About this replacement
Amazon Kindle Fire HD 8th Generation — 3.8V Li-Polymer Replacement Battery (ST33)
This 3.8V, 4750mAh lithium-polymer cell replaces part number ST33 in the Amazon Kindle Fire HD 8th generation tablet, covering device variants K72LL3 and K72LL4. It restores power to tablets where the original cell no longer holds a charge or fails to power on. Capacity matches OEM spec at 18.05Wh.
- K72LL3 and K72LL4 compatibility: Both variants share the same 3.8V power rail, connector pinout, and charge IC handshake, so one cell covers the full 8th generation Fire HD 8 lineup without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on a Fire HD 8 unit. The BMS accepted the charge IC handshake on the first cycle and held voltage within spec across display-on and idle states.
- Fuel gauge recalibration after swap: After installing this battery, run the tablet from 100% down to automatic shutoff without interruption, then charge uninterrupted back to 100%. This resets the fuel gauge IC against the new cell and corrects the inaccurate percentage readings that typically appear after a battery replacement.
Kindle Fire HD 8 shutting down at 15–25% remaining
The Fire HD 8 runs a combined load from its display backlight and WiFi radio simultaneously. When the battery cell ages, voltage drops sharply under this combined draw — the BMS reads the sag as a low-voltage fault and triggers shutdown before the percentage counter reaches zero. A new cell with full capacity holds voltage flatter under load, eliminating early cutoffs. After fitting this replacement, complete one full discharge cycle so the fuel gauge IC maps the actual voltage curve of the new cell.
Fire HD 8 showing wrong battery percentage after replacement
The fuel gauge IC on the Fire HD 8 stores a learned voltage profile of the old, degraded cell. When a new cell goes in, that stored profile no longer matches reality, so the percentage reading jumps, drops, or stalls. This is not a fault with the replacement battery — it is a calibration mismatch. Fix it by discharging fully to automatic shutoff, then charging uninterrupted to 100%; the gauge IC resets its reference at 4.2V full charge and recalibrates from there.