DC-DC converter
These converters are used to combine different battery types in terms of charging. It is also the best way to replace the service batteries with lithium batteries, as a special charging curve can be programmed for this. Neither the lithium generator nor an old charger needs to be replaced.

DC-Master 24/12 - 6 A

Victron - Orion DC-DC Converter - 12V->24V 20A

DC-Master 24/12 - 24 A

Victron - Orion XS 12/12-50A DC-DC charger