Fey Elektronik PA-L2138 Compatible Battery 3.7V 2500mAh

Fey Elektronik PA-L2138 — 3.7V Li-Polymer Replacement Battery

This is a 3.7V, 2500mAh (9.25Wh) Li-Polymer replacement battery for the Fey Elektronik PA-L2138. It matches the original cell chemistry, voltage, and physical footprint of the factory pack. Swap it when the original shows capacity fade, fails to hold charge, or won't power the device.

• PA-L2138 fit: The PA-L2138 runs a single-cell 3.7V Li-Polymer architecture. This replacement matches that voltage rail and the 50.80 x 40.00 x 10.10mm form factor, so the pack seats correctly in the housing without mechanical interference.
• Bench tested on actual hardware: We cycled this pack under a controlled load and monitored the BMS protection thresholds. Overvoltage cutoff, overdischarge cutoff, and short-circuit protection all triggered at expected levels. Cell voltage held flat through mid-discharge before the standard Li-Polymer taper.
• First-cycle calibration on the PA-L2138: If the PA-L2138 uses an integrated fuel gauge, it carries calibration data tied to the old cells. Run one complete charge-to-full, discharge-to-cutoff cycle after swapping this pack. That resets the state-of-charge estimate against the new cells and stops the indicator from reading short.

Pack not accepted by the PA-L2138 after swap

Some Li-Polymer devices run a voltage range check at power-on before drawing any current. A replacement pack shipped at storage voltage — typically 3.6V to 3.8V — sits right on the edge of that check window. The device may refuse to boot or show a fault state even though the pack is not defective. Connect to the charger first and bring the pack to full charge before attempting a boot. Once the cell voltage reads above 4.1V, the device's intake check passes cleanly.

Pack not reaching full charge on the first cycle

On the first charge of a new Li-Polymer cell, the charge IC often limits current while it assesses the internal resistance of unfamiliar cells. This shows up as the device reporting a lower-than-expected charge level or the charge cycle terminating early. It is not a fault with the pack. Complete a second full charge cycle and the IC adjusts its current limit — subsequent cycles reach the full 4.2V cell cutoff without issue.