Yaesu FNB-58 VX-5R Replacement Battery 7.4V 1400mAh

Yaesu VX-5R / VX-6R / VX-7R Series — 7.4V Li-ion Replacement Battery (FNB-58 / FNB-80)

This is a 7.4V, 1400mAh Li-ion replacement battery for the Yaesu VX-5R, VX-6R, VX-7R, VX-5E, and related handheld transceivers. It replaces OEM part numbers FNB-58, FNB-58Li, FNB-80, and FNB-80Li. The cell dimensions match the original pack at 48.50 × 57.60 × 20.00mm, so it seats correctly in the battery bay without modification.

• VX-5R, VX-6R, VX-7R platform fit: These models share the same battery bay geometry, contact pin layout, and 7.4V supply rail. The BMS handshake protocol is consistent across the series, so one pack covers the entire family without re-programming or adapter plates.
• Bench tested on actual hardware: We ran this pack through transmit-cycle testing on the VX-6R. The BMS held steady through repeated PTT bursts without triggering overcurrent cutoff, and charge acceptance via the standard desktop charger completed cleanly.
• First-insertion contact check: If the desktop charger or vehicle cradle shows a fault LED on first insertion, remove the pack and wipe the gold contact strip with a dry cloth before reseating. The Yaesu charging dock requires a clean contact cycle to recognise the new BMS before it begins charging.

Why the VX-5R cuts out mid-transmission on a new battery

The VX-5R draws a sharp current spike the moment PTT is pressed — the RF amplifier stage pulls hard for the first few milliseconds before stabilising. If the BMS reads that spike as an overcurrent fault, it trips the protection circuit and drops the pack offline instantly. This looks identical to a dead battery, but the radio recovers the moment you release PTT. A new pack at storage voltage (around 3.7V per cell) is more prone to this on first use because internal resistance is slightly elevated until the cells cycle once. Run one full charge before field use and the trip threshold stabilises.

Bar indicator shows one fewer bar than expected after a full charge

The VX-5R and VX-6R use a simple voltage-threshold bar display — each bar maps to a fixed voltage step, not a calculated capacity percentage. A new Li-ion pack shipped at storage voltage reads one bar low until it completes a full charge cycle and the resting voltage settles above the top threshold. This is not a cell fault. Charge the pack fully in the desktop charger, allow it to rest for 15 minutes after the charge LED goes green, then reinsert — the bar display should read correctly at or above 8.2V resting voltage.