O2 XDA III Replacement Battery 3.7V 4200mAh PH26B

O2 XDA III / XDA IIs — 3.7V Li-ion Replacement Battery (PH26B)

This is a 3.7V Li-ion battery rated at 4200mAh (15.54Wh), fitting the O2 XDA III and XDA IIs Windows Mobile PDA phones. Both devices share the same battery bay dimensions and connector spec, making a single cell compatible across the two models. OEM part numbers PH26B and AHTXDSSN both cross-reference to this unit.

• XDA III and XDA IIs compatibility: Both devices run the same voltage rail and use the same physical contact layout. The BMS in each handset communicates over the same three-pin thermistor line, so there is no handshake mismatch between models.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the XDA III platform. The BMS held charge termination cleanly at 4.2V and the protection circuit tripped correctly at low-voltage threshold without locking the host device.
• Fuel gauge recalibration after installation: On first use, disable any power-saving shortcuts and run one complete discharge-charge cycle without interruption. The XDA III fuel gauge IC calibrates against the new cell's discharge curve during this cycle — skipping it produces erratic percentage readings for the first several days.

Sudden shutdown at 20–30% on the XDA III

The XDA III's modem and display draw brief high-current spikes that a degraded or uncalibrated cell cannot sustain. When the cell voltage drops sharply under that load — even while the fuel gauge still shows 20–30% — the protection circuit cuts output to prevent over-discharge. This is a voltage cliff, not a capacity fault. Run one uninterrupted full discharge-charge cycle so the coulomb counter re-maps the actual cell voltage curve, then verify the device holds steady past the 25% mark.

Device will not power on after sitting in storage

Li-ion cells self-discharge slowly in storage. If the XDA III has been unused for several months, the cell may have dropped below 2.5V per cell — the threshold where the BMS engages a lockout to prevent damage. The device will show no response to the power button and may not register on charge immediately. Connect it to a charger and leave it for 20–30 minutes before attempting to power on; the charge IC needs to push enough current into the cell to bring it above the 2.5V recovery threshold before the BMS releases.