Do I need a Renogy DC-DC charger if I already have solar?

Think about when each source actually delivers, rather than how many amps each is rated for.

Solar works when you are parked somewhere open and the sun is up. It gives you nothing at night, very little under heavy cloud, and very little under trees — which is where a lot of pleasant campsites happen to be. It also gives you least in winter, when the days are short and the sun is low, and that is the same season your heating load is highest.

The alternator works whenever the engine runs. It does not care about weather, shade, season or time of day. What it does care about is being connected correctly, which is the job a DC-DC charger does.

Why not just connect the alternator to the house bank

Because a lithium house bank will accept very nearly all the current the alternator can produce, for as long as it takes to fill, and alternators are not built for that sustained load. A DC-DC charger limits the current to a level the alternator can survive and applies a proper charge profile to the house battery instead of dumping raw alternator output at it.

Some Renogy DC-DC chargers include MPPT solar input as well, so one unit manages both the alternator and the array. That is worth knowing if you are still specifying the system — it saves a separate controller and a set of wiring.

The honest answer for most rigs

If you park in open sites for days at a time and have plenty of roof, solar alone can genuinely be enough, and adding a DC-DC charger buys you little.

If you drive most days, camp under trees, or use the rig through winter, solar alone will keep disappointing you and no amount of extra panel fixes shade and short days. That is the case where alternator charging does the heavy lifting and solar becomes the top-up.

Rigs that do both — travel days followed by a week parked — get the most from having both sources, because each one covers the other's weak spell.

What to check first

Your alternator's rating and how the vehicle manages it. A modern smart alternator varies its output deliberately, which changes how a DC-DC charger should be wired and triggered. The complete guide linked below covers that wiring in detail.