The battery charger is connected in parallel to the engine battery (like a relay) and therefore does not require any modification of the charging circuit (the alternator is connected directly to the engine battery).
Some models provide voltage conversion, so it is possible to recharge a 24V battery such as the bow thruster from a 12V alternator or recharge a 12V service battery directly from a 24V alternator. Note also that the battery chargers allow to limit the current, this feature is useful to respect the charging constraints of Lithium batteries or to limit the output power of the alternator and preserve it from a premature wear.
The new models also allow :
To reduce the charging voltage of some alternators (with charging voltage above 14v)
To have an adapted maintenance phase.
To ensure the recharge for vehicles equipped with regenerative brakes (Euro 6 standards).