So I finally pulled the trigger on a Renogy 40A DC-DC charger to replace the basic split charge relay that came with my Transit-based van build. The old relay setup was just dumping alternator voltage straight into my 100Ah lithium (a Fogstar Drift), which I knew wasn't ideal — lithium needs a proper charge profile and the relay was just giving it whatever the alternator threw out, usually around 14.2–14.4V with no real control.
Fitting it was straightforward enough, took about two hours including routing the cables properly and adding an inline fuse on the input side. It's now been about three weeks and I can see on my Victron BMV-712 that the battery is hitting 100% SOC on longer drives where it wasn't always getting there before. The charger seems to be doing a proper absorption and float stage, which the relay obviously never did.
What I'm less sure about is whether it's actually protecting the alternator any better. I've read a lot about DC-DC chargers reducing the load on modern smart alternators (my Transit is a 2019 so it's got the variable voltage setup), but honestly I can't tell any difference from the driver's seat. Anyone else made this switch and noticed a real-world difference, either in alternator health or charge times?