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How to Charge Batteries Off Your Engine and Stock Alternator

How to Charge Batteries Off Your Engine and Stock Alternator

What's the best way to properly charge a bank of lithium batteries in a van, trailer, or RV?

If you have existing equipment in an engine bay, it's hard to get excited about modifying that. Charging your service battery directly off of an alternator is not a recommended, as it's easy to overload both the alternator and the house battery.

Meanwhile, all batteries, especially lithium batteries, come with specific charging parameters that must be met to obtain full performance and maximize the battery's lifespan. Improperly charging a battery can void the warranty altogether.

An Orion-Tr Smart DC-DC charger offers an ideal solution for this application. It regulates the voltage to your house battery without modifying your under-the-hood electrical system.

Before we jump into the Orion, it's important to reference alternative, historical technology for charging a house battery from a vehicle's starter battery. Battery Isolators, battery combiners, and charging relays have a common weakness: while they can adjust current by turning on and off, they can't adjust voltage from the alternator. 


In the past, there was one kind of battery (lead acid), and every battery in the system accepted similar charge parameters. Today's challenge is that not all batteries want the same charging parameters. Without an Orion in the system, it's very unlikely that a Lithium battery bank will be charged properly off an alternator system designed to charge lead acid batteries. 


Another challenge is that alternators have become much more sophisticated, and it's highly recommended that you don't mess with them by adding excitation/energizing cables to your smart alternator. 


So how do you get power out of an alternator you can't control? That's where the Orion comes in, and once you understand how it works, it's easy to see why it's been so successful in the mobile market.


Victron Orion-Tr Smart DC-DC charger isolated

Typical connection setup as DC-DC charger, Victron Orion-TR Smart DC-DC Charger isolated


How Orion DC-DC Chargers Operate


Victron's Orion DC-DC chargers have an "engine running detection" system. A voltage threshold indicates if the alternator is putting out current. When the engine runs, the Orion pulls power to charge the house battery. When the engine stops running and the voltage drops, it stops charging the house battery. The Orion DC-DC charger ensures that the house battery will only charge when the engine is running and prevent your starter battery from discharging while you're out camping or looking for fish.


Those set points are adjustable. Like all Victron Smart devices with built-in Bluetooth, you can monitor and program charging parameters to your heart's content using the VictronConnect app, ensuring that the battery gets exactly what it requires.  

Engine Detection Shutdown


There is a remote ON/OFF contact. When connected, the Orion will be enabled to run. When disconnected, Orion will not charge your batteries. The ability to turn the Orion ON and OFF is critical for lithium battery designs that allow for Allow-to-Charge (ATC) control. The Victron lithium battery system uses this ATC contact directly from its BMS (battery management system). The Pylontech RT12100G31 also has a contact for the ATC function that can enable the Orion. The ON/OFF contact can also be controlled with a manual switch. 


Many batteries, by design, will isolate themselves from the charge source when needed. Remember: even when a lithium gets below 32°F, it will keep putting out power. If you try to charge it at very low temperatures, it will isolate to protect itself.


Photo by Adventure Van Systems

When the internal temperature gets too high or too low, a typical drop-in battery will isolate. If you have three networked together, they will disconnect one at a time until you have no power. It's fairly common for a three-battery bank to partially isolate the batteries, and in many cases, those batteries don't come back online right away and the bank gets imbalanced.


You can see how the entire system can start to suffer long-term. It's always better to shut off the charger and keep batteries online rather than let them disconnect themselves from the system. With more intelligent communicating batteries, we can politely ask the charger to stop and avoid all kinds of pain. 


Orion is attractive because it's straightforward to install without heavily modifying the wiring. It can stand independently and do its job. And with the relay, it can be controlled by the rest of the system as needed.

How to pick your Orion DC-DC Charger


Orions come in a variety of options. The first question would be: How important is it that your grounds are isolated or non-isolated? You would use an Isolated DC-DC charger if your system had two grounds. This is useful in tow-behind systems where your vehicle is grounded to its chassis, while in your trailer or 5th-wheel camper, that electrical system has its own ground (typically its frame). We also find this useful in marine applications where corrosion caused by shore power connections in a marina can be a significant issue.


If you only have one ground, you'll need a Non-Isolated charger. The difference physically is that the isolated Orion has two negative connections that are not connected to each other. Some people use smaller isolated Orions in non-isolated applications. For example, Victron makes a nice 18 Amp Isolated Orion which is great when your extra alternator capacity is limited. That's not a problem. You would simply ground both negatives to the same place, thereby making it non-isolated. Most RVs and Campers/Camper Vans have one ground, so the non-isolated is ideal.


Orions can also bump the voltage from 12 Volts to 24 Volts which is extremely handy for 24 Volt systems. Generally speaking, most stock alternators recommend not having more than 70% of their load capacity occupied. Overloading your alternator is never a good idea, of course. But it's worth noting that an Orion's Amp draw, which depending on the unit, could be 30 Amps, is not adjustable. That's where the 18 Amp option is attractive. If you have plenty of alternator capacity, you can parallel two Orions and get 60 Amps of charging. 


If you turn all your loads on (rally spotlights, boom box, etc.) and clamp your alternator leads, you can get a good idea of your vehicle's electrical requirements and how much extra capacity you have to play with.


The options are relatively limited once you identify Isolated vs. Non-isolated. You might consider the 18 Amp model if your alternator is taxed or your wire is smaller. Feel free to reach out to us for help choosing the right Orion for your system. 

Two Orion-TR Smart DC-DC Chargers in parallel in Sprinter Van

Installation Notes

Wiring an Orion is not complicated. You can often connect your Orion directly to the starter battery under the seat of, say, a Sprinter Van. A secondary plug with wiring that can handle the Voltage/Amperage is often used in 5th-wheel or pull-behind trailers where your run may be significantly longer than in a camper or camper van.

The screw terminals on the Orion can accept a maximum cable size of 6 AWG. A properly sized fuse or breaker is required for both the input and output of the Orion. Remember, the fuses are to protect the wire; the Orion will not over-amp itself. If the Orion is rated for 30 amps, the short circuit current is 60 amps. Thus, a 60 amp fuse or breaker will be ideal. These fuses can come in MegaFuse form or automotive style.

Related Reading >>> Luxury Fifth Wheel Camping, Powered by Victron Energy

Victron + Pylontech mobile system with Orion photo by Adventure Van Systems

What about Buck-Boosts?

Buck-Boost DC-DC Converters are basically bigger Orions with the ability to control current. They are programmed differently (with a laptop and a particular program). Because they offer up to 100 Amps of DC-DC charging, we recommend these for use by professional installers only.

In a 100 amp use case scenario, you need a dedicated secondary alternator. It's common practice for us to back the charge current off of 100 Amp Buck Boosts to, say, 80 Amps to prolong the life of the alternator. That said, charging a 12V system at 100 Amps (1,200 Watts) or a 24V system at 50 Amps is an impressive charge range and is favored when systems get large and need to be charged quickly (think emergency response vehicles and semi trucks with big lithium battery banks to power refrigeration units and air conditioners). 



In the last 30 years, we've seen various charging methods, such as FETs and isolated charging systems, that can get the job done at a lower cost than the Orion or Buck-Boosts. Even so, these are not our first choice when caring for battery systems, especially when working with higher efficiency, electronic-controlled alternators like those often found in Mercedes Sprinter Vans.


At the end of the day, we want to protect our investment, and we want reliable power systems that do what they say they're going to do again and again over the life of the vehicle. We've found that Victron power systems—when properly implemented—offer the best chance of achieving that goal.


Victron offers many different charging options. If you've got an application question or need design support for your build, we're happy to help.

Send us your question here.
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