Acer Chromebook 311 C721 Replacement Battery AP18K4K 11.4V

Acer Chromebook 311 C721 / R721T — 11.4V Li-Polymer Replacement Battery (AP18K4K)

This is a 11.4V, 4100mAh (46.74Wh) Li-Polymer replacement battery for the Acer Chromebook 311 C721, Chromebook Spin 311 R721T, and the broader AP18K4K platform. It replaces OEM part numbers AP18K4K and KT.0030.4013. If the Chromebook dies fast, won't hold charge, or reports poor battery health in ChromeOS, this is the cell to swap.

• Chromebook 311 and Spin 311 platform fitment: The C721, R721T, CB311-9H-C4PP, and 311 C721-48BR all run the same 11.4V three-cell Li-Polymer pack with a shared BMS connector pinout and identical EEPROM handshake. That's why one SKU covers the full range — the BMS authentication logic is identical across all these models.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on Chromebook 311 hardware. The BMS negotiated correctly with the EC firmware — no authentication errors, no charge refusal, and the pack reached full voltage without thermal events.
• Post-swap calibration cycle: After fitting this battery, run the Chromebook down until it hibernates on its own, then charge uninterrupted to 100% without using the device. This forces ChromeOS to reset the fuel gauge IC against the new cell and clears the inaccurate "poor battery health" warning that almost always appears after a cell swap.

ChromeOS reporting battery health as poor immediately after fitting a new cell

The Acer Chromebook EC stores historical capacity data from the previous cell in the battery's EEPROM. When a new cell goes in, that stale data makes the fuel gauge IC report degraded health — even on a brand-new pack. This is not a fault with the replacement cell. One full discharge-to-hibernate followed by an uninterrupted charge to 100% resets the learn cycle. After two to three cycles, the health reading normalises.

Chromebook 311 shutting down at 20–30% charge shown on screen

This happens when the fuel gauge IC hasn't yet calibrated to the new cell's actual voltage curve. The displayed percentage doesn't match the real cell voltage, so the EC cuts power before the gauge reaches zero. It isn't a faulty battery — it's a calibration lag. Run two full discharge-to-hibernate cycles and charge back to 100% each time. By the third cycle, the EC's internal model catches up and the shutdown threshold aligns with actual cell voltage.