Asus VivoBook E12 E203NA Compatible Battery 7.7V 4900mAh

Asus VivoBook E12 E203NA Series — 7.7V Li-Polymer Replacement Battery (C21N1807)

This is a 7.7V, 4900mAh (37.73Wh) Li-Polymer replacement battery for the Asus VivoBook E12 E203NA-FD020TS and a range of related E203 and C223NA models. It slots into the same bay as the original C21N1807 cell and connects via the same 2-cell BMS board. Voltage and connector match the OEM spec exactly.

• E203 and C223NA platform fit: These models share a 2-cell 7.7V Li-Polymer architecture with the same physical footprint (217.10 × 154.58 × 6.50mm), identical connector pinout, and the same BMS handshake protocol. Swapping between E203NA, E203MAH, E203NA-FD026T, and C223NA-1A variants works because the charge controller on the board expects the same cell chemistry and voltage rail.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on an E203NA board. The BMS accepted the cell without fault codes, the fuel gauge IC communicated correctly over SMBus, and charge current stepped down at the expected voltage threshold near top-of-charge.
• Post-install calibration for the E203NA: After fitting this cell, run one full discharge down to hibernate-cutoff, then charge uninterrupted to 100%. The E203NA fuel gauge IC needs this cycle to re-anchor its capacity estimate against the new cell — skipping it causes the health indicator in MyASUS or Windows Battery Report to flag a false low-capacity warning for weeks.

BIOS reporting battery health as poor immediately after swap

The E203NA stores learned capacity data in the battery's EEPROM. When you install a new cell, the BIOS reads the blank or mismatched EEPROM and flags the battery as degraded before any charge cycle has run. This is a data mismatch, not a cell fault. One complete discharge-to-hibernate followed by an uninterrupted charge to 100% rewrites the fuel gauge reference point. After that single learn cycle, the health warning clears and the reported Wh value aligns with the 37.73Wh rating of this cell.

VivoBook E203NA shutting down at 20–30% charge shown

This happens when the fuel gauge IC still holds calibration data from the old, degraded cell. The percentage reading looks valid, but the actual cell voltage drops below the BMS cutoff threshold under combined CPU and display load before the gauge reaches zero. The laptop cuts power because the voltage cliff arrives earlier than the stale calibration expects. Force a full discharge to hibernate-cutoff, then charge to 100% without interruption — this re-syncs the gauge to the new cell's actual voltage curve and eliminates the early shutdown.