Hi Steve,
But the existing alternator is not going to "see" the LI bank. And the existing alternator is controlled by the Chassis ECM.
In the E-450 the OEM house bank charging circuit is fused at 60 amps (for my unit). The 60 amp are hard to find--so I replaced mine with an auto reset circuit breaker rated at 50 amps. I did add a 2nd charging path with a 2nd circuit breaker, direct to the chassis battery. The wire in both circuits is #8.
Back in the old days when I was using double conversion for limiting shore power, the twin 30 amp fuses on the PD (30 amp) converter did "deform" (i.e. quasi melted). They did not, however, fail.
When I tried double conversion on a GFCI circuit that the Magnum would instantly trip, the surge current was such that the reverse polarity fuses went *poof* more or less instantaneously, due to inrush current from the Magnum.
So, I suspect, that an OEM charging system with replacement LI batteries would blow the fuse before the alternator would fail.