Battery to battery chargers (or DC/DC chargers as you may see them elsewhere) are the best way of charging batteries in many modern power systems.
They're the only way to safely charge Lithium batteries from an alternator and still protect against all the possible issues that could arise... They're the only way to charge from a modern Smart/Euro-6/7 alternator... and they're the only way to charge across voltage ranges (12V to 48V for example).
They're intentionally current limited, and their ratings (IE BB1225, = 12V 25A) are based on their input current so you can be certain you are rating everything correctly.
Our historic guidelines have been to not exceed ~75% of an alternators rating when recommending RE: battery to battery chargers and their installs.
Example : A 120A alternator is most likely not going to be happy running from our 120A battery to battery charger! We would recommend turning it down to 65% operation (via the BBR remote) or to fit a BB1260 or BB1270.
