I have a different take on this
The only reason for a B2B charger is if you have a smart alternator, as a Smart alternator is ECU controlled you are not able to add an unknown discharge current, as this could damage the alternator and the ECU. Without a B2B it could also spike the house batteries with a voltage higher than 14.6v which is not ideal. The main aim of a B2B is to fool the ECU into thinking there is a charge current needed by the vehicle battery, this charge current can then be used by the house battery.
The only advice is not to run the engine on tickover to charge depleted batteries for more than 15 mins as the Alternator could get hot.
The first comment insinuates that a B2B chargers have the capacity to stop a Lithium battery accepting a charge. This is simply not the case as there is no data transfer between the B2B and battery BMS. Even if you had a B2B you would still have to rely on the battery BMS to switch off the ‘allow to charge’ function when the battery is fully charged, a B2B is not capable of doing this. By installing a Cyrix and a battery protect we are performing all necessary features of a B2B at a far lower cost.
In answer to the 2nd statement. If you tried to regulate the amp draw from an alternator by reducing the cable size it would simply melt the cable, the cable sizing in my installation will allow the safe transfer of available power. As I mentioned before there has never been an issue with these systems damaging alternators.
Point to note these are not cheap batteries with a cheap BMS and are capable of accepting a C1 charge and carry a ten year warranty. All the other equipment fitted carries a five year warranty. Try looking on the Transporter batteries website