This is an old revision of the document!
Devil’s advocate proposition: when combined with nontrivial amounts of solar, DC-DC charging is a decreasing return expense. Normal isolator charging would do the Bulk-style heavy lifting adequately and much more cheaply.
Reasoning: normal isolators raise the bank to alternator voltage (Valt, something like 13.8v, maybe higher). The primary value of DC-DC charging is that it DC-DC upconverts voltage to reach Vabs (Pb) or to induce top-balancing via overcharge (Li). Let’s look at the cost/benefit, assuming the 300Ah lead bank AGM absorps at 14.4v.
a $200-400 DC-DC charger (CTEK, Renogy, Sterling, etc) would bring battery voltage right up to Vabs (14.4v).
What we are paying $100-$175 extra for is raising voltage from 13.8v → 14.6v faster than solar would have done it alone.
Exceptions (where solar + DC-DC can be valuable):