Death and taxes are mandatory. Gravity is mandatory.

In many/most situations direct-charging the house bank with a combiner (relay, isolator, VSR) is both effective and inexpensive. Even with lithium banks.

There are scenarios where one may want to disable the combiner.

• when the vehicle has a smart (variable-voltage) alternator, more common in Europe
• small alternators vs. ginormous battery banks of any chemistry
• exotic alternator or battery bank voltages, including setups like 24v house bank and 12v alternator (or vice versa)
• if one needs relatively-stable charge rates. All other things being equal, direct-charging current is dictated by the difference in voltage (“delta”) between alternator and battery bank (I=V/R). Therefore…
  • …anything that increases the difference (higher alt voltage, lower bank voltage, loads on the bank) will increase current drawn from the alternator
  • … and anything that decreases the difference (like rising bank State of Charge) will decrease the current. So direct-charged banks tend to start out with higher current and “taper” off to lower and lower currrent as they charge
  • DC-DC chargers boost alternator voltage, maintaining sufficient voltage delta for more consistent current output as long as the bank can accept it.
• if you want to impose an arbitrary current limit¹⁾
• when an influencer says so
• when Chicken Little claims you need one or the sky will fall

• lead house bank when alternator is the only charging source

In this case DC-DC won't help with Absorption duration²⁾ but it can help with Absorption voltage.