fast charge protocols. If the new cell's BMS returns an unexpected state — common when the cell has been in storage — the IC falls back to standard 5V charging as a safety measure. This is not a fault with the replacement cell. Disconnect the charger, power the device off, reconnect, and allow the first charge to complete at standard rate. Fast charge will activate normally on subsequent cycles once the BMS has logged a full charge event.