Sprint AirCard 771S W-5 Replacement Battery 3.7V 2500mAh

Sprint AirCard 771S / 770S — 3.7V Li-ion Replacement Battery (W-5)

This 3.7V 2500mAh Li-ion battery replaces the original W-5 pack in the Sprint AirCard 771S and AirCard 770S mobile hotspots. Both devices share the same battery bay dimensions, connector, and BMS communication protocol, so one part fits both. Use the capacity figure above — 2500mAh — as your reference.

• AirCard 771S and 770S compatibility: Both models run the same 3.7V battery rail with an identical physical footprint (77.20 × 54.00 × 6.20mm) and the same BMS handshake. The modem and Wi-Fi radio draw from the same cell, so the BMS on both devices expects identical charge termination thresholds.
• Bench tested on actual hardware: We cycled this battery under simultaneous cellular and Wi-Fi load — five connected clients, poor signal simulation — and confirmed the BMS held charge termination at 4.2V without tripping a false low-voltage cutoff at load.
• Ventilation during extended hotspot sessions: When connected clients are far from the hotspot, the cellular radio runs at maximum transmit power. Place the AirCard on a hard, flat surface — not a cushion or bag pocket — to prevent heat from building against the battery cell during long sessions.

Why the AirCard 771S drops Wi-Fi clients mid-session under full load

When all five Wi-Fi slots are active and the cellular signal is weak, the modem pushes transmit power to its ceiling. That combined draw — modem plus Wi-Fi radio plus BMS overhead — can pull the cell voltage below 3.5V momentarily. The BMS reads that sag as a fault and shuts the output rail, disconnecting every client at once. A fresh cell with low internal resistance handles the surge without sagging. If disconnections persist after fitting this battery, check that the device firmware is current, as some builds have aggressive low-voltage thresholds that a firmware update corrects.

AirCard 771S won't power on after sitting unused for several weeks

Li-ion cells self-discharge at roughly 1–2% per day. After several weeks in storage, a depleted pack can fall below the 3.0V minimum boot threshold the AirCard's BMS requires to start. The device shows nothing — no LED, no screen — because the BMS blocks the boot sequence rather than risk drawing from a deeply discharged cell. Connect the charger and leave it for at least 20–30 minutes before pressing power. If the charge LED never illuminates, the cell is below recovery voltage; verify the charger output reads 5V before assuming the battery is at fault.