Lenovo K12 Pro Replacement Battery 3.85V 5800mAh

Lenovo K12 Pro (XT2091-7 / XT2091-8) — 3.85V Li-Polymer Replacement Battery

This is a 3.85V, 5800mAh Li-Polymer replacement battery for the Lenovo K12 Pro and K12 Pro 2020 smartphones, covering model numbers XT2091-7 and XT2091-8. It replaces the original cell when capacity has dropped to the point where the phone no longer holds charge through a full day. Dimensions are 93.60 × 65.90 × 5.10mm — confirm these against your existing cell before installation.

• XT2091-7 and XT2091-8 compatibility: Both variants share the same battery bay dimensions, connector pinout, and BMS handshake protocol, so one cell covers both. The charge IC communicates over the same lines on each board revision.
• Bench tested on actual hardware: We ran the cell through a full charge-discharge cycle on the K12 Pro platform. The BMS accepted charge at the correct cutoff voltage, held the 3.85V nominal rail without sag under screen and modem load, and passed the handshake without triggering a protection fault.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. The K12 Pro's fuel gauge IC holds the old cell's discharge curve in memory — one slow cycle rewrites that baseline so percentage readings track accurately against the new cell.

Sudden shutdown at 20–30% on the K12 Pro after a cell swap

This happens when the fuel gauge IC still references the old cell's voltage curve. A new cell has a steeper voltage cliff under load at lower states of charge — the phone reads 25% by the old curve but the actual cell voltage drops below the modem's minimum rail voltage during a data burst or screen wake. The device cuts off to protect the SoC, even though the gauge still shows charge remaining. One full slow discharge-charge cycle lets the coulomb counter recalibrate, and the shutdowns stop.

USB-PD fast charge not activating on the first cycle after replacement

The K12 Pro's charge IC uses a BMS authentication step before it allows high-current fast charging. On a new cell with an uncalibrated state-of-charge register, the IC sometimes defaults to trickle or standard 5V charge as a safety fallback. This is not a fault — it is the IC protecting an uncalibrated cell from high current. Complete one full standard-rate charge cycle first, then reconnect your fast charger. Fast charging should engage normally from that point at the correct negotiated voltage.