Normally, 18650 lithium-ion batteries are used, and in the case of a design focused to be thin, square lithium polymer batteries are used but it is not common.
The capacity of 18650 cells has been increasing, and the current cutting edge is 3600mAh.
Charging current and maximum discharging current are not necessarily increasing in proportion to the increase in capacity, but it seems that as the capacity increases the charging time becomes longer, and the maximum discharging current seems to be kept at the same value as the conventional product.
Capacity of 3000mAh or more for 18650 size seemed to be impossible capacity before, but it is realized by research and development of each manufacturer such as the change of positive electrode material.
There are various combinations such as 3S1P and 4S1P.
Constant current and constant voltage control is absolutely necessary for charging.
For a charger, a method of directly charging the battery pack was common, but recently there are those in which constant current and constant voltage control circuit is built in lithium-ion batteries and a charger is a simple AC adapter, but it is not recommended.