inspired by this thread on the promaster forum, in which OP was thinking about what DC-DC charger to add to a 300Ah bank and 30A of solar charging.
The DC-DC charger will be a back up to solar charging.
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 $25 constant duty solenoid would get the bank up to Valt, at which point 70% of the current needed to charge Pb and 100% of the current needed to fully charge Li has already been provided. Solar would have to lug voltage up 13.8v -> 14.4v for Absorption/balancing.
a $100 voltage sensing relay would do the same. Cost more for the autoswitching magic and possible crude overcurrent protection via internal autoresetting breaker
a $200-400 DC-DC charger (CTEK, Renogy, Sterling, etc) would bring battery voltage right up to Vabs (14.4v). Solar would just handle the high voltage, decreasing current Absorption proper.
What we are paying $100-$175 extra for here is raising voltage from 13.8v -> 14.6v faster than solar would have done it alone.
Exceptions (where solar + DC-DC can be valuable):
underpaneled systems that would struggle to make the 13.8v -> 14.6v slog on their own in normal insolation. Going from 800w of panel down to 200w + DC-DC can save hundreds of dollars under optimal use
using a smaller DC-DC charger to limit the amount of power Li or large AGM banks can pull from the alternator
people who drive for hours at night (delivery folks, etc)
people who live in extremely challenging insolation conditions (Alaska or the PNW in the winter) where the difference between Valt and Vabs can be hard for even hefty solar configs to bridge