How to calculate the battery charging time?

If your parameters about the charger are correct, then you can be sure that the data of your charger is false, because the greater the charging current of the charger, the higher the price, and some manufacturers will fake it. It is very simple to see the charging current from the charging time, and the simpler method is to know it with a multimeter.

You said that if you charge it for a few hours, if you don't take it off, there will be only half of the power left. There may be two reasons for this. One is that the charger discharges, and the other may be that the battery has large internal resistance and self-discharge.

Maximum charging time (hours) = (battery capacity mah/ charging current ma)* 1.5