About this replacement
Asus Transformer Book T300chi — 7.6V Li-Polymer Replacement Battery (C21N1421)
This 7.6V, 4900mAh (37.24Wh) Li-Polymer battery replaces the original cell in the Asus Transformer Book T300chi detachable tablet. It covers the T300CHI-FL121T, T300CHI-FL005T, T300CHI-FL073H, and over 37 additional T300CHI variants. Cross-references include C21N1421, 0B200-01520000, 0B200-00570200, and C21N1418.
- T300CHI tablet unit compatibility: All T300CHI variants share the same 7.6V battery rail, connector pinout, and BMS handshake protocol. The tablet component houses this cell independently from any keyboard dock — only the tablet half uses this battery.
- Bench tested on actual hardware: We ran the C21N1421 cell through a full charge and discharge cycle on T300CHI hardware. The BMS accepted the cell without fault codes, balanced both internal cells correctly, and reached cutoff voltage cleanly at both ends.
- Fuel gauge reset after installation: After fitting this battery, run one full discharge to automatic shutoff, then charge uninterrupted to 100% without using the device. This resets the fuel gauge IC calibration against the new cell and clears the inaccurate percentage readings that typically appear after a swap.
T300CHI shutting down at 15–25% remaining
The T300CHI combines a high-resolution display with active WiFi — both pull current simultaneously when in normal use. Under that combined load, voltage drops faster than the fuel gauge predicts, hitting the BMS low-voltage cutoff while the OS still reads 15–25%. This is a calibration gap, not a faulty cell. One full discharge-to-shutoff followed by an uninterrupted full charge resets the fuel gauge IC and aligns the percentage curve to the new cell's actual voltage profile.
Fast charging not available after battery swap
The T300CHI's charge IC runs a USB-PD negotiation handshake before enabling elevated charge rates. On a new cell, that negotiation sometimes fails on the first accepted cycle because the BMS has no charge history to reference. Plug in using the original Asus adapter and let the device complete one full uninterrupted charge from below 10% to 100%. After that cycle, the charge IC logs an accepted profile and fast charging becomes available on subsequent sessions.