About this replacement
COSMED Pony FX NTA2531 — 7.4V Li-ion Replacement Battery (A-410-750-002)
This is a 7.4V, 3400mAh Li-ion replacement battery for the COSMED Pony FX NTA2531 portable spirometer. It replaces OEM part number A-410-750-002 and fits directly into the Pony FX used for pulmonary function testing in clinical and field diagnostic settings. Capacity is 3400mAh (25.16Wh) — matched to the OEM specification.
- Pony FX NTA2531 fitment: The Pony FX uses a dedicated battery bay with a keyed connector tied to the device's BMS handshake. This cell matches the voltage rail, physical footprint (66.65 × 37.00 × 19.50mm), and communication profile the firmware expects at startup.
- Bench tested on actual hardware: We cycled this cell through the Pony FX power-on self-test and full charge sequence. The BMS accepted the cell without flagging a battery fault, and the charge IC brought the pack to full capacity without capping short on the first pass.
- Post-installation self-test protocol: After inserting the battery, allow the Pony FX to complete its full power-on self-test cycle without interruption. The device runs a BMS verification routine at boot — cutting power mid-sequence registers a false battery fault that persists until the next clean reboot.
Why the Pony FX flags a battery fault after a fresh cell swap
The Pony FX BMS stores the last known battery state and compares it against the new cell's reported parameters at boot. A replacement cell starts with a blank charge history, so the firmware may flag a mismatch on first power-on. This is not a defective cell — it is the BMS running its learn cycle. Run one complete charge-to-full followed by a controlled discharge, and the firmware updates its reference data. After that cycle, the fault flag clears and the battery indicator reads accurately.
Charge indicator stalls below 100% on first charge
On a new Li-ion cell, the Pony FX charge IC applies a conservative current limit until it has mapped the cell's internal resistance. This causes the charge indicator to slow significantly above 90% and sometimes appear to stall. The cell is still charging — the IC is completing a conditioning taper. Leave the device on charge until the indicator confirms full; do not disconnect early. After the first full charge cycle, subsequent charges progress at normal speed through to 4.20V per cell.