Pantech Jest TXT8040 Replacement Battery BTR-8030 3.7V 800mAh

Pantech Jest TXT8040 — 3.7V Li-ion Replacement Battery (BTR-8030)

This 3.7V, 800mAh Li-ion battery replaces the original cell in the Pantech Jest TXT8040 and Razzle Jest series. It also covers OEM part numbers BTR-8030B, TXT8030, and BTR8045B. If your original cell no longer holds charge or the phone shuts down unexpectedly, this is the direct swap.

• Jest TXT8040 and Razzle Jest compatibility: These models share the same voltage rail, connector pinout, and BMS handshake protocol. The physical footprint — 51.80 x 37.60 x 4.70mm — matches the battery bay exactly, so no modifications are needed during installation.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Jest platform. The BMS accepted the handshake without fault codes, and charge termination triggered correctly at full capacity.
• Fuel gauge recalibration on first use: After installation, run one complete discharge to near-empty before recharging fully. The Jest's fuel gauge IC was calibrated to the original cell's discharge curve — a full cycle against the new cell lets it reset that baseline for accurate percentage readings.

Sudden shutdown at 20–30% on the Jest TXT8040

This is a voltage-cliff failure. At 20–30% reported charge, the new cell's voltage drops faster than the phone's fuel gauge expects under load from the screen or radio module. The phone interprets this as a fatal undervoltage condition and shuts down rather than browning out the display. The fix is one complete discharge-charge cycle so the coulomb counter maps the actual discharge curve of the new cell. After that cycle, the reported percentage should align with real remaining capacity.

Phone showing wrong battery percentage after cell swap

The Jest's fuel gauge IC stores a discharge model built around the original cell's internal resistance and capacity curve. A new cell has different characteristics, so the IC reports inaccurate percentages — often jumping erratically or reading full charge well before the cell is actually topped up. Run the phone from 100% down to automatic shutdown, then charge uninterrupted to 100%. This single full cycle forces the IC to recalibrate against the new cell. Percentage readings stabilise at 4.2V full charge.