T-Mobile Prism Replacement Battery 3.7V 1200mAh Li-ion

T-Mobile Prism / Summit / Astro — 3.7V Li-ion 1200mAh Replacement Battery

This is a 3.7V, 1200mAh Li-ion replacement battery for the T-Mobile Prism, Astro, Summit, and Summit U8651S. It fits the original battery bay directly and restores power to devices where the factory cell has degraded through repeated charge cycles. Capacity figure is 1200mAh as spec'd — do not substitute values from third-party listings.

• Prism / Summit / Astro platform fit: These models share the same battery bay dimensions, connector pinout, and voltage rail. A single cell — 49.50 × 44.20 × 4.50mm — fits all listed variants without modification to contacts or housing.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a Prism-class device and confirmed the BMS completed a full charge cycle without thermal cutoff or premature termination. Charge acceptance was stable across the full voltage window from low cell to termination at 4.2V.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging if available and run one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before reporting percentage to the OS — skipping this step causes the phone to report inaccurate state-of-charge figures from day one.

Why the Prism reports wrong battery percentage after a cell swap

The fuel gauge IC on these devices uses a stored discharge curve calibrated to the original cell. When a new cell goes in, the IC has no reference for how the new cell's voltage maps to actual capacity. Until one full discharge-charge cycle completes, the OS reads off an outdated curve and shows percentage that does not match real charge state. This is not a fault with the replacement cell — it clears after one calibration cycle.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under load — screen on, active data, or a call — before the fuel gauge reaches what it thinks is 0%. The cell hits the BMS undervoltage cutoff threshold, typically around 3.0V per cell, and the device shuts off to protect the cell. The fuel gauge still showed 20–30% because it had not been recalibrated to the new cell's voltage cliff. Run one full discharge cycle down to automatic shutoff, then charge uninterrupted to 100% — this resets the coulomb counter reference and moves the reported cutoff point to match actual cell behaviour.