Samsung SPH-A110 Replacement Battery BST5339WA 3.7V 750mAh

Samsung SPH-A110 / SPH-A120 — 3.7V Li-ion Replacement Battery (BST5339WA)

This is a 3.7V, 750mAh Li-ion cell replacing the original BST5339WA in the Samsung SPH-A110 and SPH-A120 flip phones. Both models use an identical battery bay, connector, and contact arrangement. Fits either handset without modification.

• SPH-A110 and SPH-A120 compatibility: Samsung built the A110 and A120 on the same hardware platform with a shared battery bay geometry, contact pitch, and voltage rail. One cell covers both handsets because the BMS handshake and physical form factor are identical across the pair.
• Bench tested on actual hardware: We cycled this cell through a full discharge and charge sequence on a compatible SPH-series handset. The BMS accepted the cell without fault codes, charge termination triggered correctly at 4.2V, and the protection circuit tripped as expected at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: After installing this cell, run the phone down to auto-shutoff, then charge it to 100% uninterrupted before normal use. The A110/A120 fuel gauge IC calibrates its coulomb counter against that first complete discharge curve — skipping this step causes the percentage indicator to read inaccurately for several subsequent cycles.

Why the SPH-A110 reports the wrong battery percentage after a cell swap

The fuel gauge IC on these early-2000s Samsung handsets stores a discharge curve from the original factory cell. When a new cell goes in, that stored curve no longer matches the actual cell chemistry profile. The gauge reads voltage and estimates percentage against outdated reference data, so the number on screen drifts. One full discharge-to-shutoff followed by a full charge resets the reference point and brings the percentage indicator back into alignment.

Sudden shutdown at 20–30% remaining on the SPH-A110

This is a voltage cliff failure. As a Li-ion cell ages — or before a new cell's gauge IC has calibrated — the reported percentage can be 25% while the actual cell voltage under call load drops below the BMS cutoff threshold. The phone shuts down not because the fuel gauge says zero, but because the protection circuit sees a voltage dip it cannot allow. After the first full calibration cycle, the gauge tracks actual cell voltage more accurately and the premature shutdowns stop. If they persist, check that the battery contacts on the handset are clean and making firm contact — resistance at the pins causes the same voltage sag under load.