BL-44JS Sprint LS840 Replacement Battery 3.7V 2400mAh

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

This is a 3.7V, 2400mAh Li-ion replacement cell for the Sprint LS840 and LS840 Viper Android smartphones. It fits both variants and matches the OEM footprint at 64.90 × 43.70 × 10.60mm. The original BL-44JS chemistry and connector are replicated so the phone's charge IC accepts the cell without modification.

• LS840 and LS840 Viper compatibility: Both models run the same voltage rail, share the same battery bay dimensions, and use identical connector pinouts — one cell covers both. The BMS handshake and NTC thermistor line are wired identically across the two variants.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the LS840 platform. The charge IC accepted the cell without error flags, the BMS held cutoff at the correct low-voltage threshold, and the thermistor line reported normal temperature to the OS throughout.
• Fuel gauge recalibration on first use: Disable fast charging if your handset supports it, then run one full discharge down to automatic shutdown before charging back to 100%. The LS840 fuel gauge IC was calibrated to the original cell's discharge curve — a single full cycle resets the coulomb counter to the new cell's actual capacity.

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

The LS840's Snapdragon S2 and AMOLED display together create a high-current draw spike that older or freshly installed cells struggle to sustain. When battery voltage drops below roughly 3.5V under load, the BMS pulls the cell offline to prevent damage — even if the fuel gauge is still reading 20% or higher. This is a voltage cliff, not a capacity defect. One full discharge-to-shutdown and full recharge cycle gives the fuel gauge IC a calibrated reference point and usually eliminates the premature cutoff.

Phone warm near the battery bay during the first few charges

A new cell arrives with higher internal impedance than a broken-in cell, so the charge IC on the LS840 pushes slightly more voltage to maintain its target charge current. That extra voltage drop across the cell's internal resistance generates heat. It is most noticeable during the first two or three charge cycles and reduces as the cell breaks in. If the back cover stays consistently hot — above uncomfortable to the touch — after three full cycles, check that the NTC thermistor tab is seated flat against the battery contact, not bent away.