Short answer: a DC-DC charger charges your house battery from the vehicle while you drive. A solar controller charges it from your panels while the sun's on them. One can't do the other's job, so if you drive some days and sit still on others, you'll most likely want both, either as two separate units or one combined box.
If you rarely move the vehicle, solar is the main act. If you're always on the move, the DC-DC charger does the heavy lifting.
What does a DC-DC charger actually do?
It sits between the starter battery (and the alternator charging it) and your house battery, and charges the house battery properly instead of just joining the two together. Victron's manual for the Orion XS 12/12-50A lists the jobs:
- It charges properly from a smart alternator. Euro 5 and 6 vehicles with smart alternators often put out a charging voltage that's too low even with the engine running, and long cables make it worse. A DC-DC charger fixes that.
- It protects the alternator with lithium. A lithium battery can pull a high current out of the alternator. Controlled charging stops it overloading.
- It keeps the starter battery separate. Flat house battery, car still starts.
- It only charges when the engine's running. The Orion XS has built-in engine shutdown detection, so it only charges when the alternator is actually supplying power. No extra engine sensor or wiring change needed, though you can use an engine-running signal from the vehicle to force it on.
- It charges in stages. Bulk, absorption, float and storage, with lithium absorption fixed at 2 hours by default, and a low-temperature shutdown to protect lithium batteries in the cold.
The Orion-Tr Smart does much the same. It can charge only when the engine's running or be switched on by a signal such as the ignition, it won't drain the starter battery when the alternator isn't supplying power, and it separates the two batteries when the engine's off.
Smart alternators are a topic on their own, including whether to use the ignition wire. That's in our guide to smart alternators and DC-DC chargers.
What does a solar controller do?
It sits between the panels and the battery and turns what the panels make into a charge the battery can take. Victron's SmartSolar is an MPPT (maximum power point tracking) controller, and two things are worth knowing:
- The name tells you the limits. A SmartSolar 100/30 takes up to 100 V from the panels and puts up to 30 A into the battery.
- It can charge a lower-voltage battery from higher-voltage panels. It adjusts to the battery, charges at up to its rated current, and keeps hunting for the panels' best operating point, including under partial shade.
There's a simpler, cheaper type called PWM. When that's fine and when it isn't is in MPPT vs PWM solar controllers.
Can a solar controller charge from the vehicle instead?
No. Victron's SmartSolar manual says to avoid using it as a DC-DC charger and to use a dedicated DC-DC charger instead. If you want one box that takes both the vehicle and the panels, that's what a combined unit is for (below).
So do I need both?
Think about how your trips actually go:
- Mostly driving, short stops. A day's drive between camps gives a DC-DC charger hours to work. Solar is a bonus. Some people get away with the DC-DC charger alone.
- Drive in, then stay put for days. The DC-DC charger fills you up on the way in, then sits idle. Solar has to carry the fridge from then on, so it needs to be big enough for your daily use.
- A mix of both, which is most touring. Both. The car does the heavy lifting on travel days, the panels hold you when you stop.
"Big enough" comes from your daily use. How to size a 12V system does the load list, and how much solar do you need picks up from there.
Two separate units or one combined box?
Both work. They behave a little differently.
Separate units (for example, a Victron Orion and a SmartSolar)
Each charger does its own job and is wired and fused as its maker says. You can size each one on its own: a bigger DC-DC charger for a long-range tourer, a bigger solar controller for a big roof.
On networking: SmartSolar chargers on a VE.Smart Network share the charging as if they were one large charger. The Orion XS isn't on Victron's list of chargers that join in like that; it uses VE.Smart Networking only to receive battery voltage, temperature and current data from a BMV, SmartShunt or Smart Battery Sense.
Victron's manuals don't spell out either way whether an Orion and a SmartSolar should charge the same battery at the same time. If you're planning it, have the maker, us or an auto-electrician check your setup.
We've installed plenty of SmartSolar controllers and Victron DC-DC chargers. One plain upside of two boxes: if one needs attention, the other can keep doing its job.
One combined unit (for example, the Enerdrive DC2DC40+)
Enerdrive's ePower DC2DC/MPPT charger is one charger with two inputs: the alternator and start battery, and solar through its own built-in MPPT controller. From Enerdrive's manual:
- It picks one source, automatically. The engine while you're driving, the panels when you're stopped. It doesn't add the two together.
- It drops back to solar if the vehicle voltage sags. Below 12.8 V (25.6 V on 24 V) it disconnects from the vehicle and charges from solar until the vehicle side is back above 13.2 V (26.4 V on 24 V).
- The solar side has limits. PV input 14.5 to 45 V, and a recommended maximum of 500 W of solar.
- Smart alternators need the ignition wire. Connect a switched ignition source to the ignition input; that enables the lower start-up voltage designed for smart alternators.
The appeal is one box, one set of wiring and one place to look. The trade-off: one source at a time, and the solar side stops at 500 W recommended and 45 V input. If you want a bigger roof array later, separate units leave more room to grow.
What's the catch?
- A DC-DC charger is no use parked up for a week. Solar is no use on a grey day of driving. That's why most touring setups end up with both.
- If you only do weekenders with a long drive each way, a decent DC-DC charger on its own might be all you need. Add solar later if the fridge starts running short.
- Whichever you choose, wire it, fuse it and size the cable the way the maker's manual says. If the vehicle side is beyond you, that's an auto-electrician's job.
Our DC-DC chargers and MPPT solar controllers are grouped by type if you want to compare.