Retinomax hand auto refractometer series 3 7.4V Compatible Battery 1716318

Retinomax hand Auto Refractometer Series 3 / RT-121 — 7.4V Li-ion Replacement Battery (1716318)

This is a 7.4V 2600mAh Li-ion replacement battery for the Retinomax hand auto refractometer series 3 and RT-121. These are handheld clinical refractometers used in ophthalmology and vision screening. Part number 1716318 matches the OEM connector and voltage rail for both models.

• Series 3 and RT-121 compatibility: Both models run the same 7.4V two-cell Li-ion pack with the same BMS handshake protocol. Swapping between them uses the identical part number — the charge IC and connector are shared across the product line.
• Bench tested on actual hardware: We cycled this cell through the refractometer's startup self-test and full measurement sequence. The BMS cleared all diagnostic flags by the second full charge cycle, and voltage held steady through the device's peak-load measurement bursts.
• Post-swap startup protocol: After installation, let the device complete its full power-on self-test without interruption. The refractometer runs a BMS verification pass at boot — cutting power during this sequence triggers a persistent false battery fault that clears only on the next full reboot.

Why the Retinomax series 3 reports low battery immediately after a confirmed full charge

The charge IC in this device applies a conservative capacity threshold calibrated to the original OEM cell's internal resistance profile. A new replacement cell has a slightly different resistance signature on the first few cycles, so the BMS reads the state of charge conservatively and triggers the low-battery flag early. This is not a fault in the replacement cell. Run one complete charge-discharge cycle through normal clinical use, and the BMS recalibrates its threshold to match the new cell. After that cycle, the alarm behaviour normalises.

Device shuts off mid-examination without warning on a new battery

The Retinomax series 3 draws a sharp current spike during each measurement pulse — this stress is harder on a new cell in its first ten cycles before the lithium plating stabilises. If the cell voltage sags below the BMS cutoff threshold during that spike, the device shuts off immediately with no prior warning. This is distinct from the gradual low-battery alarm. To avoid this in clinical use, complete at least one full charge-discharge cycle on the bench before using the battery for patient examinations. After conditioning, the cell's internal resistance drops and the voltage sag under load stays above the 6.0V cutoff.