ERA PH26B MDA III Replacement Battery 3.7V 4200mAh

ERA MDA III — 3.7V Li-ion Replacement Battery (PH26B)

This 3.7V, 4200mAh Li-ion cell replaces the original battery in the ERA MDA III Windows Mobile pocket PC phone. It matches the OEM part numbers PH26B and AHTXDSSN. Fit it when the original cell no longer holds a charge or the device shuts down unexpectedly under load.

• MDA III platform fit: The MDA III uses a fixed connector pinout and a BMS handshake tied to the 3.7V single-cell voltage rail. This cell matches that rail and passes the handshake check the device runs at power-on.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge passes on the MDA III. The BMS engaged correctly at both ends — top-of-charge cutoff at 4.2V and low-voltage cutoff before the cell dropped below safe floor.
• Fuel gauge recalibration after swap: On first use, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100% without interruption. The MDA III fuel gauge IC recalibrates its coulomb counter against the new cell's discharge curve during that first full cycle.

Sudden shutdown at 20–30% on the MDA III after a cell swap

The MDA III fuel gauge IC stores a discharge curve from the previous cell. When a new cell is installed, the old curve no longer reflects actual cell voltage under load. The modem radio and screen together pull a current spike the aged curve didn't account for, and the phone reads a false voltage cliff — triggering shutdown while apparent charge remains. One full discharge-to-shutdown followed by an uninterrupted charge to 100% rewrites the coulomb counter reference. After that cycle, the percentage reading tracks closely with actual remaining capacity.

MDA III not powering on after the replacement cell sat in storage

Li-ion cells discharged below roughly 2.5V trip a BMS lockout to prevent cell damage. A cell shipped or stored partially discharged can cross that threshold before it ever reaches the device. The MDA III will show no response — no boot screen, no charging indicator — when the BMS is locked. Connect the device to a charger and leave it untouched for 15–20 minutes; the charge IC delivers a low trickle current that nudges the cell voltage above the BMS recovery threshold, typically 2.9–3.0V, after which the BMS unlocks and normal charging resumes.