Short answer: a smart alternator changes its voltage to suit the engine and switches itself off when the vehicle doesn't need it. Good for fuel, bad for a second battery in the back, which can end up undercharged even after a long drive. A DC-DC charger fixes that by charging the auxiliary battery properly whatever the alternator is doing. The one wire that trips people up is the engine-running trigger, and Victron and Enerdrive give opposite advice on it for their own chargers.
What is a smart alternator?
It's an alternator the engine computer controls, fitted to save fuel and cut emissions. It gives a variable voltage and shuts down when it isn't needed.
How much does the voltage move? Victron put a conventional alternator at a fixed voltage, say 14 V, while a smart one on a 12 V system can wander anywhere from 12.5 V to 15 V. At the low end of that, it isn't really charging anything.
Why can't I wire the second battery straight to the alternator?
Two reasons.
- The voltage is often too low. Victron say Euro 5 and 6 vehicles with smart alternators often don't give enough charging voltage even with the engine running, and long cable runs make it worse. (Our cable sizing guide covers voltage drop.)
- Lithium can overload the alternator. A lithium battery's low impedance pulls a lot of current, and controlled charging protects the alternator from that. If you're weighing up lithium, our lithium vs AGM guide has the rest.
A DC-DC charger sits between the two batteries and keeps them apart, then charges the auxiliary battery at the right voltage. With a smart alternator, Victron's Orion-Tr Smart manual calls controlled charging "indispensable".
How does a DC-DC charger know the engine is running?
Both makers' chargers can watch the voltage. When the alternator is charging, the starter battery's voltage rises; when the engine stops, it falls. Victron's Orion XS does this out of the box, with no extra wiring or engine sensor.
Here are the Orion XS factory thresholds on a 12 V system, from Victron's manual:
| Setting | Smart alternator (the default) | Regular alternator |
|---|---|---|
| Start charging at | 14 V | 14 V |
| Or start after 120 s above | 13.3 V | 13.8 V |
| Stop charging below | 13.1 V | 13.5 V |
It doesn't stop the instant the voltage dips: the voltage has to stay below the stop level for more than a minute. Separately, an input lock-out stops it drawing below 12.5 V and lets it restart at 12.8 V, so it can't flatten the starter battery.
Enerdrive's DC2DC works on the same idea with different numbers: it normally starts charging above 13.2 V on a 12 V vehicle, and accepts 10.5 V to 16.0 V in.
Why does my DC-DC charger stop while I'm driving?
This is the classic Euro 6 problem. When the alternator switches off mid-drive, the starter battery falls back to its resting voltage, around 12.6 V. The charger sees that, decides the engine has stopped, and stops charging. The auxiliary battery ends up short.
Victron also warn that charging strategies differ from vehicle to vehicle, even within the same brand and model. So what works in your mate's ute may not work in yours.
The fix, where the alternator is off for long stretches and the base load is low, is an external engine-running signal.
Should I wire the ignition to my DC-DC charger?
It depends on whose charger it is. The two makers disagree, and each is right for its own design.
Victron Orion XS: don't use the ignition
Victron "do not recommend using the ignition switch to enable charging." With the ignition on and the engine off, the Orion XS would be draining the starter battery.
Instead, they list better signals for the remote connector's L-pin, best first:
- the vehicle's own engine-running signal (the most accurate, because it follows the engine directly)
- a pulled-low signal via a relay
- D+ (often no longer provided on Euro 6 vehicles)
- a vibration sensor
- a CAN-bus interface.
More than 8 V on the L-pin forces charging on. Two details matter:
- The signal only forces charging on. Stopping when the engine's off is still the job of the shutdown detection, so leave that switched on.
- The L-pin doesn't replace the remote on/off; that still has to be on.
If you switch detection off altogether, the Orion XS keeps drawing from the starter battery with the engine off, and the 12.5 V input lock-out is the only thing protecting it. Victron say that's low enough in most applications.
Enerdrive DC2DC: the ignition is the ideal way
Enerdrive's quick start for smart-alternator vehicles says to connect a switched ignition source to the ignition input. Their support centre calls wiring the DC2DC's ignition sense terminal to the vehicle ignition "the ideal way" to turn on Priority Mode.
Priority Mode is Enerdrive's answer to the smart alternator. It lets the DC2DC start charging at 12.3 V instead of 13.2 V. It still protects the starter battery: below 12.0 V it disconnects from the vehicle and switches back to solar if it's fitted. The ignition terminal needs at least 10.5 V to trigger it.
Don't take the shortcut of a permanent jumper to keep Priority Mode always on: Enerdrive warn it will keep draining the start battery with the vehicle off. And on smart-alternator vehicles, the DC2DC's negative must go to the vehicle side of the Battery Sense Module.
So which is right?
Both, for their own chargers. They're different designs with different protections, so follow the manual of the charger you've got, not a forum post about someone else's.
Do I need an isolated DC-DC charger?
Victron's reason for the isolated Orion-Tr Smart is short: it lets the input and output have separate chassis grounds. Its specs list 200 V DC of galvanic isolation between input, output and case. Victron don't say much more about when you'd need one, so if your build might need separate grounds, ask an auto-electrician rather than guess.
Victron make both, for example the Orion-Tr Smart 12/12-30A isolated and the Orion-Tr Smart 12/12-30A non-isolated.
Charging a camper or trailer?
Enerdrive's smart-alternator diagram for a DC2DC without an ignition feed has a clear warning about the relay that feeds the trailer: it must be ignition-switched, not a VSR, or you'll get poor charging or a flat start battery.
When should an auto-electrician do it?
Mounting a charger and running battery cables is a reasonable DIY job if you're handy and follow the manual's cable and fuse sizes. Finding the vehicle's engine-running signal, tapping into CAN-bus, or working around a battery sense module on a late-model vehicle is where an auto-electrician earns their money. We've fitted plenty of Victron DC-DC chargers, and Victron's warning holds: every vehicle is a bit different.
If you're deciding between the car, the sun or both, start with DC-DC charger, solar controller or both? The Victron Orion XS 12/12-50A, the Enerdrive DC2DC 40A and the rest are in DC-DC chargers.