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Note this is a companion to the Is Solar Mandatory? article
Is DC-DC charging mandatory?
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.
However, more 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.
Situations where DC-DC is effectively required
When the vehicle has a smart (variable-voltage) alternator
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
1)
If you want to impose an arbitrary current limit
2)
when an influencer says so
when Chicken Little claims you need one or the sky will fall
Situations where DC-DC is preferable
In this case DC-DC won't help with Absorption duration3) but it can help with Absorption voltage.
List of popular DC-to-DC chargers
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The largest DC-to-DC chargers on the market, offering up to 200 amp chargers. Some offer nice features like bi-directional charging, and the offer a nice remote control for the chargers which makes monitoring/configuring them much easier.
Well made, with good customer service and after-sales support.
Expensive, like really expensive.