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 advanced situations 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 an influencer says so
• when Chicken Little claims you need one or the sky will fall

• Lead chemistry house batteries when not using solar, and the alternator is the only charging source

In this case DC-DC won't help with Absorption duration³⁾ but it can help with Absorption voltage.

• Victron DC-to-DC converters
  • More expensive brand, but generally good quality with good customer support/service
• Renogy DC to DC Chargers
  • Less expensive, but kinda so-so quality and poor after-sale support
    • Editors Note: Quality/support issues aside, I am a huge fan of the Renogy DCC50S. It combines a 50 amp DC-to-DC charger along with a highly efficient 50 amp MPPT solar charge controller in a single unit. This can take a lot of the complexity out of an electrical system design, and does it for an extremely good price.
      • Weird “gotcha” that isn't documented in any of the manuals; the “Trickle Charge” function that allows the solar panels to charge the vehicle battery does not work if the house battery type is Lithium.
    • IMO, this one device can probably supply the needs of 90% of most van electrical system builds.
  • They also sell a smaller 30 amp version, which is good for smaller vans/cars that might not need as much capacity
• Sterling Power
  • The largest DC-to-DC chargers on the market, offering up to 200 amp chargers. Some offer nice features like bi-directional charging, and an optional remote control which makes monitoring/configuring the chargers much easier.
  • Well made, with good customer service and after-sales support.
  • Expensive, like really expensive.