Horizon HX300 FNB-122Li Compatible Battery 3.7V 1800mAh

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

This 3.7V, 1800mAh lithium-polymer battery replaces the FNB-122Li and SBR-27LI packs in the Horizon HX300 handheld two-way radio. It fits the HX300's battery bay directly and uses the same connector and BMS communication protocol as the original pack. Capacity figure is 1800mAh (6.66Wh) as specified in the product data.

• HX300 platform fitment: The HX300 runs a single-cell 3.7V lithium-polymer architecture. This pack matches that voltage rail and uses the same three-pin contact layout, so the radio's BMS handshake completes on first insertion without reconfiguration.
• Bench tested on actual hardware: We cycled this pack through transmit and receive loads representative of the HX300's RF output stage. The BMS held voltage within the expected window under sustained PTT presses and did not trigger premature cutoff during back-to-back transmissions.
• First-insertion contact check: If the charger dock shows a fault LED after inserting this pack, remove the battery and wipe the gold contact strip with a dry cloth before reseating. The HX300 charger dock requires a clean contact cycle to complete the BMS handshake before it begins charging.

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

A new lithium-polymer cell ships at storage voltage — typically around 3.6–3.8V, not at full charge. When the HX300 fires its transmit stage, the current draw spikes sharply. At storage voltage, the cell's available headroom is narrow, and the BMS interprets the voltage sag as an undervoltage condition. It trips the output, and the radio drops out of transmission. Running one full charge cycle before heavy PTT use resolves this — the cell reaches rated capacity and the BMS has room to absorb the transmit spike without cutting off.

Bar indicator showing one fewer bar than expected after install

The HX300 uses a voltage-threshold bar indicator — it reads pack voltage directly and maps it to display bars. A new pack at storage voltage sits at a threshold that the radio reads as one step below full. This is not a fault with the pack or the radio. Charge the battery fully in the dock until the charger shows a green light, then power on the HX300. The bar indicator will reflect the correct level once the cell is at rated voltage.