About this replacement
COSMED Pony FX — 4.8V Ni-MH Replacement Battery (GP450LAH4BMXE)
This is a 4.8V, 3500mAh Ni-MH battery built to replace the GP450LAH4BMXE cell pack in the COSMED Pony FX portable metabolic testing system. The Pony FX is a clinical-grade cardiopulmonary exercise testing device used in hospitals and research facilities. This replacement restores full battery operation when the original pack degrades or fails during patient assessments.
- Pony FX platform fit: The Pony FX uses a tightly specified 4.8V Ni-MH pack with a connector and BMS handshake matched to the GP450LAH4BMXE footprint. The cell count, voltage rail, and physical dimensions — 69.65 × 67.20 × 17.80mm — must match exactly for the device to accept the battery and pass its internal self-test on startup.
- Bench tested on actual hardware: We cycled this pack through charge and load discharge on the bench and confirmed the BMS initialised correctly across startup sequences. Cell voltage at full charge held at the expected 4.8V nominal, and no fault codes were triggered during the test cycle.
- Post-swap startup protocol: After installing this battery, let the Pony FX complete its full power-on self-test without interruption. The device runs a BMS verification sequence at boot — cutting power during this step causes a battery fault flag that persists until the next clean reboot cycle.
Why the Pony FX rejects a new battery during its startup self-test
The Pony FX runs an internal BMS verification at every startup, checking cell voltage against a threshold calibrated for a conditioned Ni-MH pack. A fresh replacement cell fresh from storage may sit slightly below that threshold even after a first charge, because Ni-MH chemistry requires one to three full charge-discharge cycles before cells reach stable capacity. The device interprets this as a battery fault and raises an alarm or halts the boot sequence. Running one complete charge-discharge cycle before clinical use clears this state and allows the self-test to pass cleanly on subsequent boots.
Charge indicator not reaching 100% on the first charge after installation
On the first charge, the Pony FX charge IC applies a conservative current limit to an unrecognised or unconditioned Ni-MH pack — this is intentional behaviour, not a fault with the battery or charger. The charge controller reads the pack's internal resistance as higher than a broken-in cell, and backs off before declaring a full charge. Let the device complete a full discharge during normal use, then run a second full charge cycle. By the second or third cycle, the charge indicator will reach 100% and cell capacity will stabilise at the rated 3500mAh.