Straight Talk UMX U240C Replacement Battery MXE-650 3.7V

Straight Talk UMX U240C — 3.7V Li-ion Replacement Battery (MXE-650)

This is a 3.7V, 1500mAh Li-ion battery for the Straight Talk UMX U240C portable Wi-Fi hotspot. It replaces OEM part MXE-650 directly. The U240C broadcasts a shared wireless network while the modem and radio run simultaneously — the battery handles both loads at once.

• UMX U240C platform fit: The U240C runs a combined cellular modem and Wi-Fi radio off one 3.7V cell. Both subsystems share the same power rail, so the battery BMS must handle simultaneous draw spikes without tripping. This cell matches the voltage rail and connector spec to keep both radios up.
• Bench tested on actual hardware: We cycled this cell under combined modem and Wi-Fi load — five connected clients pulling data while the modem held a 4G signal. The BMS held cutoff thresholds steady and did not trip on radio transmit spikes.
• Ventilation during extended sessions: Keep the U240C on a hard, open surface when running extended sessions. When client devices are at range limit, the cellular radio operates at peak output continuously, pushing heat directly into the cell. Trapping that heat against fabric or in a bag accelerates capacity loss.

Why the U240C drops all connected clients mid-session

The U240C runs both the LTE modem and the Wi-Fi access point off a single 3.7V cell. At maximum client connections, total current draw spikes hard — the modem transmits while every client pulls data simultaneously. If the cell voltage sags below the BMS low-voltage threshold during that spike, the BMS cuts output to protect the cell, dropping every connected device instantly. A fresh, full-capacity cell sustains voltage through those peaks where a degraded original cannot.

U240C won't power on after sitting unused for weeks

Li-ion cells self-discharge at roughly 2–3% per month even when idle. If the U240C sat unused for an extended period, the cell may have dropped below the minimum boot voltage the device needs to initialise — typically around 3.0V. At that level the BMS blocks output entirely, so pressing the power button produces nothing. Charge the replacement battery for at least 30 minutes before the first boot attempt; most chargers will trickle-charge a deeply discharged cell up past the 3.0V threshold before switching to normal charge rate.