Vertex Standard VX-820 Replacement Battery FNB-V128Li 7.4V

Vertex Standard VX-820 / VX-920 Series — 7.4V Li-ion Replacement Battery (FNB-V128Li)

This 7.4V 3400mAh Li-ion battery replaces the FNB-V128Li pack used in the Vertex Standard VX-820, VX-870, VX-920, and VX-970 portable two-way radios. It fits the full UHF/VHF transceiver lineup that shares this battery form factor across professional public safety, security, and business deployments. Voltage and connector match the OEM spec exactly.

• VX-800 and VX-900 series compatibility: These radios share the same battery bay dimensions, contact layout, and BMS handshake protocol across the platform. One pack covers the VX-820, VX-870, VX-920, and VX-970 because Vertex Standard standardised the battery interface across both series generations.
• Bench tested on actual hardware: We ran this pack through full charge cycles on a VX-920 dock and monitored BMS behaviour under sustained PTT loads. The protection circuit held stable at transmit current spikes and did not trip into overcurrent lockout during back-to-back keying.
• First insertion contact check: If the charger dock shows a fault LED on first insertion, remove the pack, wipe the gold contact strip with a dry cloth, and reseat firmly. The Vertex Standard dock requires a clean contact cycle to complete the BMS handshake before it begins the charge sequence.

Why the VX-820 drops to reduced TX power mid-shift on a new battery

New cells ship at storage voltage — typically 3.7V to 3.8V per cell, which puts the pack around 7.5V total. Under sustained RF output, that starting voltage sags faster than a fully conditioned pack. The radio's internal voltage monitor interprets the sag as a low-battery condition and steps down transmit power to protect the final amplifier stage. Running the pack through two full charge cycles before heavy use lets the cells reach their rated capacity and hold voltage more firmly under TX load.

Bar indicator showing one fewer bar than expected after inserting a new FNB-V128Li

The VX-series radios use a simple voltage-threshold bar indicator — each bar maps to a voltage band, not a percentage from a fuel gauge chip. A new pack at storage voltage reads lower than a freshly charged pack, so the indicator often shows one bar fewer than maximum on first power-up. This is not a cell defect. Charge the battery to full in the dock first, then power on the radio — the indicator should step up to its full reading once the pack is at or above 8.2V.