Motorola VX-261 Replacement Battery FNB-V133Li 7.4V 2600mAh

Motorola VX-261 / VX-451 Series — 7.4V Li-ion Replacement Battery (FNB-V133Li)

This is a 7.4V, 2600mAh Li-ion battery for the Motorola VX-261, VX-264, VX-451, and EVX-531 portable two-way radios. It replaces OEM part numbers FNB-V133Li, FNB-V134Li, FNB-V138Li, AAJ67X001, AAJ68X001, and AAK66X501. The pack slots into the same housing footprint and connects through the same five-pin contact strip as the factory battery.

• VX-261, VX-264, VX-451, EVX-531 fit: These models share a common 7.4V battery bay, connector pinout, and BMS handshake protocol. Swapping across this group works because the dock communicates battery status using the same voltage-threshold logic on all four platforms.
• Bench tested on actual hardware: We ran this pack on a VX-261 and cycled PTT transmit at full power. The BMS handled the transmit current spike without tripping, and the dock accepted the battery and moved to charge within two minutes of insertion.
• First insertion on a Vertex Standard dock: If the 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 platform requires a clean contact cycle to complete the BMS handshake before it will begin charging.

Why the VX-261 drops to reduced TX power mid-shift on a new battery

A new cell ships at storage voltage — typically 3.6–3.7V per cell, well below a full 4.2V charge. When the radio pulls the elevated current needed for sustained RF output at that lower starting voltage, the pack's internal resistance causes enough voltage sag to trigger the radio's low-voltage TX power reduction. This is not a faulty battery. One full charge cycle through the dock brings each cell to 4.2V and eliminates the sag behaviour.

Bar indicator showing one fewer bar than expected on a fresh FNB-V133Li

The VX-261 series reads battery level using simple voltage thresholds — each bar corresponds to a voltage band. A new pack at storage voltage sits in the second-lowest band, so the indicator shows one or two bars even though the cell is not depleted. Charge the pack fully before drawing conclusions about capacity. After a complete charge, the open-circuit voltage should measure 8.3–8.4V across the terminals, and the indicator will display full bars.