Sprint LS840 Replacement Battery BL-44JS 3.7V 1200mAh

Sprint LS840 — 3.7V Li-ion Replacement Battery (BL-44JS)

This 3.7V, 1200mAh Li-ion cell replaces the original BL-44JS battery in the Sprint LS840 smartphone. It fits the LS840 directly, restoring power to calls, messaging, and applications. Capacity figures come from the product specification — 4.44Wh at rated voltage.

• LS840 fitment: The LS840 uses a removable cell bay with a fixed connector orientation and a BMS that handshakes on a single data line. This replacement matches that connector and communicates correctly with the phone's charge IC so the OS reads cell status normally.
• Bench tested on actual hardware: We cycled this cell on an LS840 unit and logged the BMS response across charge and discharge. The protection circuit tripped correctly at low-voltage cutoff, and the charge IC accepted a full CC-CV charge profile without fault flags.
• First-cycle fuel gauge recalibration: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before fast charging pushes high current into an uncalibrated cell — otherwise the OS percentage readout will drift.

Sudden shutdown at 20–30% on the LS840 after a cell swap

The LS840's fuel gauge IC holds a discharge curve learned from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-versus-capacity profile. Under modem transmit or screen-on load, the phone sees a voltage drop the IC interprets as near-empty, and the OS forces a shutdown to protect the cell. One full discharge-charge cycle at standard rate resets the coulomb counter and re-maps the curve. After that cycle, shutdowns at 20–30% typically stop.

Phone warm near the battery during the first charge after replacement

A new cell arrives with higher internal impedance than a broken-in one. The charge IC pushes current into that resistance, and some energy converts to heat — more than you'd see on a fully conditioned cell. This is normal during the first one or two charge cycles and reduces as impedance drops with use. If the phone feels hot rather than warm, or the back becomes uncomfortable to hold, stop charging and check that the replacement cell is seated flat with no contact pressure on the corners. Surface temperature should normalise by the third full charge.