About this replacement
Edan Pro II Doppler / Sonotrax II — 7.4V Li-Polymer Replacement Battery (M21R-064131)
This 7.4V, 900mAh Li-Polymer battery replaces the OEM cell in the Edan Pro II Doppler and Sonotrax II handheld Doppler units. Both devices are portable fetal and vascular Doppler monitors used in clinical examinations. The M21R-064131 part number confirms compatibility across both models.
- Pro II Doppler and Sonotrax II shared battery platform: Both units run the same 7.4V Li-Polymer cell with an identical connector footprint and BMS handshake protocol. Edan consolidated the battery across these models, so one part number covers both.
- Bench tested on actual hardware: We cycled this cell through the Pro II Doppler's BMS verification routine. The charge IC applied a conservative current limit on the first charge, and the BMS flagged a calibration hold until the cell completed one full charge-discharge cycle — normal behaviour on first use.
- Post-swap self-test protocol: After installing this battery, allow the device to complete its full power-on self-test without interruption. The Edan Doppler runs BMS verification at startup — cutting power mid-sequence triggers a persistent false battery fault that will not clear until the next clean reboot.
Device not completing boot sequence on new M21R-064131 battery
The Edan Doppler's BMS compares cell voltage against an internal threshold table during boot. A new Li-Polymer cell that has partially self-discharged in storage can sit just below the threshold the firmware expects at startup, causing the boot sequence to stall or abort. This is not a fault with the cell — it is the BMS rejecting a voltage it has not yet learned. Charge the battery fully before the first boot attempt, then power on without interrupting the self-test cycle.
Charge indicator not reaching 100% on the first charge cycle
The Edan charge IC applies a reduced current ceiling on an unrecognised cell to prevent overvoltage on first contact — the indicator stalls below 100% because the IC is holding a conservative charge cutoff. This clears after one complete charge-discharge cycle, which lets the BMS establish the cell's actual capacity curve. Run the device on battery until the low-battery warning triggers, then charge to full. After that cycle, the indicator will reach 100% and the BMS will report capacity accurately.