Vax Air Cordless Switch AC21 20V Replacement Battery 4000mAh

Vax Air Cordless Switch AC21 / H85 — 20V Li-ion Replacement Battery

This 20V, 4000mAh (80Wh) lithium-ion battery fits the Vax Air Cordless Switch AC21 and Air Cordless Switch H85 stick vacuums. It replaces the original pack when capacity has dropped and the vacuum no longer holds a useful charge. No OEM part number exists for this cell — match by model name before ordering.

• AC21 and H85 compatibility: Both models run the same 20V motor rail and use the same battery dock connector with identical BMS handshake requirements. One cell covers both machines.
• Bench tested on actual hardware: We ran this pack through full charge and discharge cycles on the AC21 platform. The BMS communicated cleanly with the charger, accepted a full charge without fault codes, and the motor powered up at rated voltage across both suction modes.
• Dock charging habit on cordless vacuums: Do not leave the vacuum sitting on the charging dock between uses. Cordless stick vacuums on continuous dock power develop capacity fade faster than those charged only when depleted. Charge to full, then remove from the dock.

Suction dropping before the battery indicator reaches low

The AC21 motor draws significantly more current when airflow is restricted — a partially blocked filter forces the motor to work harder at the same voltage. As current draw rises, the battery voltage sags under load, and the motor loses speed even though the cell is not actually depleted. The battery indicator reads the resting voltage, not the load voltage, so it can show two or three bars while suction has already dropped. Clean or replace the filter first — if suction recovers immediately, the battery is not the fault.

Motor cutting out mid-clean and then restarting after a pause

This is a BMS overcurrent trip, not a dead battery. When the vacuum runs sustained restricted suction — carpet pile, a partial blockage, or a dirty filter — current spikes past the BMS protection threshold and it cuts the output to protect the cell. The motor returns after 10–30 seconds because the BMS resets once current demand drops. Clear any blockage in the head and wand, clean the filter, and verify the cell voltage sits above 18V before placing it back on charge.