Samsung SPH-A820 Replacement Battery 3.7V 750mAh Li-ion

Samsung SPH-A820 / SPH-A840 / PM-A840 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 750mAh (2.78Wh) Li-ion replacement battery for the Samsung SPH-A820, SPH-A840, and PM-A840 flip phones. These handsets launched on Sprint's early 2000s network and share a common battery form factor across the three model numbers. If the original cell no longer holds a charge, this unit restores the phone to normal operating condition.

• SPH-A820, SPH-A840, and PM-A840 compatibility: All three models share the same battery bay dimensions (65.84 × 41.75 × 7.35mm), voltage rail, and connector pinout. One battery fits all three without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on compatible hardware. The BMS held the charge cutoff correctly at 4.2V and the low-voltage protection tripped at the expected threshold, with no thermal event during the charge phase.
• Fuel gauge recalibration on first use: After installing this cell, run one full discharge-to-charge cycle before relying on the battery percentage indicator. The phone's fuel gauge IC is calibrated to the old cell's discharge curve. Without a full cycle, the percentage reading will be inaccurate and the phone may shut down at an unexpected charge level.

Why the SPH-A820 shuts down suddenly at 20–30% after a battery swap

This happens because the phone's fuel gauge IC still tracks the discharge curve of the old, degraded cell. When the new cell hits a voltage point the gauge associates with 20–30% on the old curve, the phone interprets it as a critical low and triggers a protective shutdown. The actual cell still has capacity — the gauge just doesn't know it yet. One complete discharge cycle, letting the phone run down to automatic shutdown and then charging uninterrupted to 100%, resets the coulomb counter against the new cell's real curve.

Phone not powering on after the replacement battery sat in storage

Li-ion cells self-discharge in storage. If this battery dropped below approximately 2.5V per cell before installation, the BMS will have entered a deep-discharge lockout state and the phone will show no response when you press the power button. Connect the phone to a charger and leave it for 20–30 minutes without pressing anything — the charge IC needs to push a low recovery current into the cell before the BMS will re-initialise. Once the BMS releases lockout, the charging indicator will appear. If the screen stays blank past 45 minutes on charge, check that the charger is outputting at least 5V.