About this replacement
Natus FreeDOP Series — 12V Ni-MH Replacement Battery (C631)
This is a 12V, 230mAh Ni-MH replacement battery for the Natus FreeDOP, DOP CT, and CT+ Doppler handheld stimulator units. The C631 powers the portable neurophysiological monitoring system used during intraoperative neuromonitoring procedures. Voltage and cell chemistry match OEM spec — 12V Ni-MH, 230mAh (2.76Wh).
- FreeDOP, DOP CT, and CT+ Doppler compatibility: All three models share the same 12V Ni-MH cell requirement, connector pinout, and BMS handshake protocol. The C631 OEM part number spans this platform because the power rail and charge IC are identical across the series.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on the FreeDOP platform. The BMS accepted the cell, completed its verification sequence, and reported no faults across repeated cycles.
- Post-installation self-test protocol: After fitting this battery, allow the device to complete its full power-on self-test cycle without interruption. The FreeDOP's BMS runs a chemistry verification at startup — cutting power during this sequence triggers a false battery fault that persists until the next complete reboot.
Why the FreeDOP BMS rejects a new C631 cell on first boot
The FreeDOP's charge IC applies conservative acceptance thresholds calibrated to a conditioned OEM cell. A fresh replacement hasn't completed a full charge-discharge cycle, so its internal resistance profile sits outside the BMS's expected window. This causes the device to flag the battery as non-compliant even when the cell is fully charged. Run one complete charge-discharge cycle before relying on the battery in a clinical setting — this allows the BMS to update its learned baseline for the new cell.
Charge indicator stalling below 100% on the first charge
On first charge, the FreeDOP's charge IC applies a reduced current ceiling to an unrecognised cell, which causes the charge indicator to plateau short of 100%. This is not a faulty battery — it's the charge controller applying a protective limit until the cell establishes a charge history. Let the first charge run to termination without removing the battery early. By the second full charge cycle, the IC will recognise the cell's delta-V signature and charge to the full 12V endpoint.