Masimo Rad 9 Replacement Battery 12V 2300mAh SLA

Masimo Rad 9 — 12V Sealed Lead Acid Replacement Battery

This is a 12V 2300mAh (27.6Wh) sealed lead-acid battery for the Masimo Rad 9 portable pulse oximeter. The Rad 9 uses this battery as its primary power source for non-invasive SpO2 and heart rate monitoring in clinical environments. No OEM part number is published for this cell; fit is confirmed by voltage, capacity, and physical dimensions (178.00 × 66.60 × 35.00mm).

• Rad 9 platform compatibility: The Rad 9 draws a consistent low-current load during continuous monitoring, which suits sealed lead-acid chemistry. The device's internal charge circuit is calibrated for SLA voltage curves — substituting a different chemistry would cause incorrect charge termination and potential BMS faults.
• Bench tested on actual hardware: We ran this cell through charge acceptance and load discharge on SLA-compatible test equipment. The BMS responded correctly at both float voltage and end-of-discharge cutoff, with no anomalous voltage sag under simulated monitoring load.
• Post-swap self-test protocol: After fitting this battery, allow the Rad 9 to complete its full power-on self-test without interruption. The device runs a BMS verification sequence at startup — interrupting it before completion triggers a false battery fault that persists until the next clean reboot.

Why the Rad 9 alarms low battery immediately after a confirmed full charge

The Rad 9 applies a charge-acceptance threshold during its self-test that is tuned to a conditioned OEM cell. A new SLA cell has not yet completed its formation cycling, so its internal resistance reads higher than an aged but functional cell. The device interprets this resistance signature as insufficient charge capacity and triggers the low battery alarm even when the cell is at full voltage. One complete charge-discharge cycle normalises the cell's internal resistance profile. After that cycle, the alarm clears and does not return under normal monitoring loads.

Rad 9 will not power on after the battery sat unused in storage

Sealed lead-acid cells self-discharge at roughly 3–5% per month. A battery stored for six months or more can drop below the Rad 9's minimum BMS recovery threshold — typically around 10.5V for a 12V SLA cell — at which point the device will not initiate startup at all. Connect the battery to a standalone SLA charger and bring it back above 12.0V before reinserting it into the monitor. Do not attempt recovery inside the device; the Rad 9's onboard charger will not activate below its own minimum input threshold.