ZTE E821 Li-ion Replacement Battery 3.7V 800mAh

ZTE E821 — 3.7V Li-ion Replacement Battery (Li3709T42P3H483757-H)

This is a 3.7V, 800mAh Li-ion cell that replaces the original battery in the ZTE E821 smartphone. It fits the E821 directly, matching the OEM connector, cell dimensions (44.80 × 37.40 × 5.60mm), and BMS handshake. Capacity is 800mAh — use the product data figure, not third-party listings that inflate it.

• E821 platform fit: The ZTE E821 uses a fixed 3.7V single-cell architecture with a specific BMS communication protocol. This cell matches that voltage rail and connector pinout. A mismatched BMS handshake causes the phone to report 0% immediately or refuse to boot — this cell avoids that.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the E821 platform. The BMS accepted the cell on first connection, reported charge state without error flags, and the protection circuit tripped correctly at low-voltage cutoff rather than allowing the cell to drain below the safe floor.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging if your firmware supports it and run one complete discharge-charge cycle before regular use. The fuel gauge IC on the E821 carries calibration data from the old cell's discharge curve. A single full cycle lets it recalibrate against this cell's actual characteristics before high-current charging is applied.

Sudden shutdown at 20–30% on the ZTE E821 after a cell swap

This is a voltage cliff issue, not a capacity problem. At 20–30% reported charge, the modem radio or screen backlight draws a surge that the new cell cannot sustain while the fuel gauge IC is still running the old discharge curve. Voltage drops sharply under that load, the protection circuit reads it as a hard undervoltage event, and the phone cuts off. Running one full discharge-charge cycle recalibrates the coulomb counter and moves the reported cutoff threshold to match where this cell's voltage actually collapses — typically below 3.4V under load.

Phone shows incorrect battery percentage after installing replacement cell

The fuel gauge IC stores a charge curve profile built from hundreds of cycles on the original cell. A new cell has a different internal resistance and a slightly different discharge slope, so the stored profile no longer maps correctly to actual state of charge. The result is percentage readings that jump, stall, or show full charge when the cell is not. To fix this, drain the phone to auto-shutdown, then charge uninterrupted to 100% without removing the cable early. After one complete cycle the coulomb counter resets its reference and percentage reporting stabilises.