Kenwood KNB-81L Two-Way Radio Replacement Battery 3.7V 1800mAh

Kenwood NX-P500 / PKT-300 / TK-3601D — 3.7V Li-ion Replacement Battery (KNB-81L)

This is a 3.7V, 1800mAh Li-ion replacement battery for the Kenwood NX-P500, PKT-300, and TK-3601D portable two-way radios. It replaces OEM part numbers KNB-81L and KNB-81LI. The pack fits the original battery bay directly and uses the same connector and BMS communication protocol as the factory unit.

• NX-P500, PKT-300, TK-3601D fit group: These three radios share the same 3.7V battery bay, connector pinout, and BMS handshake protocol. One pack covers all three without modification or adapter.
• Bench tested on actual hardware: We cycled this pack on the NX-P500 through repeated PTT transmit bursts and monitored BMS response. The overcurrent protection tripped correctly on fault simulation and reset cleanly on power cycle. Voltage held within spec across the full discharge curve.
• First insertion into the charger dock: If the dock LED shows a fault or blinks amber on first insertion, remove the pack and wipe the gold contact strip with a dry cloth. Reseat it firmly. The Kenwood dock requires a clean contact cycle to complete the BMS handshake before it will begin charging.

Why the NX-P500 drops to reduced TX power mid-shift on a new battery

The NX-P500 monitors pack voltage in real time and steps down transmit power when voltage sags below its internal threshold. A new cell shipped at storage voltage — typically around 3.6V — has less headroom than a fully charged pack at 4.2V. Under sustained RF output, that sag crosses the radio's protection floor faster than expected. Charge the pack to full before the first shift and the TX power will hold at rated output throughout normal use.

Bar indicator showing one fewer bar than expected after inserting a new KNB-81L

The NX-P500 uses voltage-threshold bar indicators, not a fuel gauge chip. A new pack leaves the warehouse at storage voltage, roughly 3.6–3.7V, which the radio correctly reads as a partially charged cell. This is not a fault — the cell has full capacity but is not at full charge voltage. Place the pack in the dock, allow a complete charge cycle until the LED goes green, and the indicator will show full bars at 4.2V.