Lenovo Yoga Book L15C2P31 Replacement Battery 3.8V 8500mAh

Lenovo Yoga Book YB1-X91F / YB1-X90F — 3.8V Li-Polymer Replacement Battery (L15C2P31)

This is a 3.8V, 8500mAh (32.3Wh) lithium-polymer replacement battery for the Lenovo Yoga Book tablet. It fits the YB1-X91F, YB1-X90F, and the broader Yoga Book line covered under OEM part numbers L15C2P31 and SB18C04740. It replaces a battery that no longer holds a charge, swells, or fails to power the tablet past a few percent.

• Yoga Book YB1-X91F and YB1-X90F compatibility: Both the Windows (X91F) and Android (X90F) variants of the Yoga Book share the same battery bay dimensions, connector pinout, and BMS communication protocol — that's why a single cell covers both. The charge IC on each model runs the same 4.35V charge termination voltage, so the replacement cell works without any firmware delta between the two.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on a Yoga Book unit and confirmed the BMS accepted the new cell, completed CC/CV charging without fault flags, and reported state-of-charge correctly after a recalibration cycle.
• Fuel gauge recalibration after swap: After installing this battery, run one full discharge to automatic shutoff — do not force it off manually — then charge uninterrupted to 100%. This gives the fuel gauge IC a complete reference window on the new cell and clears the inaccurate percentage display that almost always appears immediately after a swap.

Why the Yoga Book shuts down at 15–20% battery remaining

The Yoga Book combines an always-on display with active Wi-Fi — a combined load that pulls current hard enough to cause a rapid voltage sag near the bottom of the discharge curve. The fuel gauge IC, calibrated to the old degraded cell, misreads where that voltage cliff sits. When the new cell hits the same voltage threshold, the system interprets it as critical and cuts power even though meaningful capacity remains. Running one full discharge-to-shutoff cycle recalibrates the IC against the new cell's actual voltage curve and moves the cutoff point back to where it belongs.

Fast charging not available after battery replacement

The Yoga Book uses USB Power Delivery negotiation to enable fast charge — the charge IC and the PD controller handshake on the first accepted charge cycle after a new cell is installed. If fast charging doesn't appear immediately after the swap, that handshake hasn't completed yet. Charge the tablet from 0% to 100% uninterrupted using the original Lenovo AC adapter, not a third-party charger. After that cycle completes, USB-PD negotiation resets and fast charge becomes available again.