About this replacement
LG VS840 Lucid Series — 3.7V Li-ion Replacement Battery (BL-44JS)
This is a 3.7V, 1700mAh Li-ion replacement battery for the LG VS840 and its variants, including the Lucid, Lucid 4G, and Cayman. It uses the OEM part number BL-44JS and matches the original cell's physical dimensions at 65.00 × 43.80 × 4.50mm. Swap it in when the original cell no longer holds a usable charge through a normal day.
- VS840 / Lucid platform fit: These models share the same battery bay dimensions, connector pinout, and BMS communication protocol. One cell covers all variants in the Lucid lineup without modification.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on the VS840 platform. The BMS negotiated charge termination cleanly and held the expected voltage curve across multiple cycles without tripping overcurrent cutoff.
- Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one full discharge-charge cycle to completion. This lets the phone's fuel gauge IC map the new cell's discharge curve before drawing high current into an uncalibrated cell.
Sudden shutdown at 20–30% on the VS840 after a cell swap
The VS840's fuel gauge IC retains the learned discharge curve of the original cell. When a new cell goes in, the IC has no accurate data for where voltage drops sharply. At around 3.5V — which can correspond to 20–30% in a miscalibrated gauge — the phone hits a voltage cliff under modem or screen load and shuts off before the reported percentage reaches zero. Running two full discharge-charge cycles without interruption gives the coulomb counter enough data to correct the curve. After calibration, shutdowns at falsely high percentages stop.
Phone feels warm near the battery during the first few charges
A new Li-ion cell has slightly higher internal impedance than a broken-in cell. During the first few charge cycles, the charge IC pushes current into higher resistance, which converts more energy to heat than usual. This is temporary — impedance drops as the cell cycles. If the phone surface temperature stays below roughly 40°C and cools within a few minutes of unplugging, no action is needed. If it stays hot after unplugging, check that the charge port is clear and retry with the original cable.