About this replacement
Icom ID-31A / ID-51A Series — 7.4V Li-ion Replacement Battery (BP-722)
The BP-722 is a 7.4V, 1800mAh lithium-ion battery pack for the Icom ID-31A, ID-31E, ID-51A, and ID-51E portable amateur radio transceivers. These are D-STAR capable handhelds that draw a sharp current spike on every PTT press. This replacement matches the OEM voltage, capacity, and contact footprint.
- ID-31 and ID-51 platform fit: Both the ID-31 and ID-51 series share the BP-722 form factor — same battery bay depth, same gold contact strip layout, and the same BMS communication protocol the radio uses to verify pack identity before enabling transmit.
- Bench tested on actual hardware: We cycled the BP-722 through repeated PTT bursts at full RF output. The BMS held voltage above the radio's low-battery threshold across the full charge cycle and did not trip overcurrent during transmit spikes.
- 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 ID-31 and ID-51 docks require a clean contact cycle to complete the BMS handshake before charging begins.
Why the ID-51A cuts out mid-transmission on a freshly inserted BP-722
New lithium-ion cells ship at storage voltage — typically around 3.7V per cell, or roughly 7.4V combined. That sits right at the edge of the ID-51A's low-voltage transmit threshold. When PTT is pressed, the transmit current spike causes an immediate voltage dip, which the radio reads as an undercharge condition and responds by cutting RF output or powering down. This is not a faulty pack. Running a full charge cycle before first use brings each cell above 4.1V, which gives the BMS enough headroom to handle the transmit surge without tripping.
Bar indicator shows one fewer bar than expected after a full charge
The ID-31 and ID-51 series use a simple voltage-threshold bar display — each bar corresponds to a voltage window, not a percentage calculated by a fuel gauge chip. A new BP-722 fresh off the dock may sit at 8.3V rather than the 8.4V peak the radio associates with a full four-bar reading. This is normal cell-to-cell settling, not a capacity defect. Run one complete discharge and full recharge cycle and the bar display will align correctly with the pack's actual state of charge.