Standard Horizon HX300 Replacement Battery FNB-122Li 3.7V 1800mAh

Standard Horizon HX300 — 3.7V Li-Polymer Replacement Battery (FNB-122Li)

This is a 3.7V, 1800mAh Li-Polymer replacement battery for the Standard Horizon HX300 handheld marine VHF transceiver. It replaces OEM part numbers FNB-122Li and SBR-27LI. The HX300 is a compact waterproof radio used for vessel-to-vessel and vessel-to-coast communication on the water.

• HX300 platform fit: The HX300 uses a compact Li-Polymer cell in a slim housing with a specific connector and BMS handshake tied to the Standard Horizon charging dock. This pack matches that voltage rail and connector geometry, so the dock reads the new BMS correctly on first insertion.
• Bench tested on actual hardware: We ran this pack through transmit-load cycles at the HX300's 5W RF output. The BMS held stable across repeated PTT presses with no overcurrent trip, and the dock moved from charge to standby without fault indication.
• First insertion after extended storage: Li-Polymer cells ship at storage voltage — roughly 3.6–3.8V. If the dock shows a fault LED on first insertion, remove the pack, wipe the gold contact strip with a dry cloth, and reseat firmly. One clean contact cycle is usually enough for the dock to accept the BMS handshake and begin charging.

Why the HX300 cuts out mid-transmission on a new battery

Transmitting at 5W pulls significantly more current than standby or receive mode. A cell at storage voltage has lower internal headroom, so the BMS can trip the overcurrent threshold during that initial PTT spike before the cell has been through a full charge cycle. This is not a faulty pack — it is the BMS protecting a cell that has not yet been conditioned. Charge the battery fully in the dock before first use, then run one complete transmit session to let the BMS calibrate to the load profile.

Bar indicator showing one fewer bar than expected after fitting this pack

The HX300 reads battery level by measuring cell voltage against fixed thresholds — there is no fuel gauge chip involved. A new pack at storage voltage sits at the lower end of the 3.7V nominal range, which maps to one or two bars on the indicator. After a full charge cycle in the dock, resting voltage climbs above the top threshold and the indicator shows a full display. If it still reads low after a complete charge, check that the dock contacts are clean and making firm contact with the gold strip on the pack.