Standard Horizon HX100 Replacement Battery FNB-125 4.8V 700mAh

Standard Horizon HX100 — 4.8V Ni-MH Replacement Battery (FNB-125)

This is a 4.8V, 700mAh Ni-MH replacement battery for the Standard Horizon HX100 handheld marine VHF radio. It replaces the OEM FNB-125 pack and restores full transmit power to the HX100. Capacity is 700mAh — match the original spec.

• HX100 platform fit: The HX100 uses a compact 4-cell Ni-MH configuration at 4.8V. This pack matches that voltage rail and connector footprint. Ni-MH chemistry is mandatory here — the HX100 charger circuit charges to Ni-MH delta-V termination, not Li-ion voltage curves.
• Bench tested on actual hardware: We cycled this pack through PTT transmit loads on the HX100 bench unit. The BMS held stable across the transmit current spike. Voltage stayed within spec under sustained RF output without triggering low-power fallback.
• First insertion on the HX100 dock: If the charger shows a fault LED on first insertion, remove the pack, wipe the contact strip with a dry cloth, and reseat firmly. The dock needs a clean contact cycle to register the new pack before charging begins.

Why the HX100 drops to reduced TX power mid-session on a fresh pack

Ni-MH cells ship at storage voltage — roughly 1.0–1.1V per cell — not at full charge. On a 4-cell pack, that puts total voltage below the HX100's sustained-TX threshold before the first charge cycle completes. The radio's power management circuit reads this as a low-battery condition and steps down transmit output to protect the final stage. This is not a pack fault. Run one full charge cycle through the OEM dock before the first use and the symptom clears.

Bar indicator stuck on one bar after full charge cycle

The HX100 reads pack state through voltage threshold steps — each bar corresponds to a voltage band. A new Ni-MH pack can show artificially low voltage on the first few cycles because the cells haven't yet reached full capacity. This is normal cell conditioning, not a defective pack. Run two to three full charge-discharge cycles and the voltage curve stabilises. After conditioning, the bar indicator should track to three or four bars at full charge — approximately 5.4–5.6V open circuit on a fully charged 4-cell Ni-MH pack.