A simple estimate is charging time (hours) = battery energy (Wh) ÷ charger output (W). A 72 V, 20 Ah pack is approximately 1,440 Wh. With a 500 W charger, the ideal result is 2.9 hours; a practical empty-to-full estimate is longer because charging tapers and losses occur.
Use the correct inputs
Use nominal battery voltage and capacity only as an estimate. Charger output is not always the same as the number printed on the adapter. If the battery begins at 30%, you only need roughly 70% of its usable energy. If the pack is hot, cold, aged or near full, the BMS may reduce current.
Safety
Never leave an unknown or damaged battery charging unattended. Use the specified charger, keep connectors dry, and stop if there is unusual heat, smell, swelling or noise.