About this replacement
Standard Horizon HX400IS / HX400 — 7.4V Li-ion Replacement Battery (SBR-29LIIS)
This 7.4V, 2550mAh Li-ion battery replaces the Standard Horizon SBR-29LIIS in the HX400IS and HX400 handheld marine VHF radios. It also cross-references OEM part numbers FNB-115LIIS, SBR-45LIIS, and SBR-45LI. Voltage and capacity match factory spec exactly.
- HX400IS and HX400 fit: Both models share the same battery bay geometry, contact layout, and BMS handshake protocol — one pack covers both. The SBR-29LIIS form factor was carried across the HX400 line without revision to the connector or locking tab.
- Bench tested on actual hardware: We ran this pack through a full charge cycle and PTT transmit load test. The BMS held voltage stable through repeated transmit bursts and returned clean cutoff behaviour at low-cell threshold — no false trips under normal TX current draw.
- Marine environment contact care: Salt air and moisture accelerate oxidation on the gold contact strip. Before first use, wipe the contacts with a dry cloth and seat the pack firmly into the dock — the HX400 platform requires a clean contact cycle to initiate the BMS handshake and begin charging.
Why the HX400IS drops to reduced TX power mid-call on a new battery
The HX400IS transmits at 5W on full power, which pulls a significant current spike from the pack. If the replacement cell shipped at storage voltage (typically around 3.7V per cell), the BMS may sag under that load and throttle output before a full charge cycle has been completed. This is not a faulty pack — it is a storage-voltage limitation. Run one full charge cycle before using the radio on full TX power and the behaviour stops.
Bar indicator showing one fewer bar than expected after a fresh charge
The HX400IS uses a simple voltage-threshold bar indicator — each bar corresponds to a voltage band, not a percentage. A new cell fresh from storage sits slightly below the top threshold band even after the dock shows a full charge LED. The fix is straightforward: leave the pack on the dock for an additional 30 minutes after the charge light goes green. At that point the resting voltage should sit above 8.2V, which the radio reads as a full four-bar display.