About this replacement
Standard Horizon HX400IS / HX400 — 7.4V Li-ion Replacement Battery (SBR-29LIIS)
This 7.4V, 3200mAh Li-ion battery replaces the SBR-29LIIS, FNB-115LIIS, SBR-45LIIS, and SBR-45LI packs in the Standard Horizon HX400IS and HX400 handheld marine radios. It delivers 23.68Wh of capacity through the same BMS interface the radio expects at power-on. Voltage and connector match the OEM spec exactly.
- HX400IS and HX400 compatibility: Both models share the same battery bay geometry, connector pinout, and 7.4V voltage rail. The BMS handshake on either radio reads pack voltage and cell count before enabling transmit — this pack passes that check on both variants without modification.
- Bench tested on actual hardware: We ran the pack through power-on, PTT transmit cycles, and charger dock insertion on the HX400 platform. The BMS correctly flagged overcurrent during back-to-back transmit bursts and recovered within two seconds, which is normal protection behavior on this radio.
- First dock insertion on the HX400 charger: If the dock shows a fault LED on first seating, remove the pack, wipe the gold contact strip with a dry cloth, and reseat firmly. The HX400 charger requires a clean contact cycle to register the new BMS before it begins charging.
Why the HX400IS cuts out mid-transmission on a freshly inserted pack
The HX400IS draws a sharp current spike the moment PTT is pressed — transmit current briefly exceeds standby draw by a factor of several times. If the battery contacts are even slightly dirty or the pack is not fully seated, the BMS sees a voltage dip that triggers overcurrent protection and kills output. This is not a faulty battery. Wipe the contacts, press the pack fully into the bay until it clicks, and the cutout stops. If it continues, check that the radio's battery bay contacts are not bent or oxidised.
Bar indicator showing one fewer bar than expected on a new SBR-29LIIS
New Li-ion cells ship at storage voltage — typically around 3.7V per cell, not the 4.2V per cell that represents a full charge. The HX400 reads pack voltage against fixed thresholds to set bar count, so a new pack straight out of packaging will often show one bar lower than full. This is not a capacity defect. Place the pack in the dock and charge to completion — the indicator will read correctly once cell voltage reaches approximately 8.4V across the pack.