Vertex Standard EVX-C31 Compatible Battery FNB-Z181Li 7.4V

Vertex Standard EVX-C31 / VZ-30 Series — 7.4V Li-ion Replacement Battery (FNB-Z181Li)

The FNB-Z181Li is a 7.4V, 1800mAh Li-ion battery for the Vertex Standard EVX-C31, VZ-30, VZ-30-G6-4, and VZ-30-D0-5 two-way radios. It slots into the same battery compartment as the original and connects through the same contact strip. Capacity figure is 13.32Wh as rated.

• EVX-C31 and VZ-30 platform fit: These models share the same battery bay dimensions, three-pin contact layout, and BMS handshake protocol — which is why one pack covers all four variants. The radio reads cell voltage through those contacts to gate TX power levels.
• Bench tested on actual hardware: We cycled this pack through full charge and discharge on an EVX-C31 dock. The BMS held voltage within spec under simulated PTT load, and the dock advanced from charge to ready without fault codes.
• First-insertion contact check on the VZ-30 dock: If the dock shows a fault LED on first insertion, pull the battery and wipe the gold contact strip with a dry cloth before reseating. The VZ-30 dock requires a clean contact cycle to complete the BMS handshake before it begins charging.

Why the EVX-C31 drops to reduced TX power mid-shift on a new battery

A new Li-ion cell ships at storage voltage — typically 3.6–3.7V per cell, not full charge. Under sustained RF output, the radio's power control circuit detects voltage sag below its TX threshold and steps down transmit power automatically. This is normal behaviour from the radio's firmware, not a fault in the pack. Fully charge the battery before the first shift and the radio will hold rated TX power throughout the day.

Bar indicator showing one fewer bar than expected after fitting the FNB-Z181Li

The EVX-C31 uses simple voltage-threshold steps to drive the bar indicator — it does not read capacity directly from the cell. A pack at storage voltage sits just below the top threshold, so the radio displays one bar short of full even though the cell is not depleted. Put the battery through a full charge cycle until the dock shows ready, then reinsert it. The indicator will read correctly once cell voltage reaches 8.3–8.4V at the contacts.