Lithium battery charging in FW of truck

One thing to realize here is that almost all modern (as in > 2010 or so) alternators are 'smart'. They have a small microcontroller in them and communicate with the vehicles ECU, which can command the voltage output of the alternator and query the status of the alternator. As part of this system there is thermal monitoring of the alternator - it is gets too hot, it will sense this and drop the output voltage and thus the current to reduce the load on the alternator. In this case it is actually worse for the alternator to be connected to a DC-DC charger as opposed to directly to a battery. A DC-DC charger will actually draw MORE current as the alternator output voltage drops.