T-Mobile PH26B MDA iii Replacement Battery 3.7V 4200mAh

T-Mobile MDA iii — 3.7V Li-ion Replacement Battery (PH26B)

This is a 3.7V, 4200mAh Li-ion battery for the T-Mobile MDA iii, which is the T-Mobile-branded HTC Touch Pro2 Windows Mobile smartphone. It slots in where the original cell sits behind the rear cover. If your MDA iii no longer holds a charge or shuts down unexpectedly, this cell replaces the degraded OEM pack directly.

• MDA iii / Touch Pro2 fitment: The MDA iii shares the same battery bay, connector pinout, and BMS communication protocol as the HTC Touch Pro2. Both devices use a single-cell Li-ion pack with a three-contact connector — positive, negative, and a data line the phone uses for fuel gauge reporting.
• Bench tested on actual hardware: We ran the PH26B through charge and discharge cycles on the bench. The BMS accepted the charge IC handshake on the first cycle, and the protection circuit tripped correctly at the low-voltage cutoff threshold without any communication errors.
• Fuel gauge recalibration on first use: After installing this cell, disable any sync-heavy background tasks and run one full discharge-to-charge cycle before relying on the percentage readout. The phone's fuel gauge IC is calibrated to the old cell's discharge curve — one full cycle resets the coulomb counter against the new cell's actual capacity.

Why the MDA iii reports wrong battery percentage after a cell swap

The MDA iii uses a coulomb counter in its fuel gauge IC to track charge state. That counter holds calibration data from the old cell's discharge curve, including its internal resistance profile and voltage-to-capacity mapping. A new cell with different impedance characteristics will cause the IC to misread state of charge — sometimes by 15–25%. One full discharge down to automatic shutoff, followed by a complete charge to 100%, resets the calibration baseline against the new cell.

Sudden shutdown at 20–30% on the MDA iii after replacement

This happens when the fuel gauge IC has not yet recalibrated and the phone's reported percentage does not reflect the cell's actual voltage. Under a high-current load — screen at full brightness, active data sync, or the slide-out keyboard backlit — the cell voltage drops faster than the uncalibrated IC expects. The phone hits the hardware low-voltage cutoff before the displayed percentage reaches zero. Run one full discharge cycle, then check that the cutoff voltage displayed during that discharge reaches approximately 3.4–3.5V before shutdown.