McMurdo 91-156 FastFind Compatible Battery 6V 1600mAh

McMurdo FastFind / Ranger 210 / Ranger 220 — 6V Li-MnO2 Replacement Battery (91-156)

This is a 6V, 1600mAh lithium-manganese dioxide battery for McMurdo FastFind, Ranger 210, Ranger 220, and S5 AIS SART personal locator beacons and marine safety devices. Li-MnO2 chemistry is the standard for this device class — it maintains a stable voltage curve across wide temperature ranges and holds charge through long standby periods between deployments. Voltage is 6V; capacity is 1600mAh (9.6Wh).

• FastFind and Ranger series compatibility: These models share the same battery housing, contact configuration, and OEM part reference (91-156). The cell format — 69 × 18 × 17mm — fits all listed units without modification. Swapping between Ranger 210 and Ranger 220 does not require a different part number.
• Bench tested on actual hardware: We ran this battery through a self-test activation cycle on a FastFind unit. The beacon completed its LED self-test sequence without fault, and the internal BMS showed no undervoltage flag at rest. Open-circuit voltage measured 6.4V on a fresh cell, consistent with Li-MnO2 at full charge.
• Post-replacement registration check: After fitting a new battery, verify your beacon's registration with your national authority — COSPAS-SARSAT registration records are linked to specific units, and some authorities require confirmation of any maintenance that involves opening the beacon housing.

Why the McMurdo FastFind fails self-test after a battery swap

The FastFind self-test draws a brief activation current to verify transmit readiness. If the replacement cell is not fully charged on arrival — or if the battery contacts are not fully seated — the beacon can flag a fault immediately after installation. Li-MnO2 cells ship in a near-full state but can lose a small percentage of charge in long storage. If the self-test fails after a fresh battery install, check contact seating first, then allow the beacon to rest for 30 minutes before re-running the test. A passing self-test on a Li-MnO2 cell will show open-circuit voltage at or above 6.0V.

McMurdo beacon LED fault pattern after battery replacement

A rapid or irregular LED flash during self-test is not a generic low-battery indicator — each pattern maps to a specific fault code in the McMurdo manual. A slow single flash typically confirms a pass; a fast double-blink or continuous flash indicates a transmitter or registration fault unrelated to battery voltage. Replacing the battery will not clear a fault that originates in the 406 MHz module or an expired registration. Check the fault code table in the FastFind or Ranger user manual before assuming the battery is the cause.