Samsung SGH-X506 Replacement Battery BST5528ST 3.7V 750mAh

Samsung SGH-X506 / SGH-X507 — 3.7V Li-ion Replacement Battery (BST5528ST)

The BST5528ST is a 3.7V, 750mAh Li-ion cell that fits the Samsung SGH-X506 and SGH-X507 handsets. These mid-2000s phones share the same battery bay and connector, so both models take the same pack. If your original cell no longer holds charge through a normal day, this replacement swaps in directly.

• SGH-X506 and SGH-X507 compatibility: Both handsets run the same 3.7V supply rail and use the same physical battery bay with identical connector pinout. One cell fits both without any modification.
• Bench tested on actual hardware: We cycled the BST5528ST through charge and discharge on the SGH-X506. The protection circuit cut in correctly at low voltage, and the charge IC accepted the cell without fault flags on either model.
• Fuel gauge recalibration on first use: After fitting this cell, run one full discharge to automatic power-off, then charge uninterrupted to 100% before normal use. This lets the phone's fuel gauge IC map its percentage readings against the actual discharge curve of the new cell — skipping this step causes erratic percentage jumps for the first several cycles.

Sudden shutdown at 20–30% on the SGH-X506

This is a voltage cliff problem, not a capacity problem. When the modem transmits or the backlight fires at full brightness, current draw spikes sharply. An aged or uncalibrated cell cannot hold its terminal voltage under that load, so it drops below the protection circuit threshold and the phone shuts off — even though the fuel gauge still showed charge remaining. A full recalibration cycle resolves this on a new cell. After one complete discharge-to-cutoff and full recharge, the fuel gauge IC adjusts its low-voltage threshold to match the new cell's actual voltage curve, and the early shutdowns stop.

Phone reports wrong battery percentage after cell swap

The SGH-X506 uses a simple coulomb-counter fuel gauge that stores a reference discharge curve from the original cell. A new cell has a different internal resistance profile, so the old curve no longer maps accurately — the phone reads percentage against the wrong baseline. This shows up as the display jumping from 40% to 10% suddenly, or reading 100% and dropping fast in the first few minutes of use. The fix is one complete discharge cycle: use the phone until it shuts itself off, then charge to full without interruption. After that single cycle the fuel gauge rewrites its reference and percentage readings stabilise. Target a resting voltage of 4.2V at full charge to confirm the cycle completed correctly.