About this replacement
Codio K8 / T8 — 3.8V Li-Polymer Replacement Battery (W801)
This is a 3.8V, 450mAh lithium-polymer cell for the Codio K8 and T8 smartwatches. It replaces the original W801 battery when the watch no longer holds charge across a full day of use. At 29.50 x 27.70 x 5.00mm, it matches the footprint of the factory cell.
- K8 and T8 platform fit: Both models share the same chassis dimensions, connector position, and BMS handshake protocol — one cell covers both. The W801 part number is the same across the K8 and T8 production runs.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a K8 unit. The BMS accepted the new cell without fault flags, and the fuel gauge read correctly after a full charge cycle from near-zero.
- Fuel gauge reset after swap: After fitting this cell, place the watch straight on its charger and run it to 100% before first use. The fuel gauge IC on the K8 loses its reference point when the old cell is removed — skipping this step causes the watch to report incorrect percentages for days.
Why the K8 shows 0% immediately after a battery swap
The K8 uses a coulomb-counting fuel gauge IC that tracks charge state relative to a stored reference. Removing the old battery wipes that reference entirely. When the new cell lands in the circuit, the IC has no baseline, so it defaults to 0% or shuts the display off. A full charge from near-zero re-anchors the IC to the new cell's actual capacity. Do not judge the new battery's condition until after that first complete charge cycle.
Watch not pairing after battery replacement
Removing the battery cuts power to the BLE radio mid-session. The phone retains the old pairing record but the watch starts with a clean Bluetooth stack — the two sides no longer recognise each other. Go into your phone's Bluetooth settings, forget the K8 device entirely, then re-pair from scratch. Do not attempt to reconnect through the fitness app until the device appears as a fresh entry in your phone's Bluetooth list.