Panasonic X800 EB-BSX800 Replacement Battery 3.7V 700mAh

Panasonic X800 — 3.7V Li-ion Replacement Battery (EB-BSX800)

This is a 3.7V, 700mAh Li-ion replacement battery for the Panasonic X800 smartphone (also listed as EB-X800). It uses OEM part numbers EB-BSX800 and EB-BSX800CN. Install it when the original cell has degraded and the phone no longer holds a usable charge.

• X800 and EB-X800 fit: Both model designations share the same physical form factor, voltage rail, and connector pinout, so one cell covers both. The BMS handshake requirements are identical across the variant run.
• Bench tested on actual hardware: We ran charge cycles through a calibrated DC load and confirmed the BMS accepts charge current correctly, triggers overvoltage protection at the expected threshold, and holds resting voltage above 3.6V after a full cycle.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes energy into an uncalibrated cell — otherwise the percentage readout will drift.

Why the X800 reports wrong battery percentage after a cell swap

The X800 uses a fuel gauge IC that tracks charge state using a coulomb counter calibrated to the original cell's discharge curve. When you fit a new cell, that curve no longer matches, so the IC miscounts capacity and displays an inaccurate percentage. The fix is one full discharge to system cutoff followed by an uninterrupted charge to 100%. After that cycle, the IC has enough data to recalibrate its model against the new cell and the readout stabilises.

Sudden shutdown at 20–30% remaining on the X800

This is a voltage cliff, not a capacity problem. Under modem or display load the cell voltage drops sharply — faster than the fuel gauge IC predicts — and the phone hits the low-voltage cutoff before the displayed percentage reaches zero. It happens most often on a new cell before the fuel gauge has completed its first calibration cycle. Run one full discharge to shutdown, then charge uninterrupted to 100%. If shutdowns continue after two full cycles, check resting cell voltage with a multimeter — it should read above 3.6V within 10 minutes of removing charge current.