The capacity of the micro: bit card is limited to 3.3 v. The micro: bit card can be powered by a USB connector or a JST connector.
The first one, which has the advantage of being always within reach of the user, is not the right solution since it is not portable. However, this connector is of great use in projects where the micro: bit remains in a fixed location.
As for the second, it has the advantage of being more convenient for powering the micro: bit card since it connects two AAA batteries in series. In addition, its small size makes it ideal for carrying out portable projects.