Death and taxes are mandatory. Gravity is mandatory. Are DC-DC chargers mandatory? Ehh…

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

However, some use cases may require a DC-to-DC charger. These are much more intelligent (and expensive) devices that sit in between the vehicle batteries and the house batteries, and actively adjust both the voltage and current flowing from the vehicle's electrical system and into the house batteries.

Note: This article is just the TL;DR of DC-DC charger options. More detailed info is here.

• When the vehicle has a smart (variable-voltage) alternator
  • Common on European vehicles, and on American vehicles newer than ~2017
• 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 rates¹⁾
• If you want to impose an arbitrary current limit²⁾
• when the alternator is the only charging source for the lead-chemistry house batteries. In this case DC-DC won't help with Absorption duration³⁾ but it can help with Absorption voltage.
• when an influencer says so
• when Chicken Little claims you need one or the sky will fall

• when the alternator is the sole charging source for LiFePO4. In this situation DC-DC will deliver more predictable and often faster charging, particularly when the bank is at higher SoC.