AT&T Blade Spark Z971 Replacement Battery 3.85V 3100mAh

AT&T Blade Spark / Z971 — 3.85V Li-Polymer Replacement Battery

This 3.85V, 3100mAh Li-Polymer cell replaces the original battery in the AT&T Blade Spark and Z971 smartphones. It fits phones where the original cell has degraded or failed outright. Capacity is rated at 11.94Wh, matching the factory power specification for these models.

• Blade Spark and Z971 fitment: Both phones run on the same battery platform — identical cell dimensions (74.10 × 62.80 × 4.30mm), voltage rail, and connector. One cell covers both models without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Blade Spark platform. The BMS accepted charge handshake normally, and cell voltage held stable under screen-on and modem-active load without tripping the protection circuit.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one full discharge-to-charge cycle before resuming normal use. The fuel gauge IC is calibrated to the old cell's discharge curve — this first cycle gives it fresh reference data before high-current charging begins on the new cell.

Sudden shutdown at 20–30% on the Blade Spark after a cell swap

This happens when the fuel gauge IC still references the discharge curve of the old, degraded cell. A new cell has a steeper voltage cliff at low charge states — the phone's charge controller reads voltage collapse as a hard cutoff and shuts down before the percentage counter reaches zero. The fix is a full discharge-to-charge cycle with the screen on and mobile data active, which forces the coulomb counter to relearn the curve. After one complete cycle, shutdowns at 20–30% typically stop.

USB fast charge not activating after fitting the replacement cell

On the first cycle after a cell swap, the charge IC can refuse to negotiate fast-charge protocol because it sees an unfamiliar BMS response from the new cell. This is a protection behaviour, not a fault. Plug in using the stock cable and charger and let the phone complete a full standard-rate charge first. Once the charge IC logs one complete cycle, fast charging typically re-enables on the next plug-in without any further action needed.