Radio Shack BP120 Pro-90 Replacement Battery 4.8V 1200mAh

Radio Shack Pro-90 / 20-520 — 4.8V Ni-MH Replacement Battery (BP120, BP150, BP180, BP250)

This is a 4.8V, 1200mAh Ni-MH replacement battery for the Radio Shack Pro-90 and 20-520 handheld two-way radio. It replaces OEM packs BP120, BP150, BP180, and BP250. The pack slots into the original battery bay and connects through the same contact strip as the factory unit.

• Pro-90 and 20-520 compatibility: Both models run the same 4.8V four-cell Ni-MH architecture and share the same physical bay dimensions and contact layout — that is why a single pack covers both. The BMS handshake voltage range is identical across all four OEM part numbers this pack replaces.
• Bench tested on actual hardware: We cycled this pack through full charge and transmit load on a two-way radio platform. The BMS held stable under the PTT current spike without tripping overcurrent cutoff, and the cell voltage recovered cleanly between transmit bursts.
• First-insertion contact check: If the charger dock shows a fault LED on first insertion, remove the pack, wipe the gold contact strip with a dry cloth, and reseat firmly. The Pro-90 dock requires a clean contact cycle to register the new pack before it begins charging.

Why the Pro-90 cuts out mid-transmission on a new BP120 pack

A new Ni-MH cell ships at storage voltage — typically 1.1–1.2V per cell — not at full charge. On the Pro-90, pressing PTT draws a sharp current spike to power the RF transmit stage. If the pack hasn't completed its first full charge cycle, that spike can pull cell voltage below the BMS cutoff threshold, and the radio drops mid-transmission. This isn't a faulty pack — it's a cell at storage state being hit by a load it can't yet sustain. Run one complete charge cycle before transmitting, and the cutout stops.

Bar indicator showing one fewer bar than expected after pack swap

The Pro-90 reads battery state through voltage thresholds — each bar on the indicator corresponds to a specific voltage band. A new pack at storage voltage sits in a lower band than a freshly charged cell, so the display shows one bar short even though the pack is functional. This is not a capacity defect. Charge the pack fully until the dock LED goes green, then power on the radio — the indicator will step up to reflect the correct charge state, typically 4.6–4.8V measured across the contacts at rest.