About this replacement
Xiaomi Mijia B302cn / 2105 — 21.6V Li-ion Replacement Battery
This is a 21.6V, 4000mAh (86.4Wh) Li-ion replacement battery for the Xiaomi Mijia B302cn and 2105 cordless handheld vacuum cleaner. It fits directly into the battery bay and connects to the vacuum's motor control board at the correct voltage rail. Capacity figures are taken from the product specification — not estimated.
- B302cn and 2105 compatibility: Both the B302cn and 2105 share the same 21.6V battery platform, connector pinout, and BMS handshake protocol. A battery that communicates correctly on one will function identically on the other — no firmware or hardware difference separates them at the battery interface.
- Bench tested on actual hardware: We ran this cell through charge cycles on the B302cn platform and confirmed the BMS negotiated charge termination correctly at full capacity. The motor start surge — which briefly pulls well above steady-state draw — did not trigger an overcurrent cutoff on any test run.
- Dock charging on the B302cn: Do not leave the vacuum sitting on the charging dock between uses. Cordless vacuums on continuous dock charge develop capacity fade faster than those charged only when the battery is depleted. Charge to full, then remove it from the dock.
Suction dropping before the battery indicator reaches low
This happens when the motor draws more current than the battery can deliver at its present state of charge, causing voltage to sag under load. The vacuum's motor controller interprets the sag as low voltage and reduces power — even though the indicator still shows charge remaining. A partially blocked filter is usually the trigger: restricted airflow forces the motor to work harder, pulling sustained current that exceeds the battery's comfortable discharge rate. Clean or replace the filter first; if sag still occurs with a clear filter, the original cell has lost enough capacity that a replacement is the correct fix.
Motor cuts out mid-use and then recovers after a few seconds
This is a BMS overcurrent trip, not a dead battery. When suction is restricted — blocked nozzle, clogged filter, or heavy carpet load — the motor draws a sustained spike that exceeds the BMS protection threshold. The BMS disconnects the cell to prevent damage, then resets once current demand drops. Clear the blockage, check the filter, and resume use. If the cutout happens on a clean filter with no blockage, check that the battery's discharge rating matches what the motor expects — a replacement cell should hold the load without tripping at 21.6V nominal.