O2 XDA Terra 35H00078-02M Replacement Battery 3.7V 2400mAh

O2 XDA Terra — 3.7V Li-Polymer Replacement Battery (35H00078-02M)

This is a 3.7V, 2400mAh Li-Polymer cell that fits the O2 XDA Terra Windows Mobile smartphone. It replaces a depleted or failing original cell and restores normal operation for calls, messaging, and applications. Voltage and chemistry match the original spec exactly.

• XDA Terra fit: The Terra uses a slim Li-Polymer pack with a specific connector pinout tied to the device's charge IC and fuel gauge. This cell matches that pinout and the 3.7V nominal rail the BMS expects — no adapter or modification needed.
• Bench tested on actual hardware: We cycled this cell through charge, standby draw, and active GSM load on the Terra platform. The BMS accepted the cell without fault codes, and the charge IC completed a full CC/CV cycle without interruption.
• Fuel gauge recalibration on first use: On first use after installation, run one full discharge-charge cycle with fast charging disabled. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before reporting accurate percentages.

Why the XDA Terra reports wrong battery percentage after a cell swap

The Terra's fuel gauge IC stores a learned discharge curve from the previous cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The IC interpolates percentage from voltage readings calibrated to the old cell, so readings can be off by 15–25% until recalibrated. One complete discharge below 10% followed by a full charge corrects the stored curve and brings the readout back in line.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC hasn't recalibrated and the cell voltage drops below the BMS cutoff threshold faster than the percentage display indicates. Under GSM transmit or screen-on load, current draw spikes and voltage sags — the BMS trips the cell offline even though the display still shows charge remaining. It's not a faulty cell; it's a calibration lag. Force a full discharge to near shutdown, then charge uninterrupted to 100% — after that cycle, the cutoff and the displayed percentage will align, typically above 3.5V at the low end.