Charging is slow.
The rated charging voltage of the charger should be 72 volts, and the charging current of 5 amps is a relatively conservative slow charging situation. It is acceptable for general lithium batteries, but the charging speed. is relatively fast.
To charge a 72v45ah lithium battery, you can use a charger with a charging output of around 20A (standard charging). Of course, you can also choose a charger with a lower charging current (slow charging). Unless the battery manufacturer specifically states that a particularly low current should be used for charging, it is best to use a standard charger suitable for the battery.