transformer

No batteries are charged with ac.

Some batteries have part of the charging circuit built within the battery enclosure/assmebly/equipment - if the external "charger" has an ac output, then that part of the charging circuit will include conversion of ac to dc.

Different batteries have very different charging current and voltage profiles - which can include sensing the temperature of the battery. The electronics to provide that profile is usually best located in the equipment holding the battery, rather than an external "charger". It makes sense to include the acdc conversion within the electronics and use a very inexpensive, very simple, external, ac output, "charger".

Thank you, that makes sense ?

Or you could try this:

Batteries are charged using dc.

If the charger has an ac output, there is stuff in the equipment holding the battery that turns the ac to dc, before it gets to the battery.