Kenwood KNB-31A NX-210 Replacement Battery 7.2V 2100mAh

Kenwood NX-210 / TK-5210 Series — 7.2V Ni-MH Replacement Battery (KNB-31A)

This is a 7.2V, 2100mAh Ni-MH replacement battery for Kenwood handheld two-way radios. It fits the NX-210, NX-410, TK-5210, and TK-5210G, covering both the NX-series digital and TK-series conventional platforms. OEM part numbers covered include KNB-31A, KNB-32A, KNB-32L, and KNB-43L.

• NX and TK platform compatibility: The NX-210 and TK-5210 share the same battery bay geometry, 7.2V rail, and contact pinout. Both platforms use the same BMS handshake protocol, so one pack covers both lines without any adapter or modification.
• Bench tested on actual hardware: We cycled this pack through full charge and transmit-load discharge on the NX-210. The BMS held cutoff at the correct low-voltage threshold and did not trigger overcurrent lockout during repeated PTT bursts at rated TX power.
• First insertion into charger dock: If the dock LED flashes fault on first insertion, remove the pack, wipe the gold contact strip with a dry cloth, and reseat firmly. The Kenwood dock requires a clean contact cycle to accept the BMS handshake before charging begins — this is not a defective pack.

Why the NX-210 drops to reduced TX power mid-shift on a fresh pack

A new Ni-MH cell ships at storage voltage — typically around 1.20V per cell, not peak charge. Under sustained RF output, the pack voltage sags below the radio's TX power threshold before the first charge cycle is complete. The radio reads this as a low-battery condition and steps down transmit power to protect the finals. A full conditioning charge before the first shift resolves this. After one complete charge cycle, resting voltage stabilises above 1.25V per cell and the radio holds full TX power under load.

Bar indicator showing one fewer bar than expected after charging

The NX-210 uses a voltage-threshold bar indicator — it reads pack voltage at rest and maps that to a bar level. A new Ni-MH pack often reads one bar low immediately after a first charge because the cells haven't completed their first full electrochemical cycle and surface charge dissipates quickly. This is not a capacity fault. Run one full discharge under normal radio use, then charge completely. After that cycle, the resting voltage will settle correctly and the indicator will display the expected bar level.