====== Opinion:  adding DC-DC charging to solar ======

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 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 [[opinion:frater_secessus:panelsizesforinsolation|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