Bosch FuG10 Replacement Battery 12V 500mAh B5850

Bosch FuG10 / HFG10 — 12V Ni-MH Replacement Battery (B5850 / B5860)

This is a 12V, 500mAh Ni-MH battery for the Bosch FuG10 and HFG10 portable two-way radios. It replaces OEM part numbers B5850 and B5860 directly. These units are used in professional and industrial communication environments where the original pack has degraded past usable capacity.

• FuG10 and HFG10 platform fit: Both models share the same 12V pack format, connector pinout, and BMS handshake protocol. One replacement covers both radio variants without modification.
• Bench tested on actual hardware: We cycled this pack through transmit load on the FuG10 platform. The BMS held steady through PTT spikes and did not trip during sustained RF output bursts.
• First insertion contact cycle: If the charger dock shows a fault LED on first insertion, remove the pack, wipe the contact strip with a dry cloth, and reseat firmly. The FuG10 dock requires a clean contact cycle to accept the new BMS handshake before it will begin charging.

Why the FuG10 cuts out mid-transmission on a new battery

Ni-MH cells ship at storage voltage — typically 1.1–1.2V per cell, not full charge. Under PTT load, that voltage sags hard enough for the BMS to read it as an undervoltage fault and cut output. The radio goes silent even though the pack is new. A full charge cycle before first use brings each cell up to ~1.4V and eliminates this sag. After one full charge, the cutout behaviour stops.

Bar indicator showing fewer bars than expected after fitting this pack

The FuG10 uses voltage-threshold bar indicators — each bar corresponds to a voltage band, not a fuel gauge reading. A freshly inserted, uncharged pack sits at storage voltage and will register one or two bars below what the radio shows at full charge. This is not a fault with the cell. Charge the pack fully and the indicator will step up to its correct bar count, which for a 12V Ni-MH pack aligns at approximately 13.2–14.4V under no-load conditions.