Charging a LiFePO4 House Battery from an Outboard

One of the most common questions I receive is:

"How do I charge my new LiFePO4 house battery from my outboard?"

Fortunately, the answer is pretty straightforward.

For the vast majority of boats, the best solution is to install a DC-to-DC charger between the start battery and the new LiFePO4 house battery. This allows the outboard's charging system to safely and properly charge the house bank while the engine is running.

****Please note: This is just one piece of the puzzle. Converting your house battery to LiFePO4 involves more than simply replacing the battery or adding a DC-to-DC charger. You'll also need to evaluate your battery switching strategy, all available charging sources (shore charger, solar, alternator, etc.), and the electrical demands (loads) that will be placed on the new LiFePO4 battery. Taking a holistic approach will ensure your system is safe, reliable, and performs as intended.

Why can't I keep my existing charging relay?

Let's start with how most outboard boats are wired from the factory.

A typical setup consists of:

  • One lead-acid starting battery
  • One lead-acid house battery

(For this article, "lead-acid" includes flooded, AGM, and Gel batteries. In reality, most boats are running flooded or AGM batteries.)

To keep the house battery charged while underway, manufacturers commonly install an Automatic Charging Relay (ACR) or Voltage Sensitive Relay (VSR). Some of the most common examples are the Blue Sea ACR and BEP VSR.

These relays work great when both batteries are lead-acid.

What changes when you install a LiFePO4 battery?

Here's where things change.

LiFePO4 batteries have a much higher resting voltage than lead-acid batteries.

A charging relay simply monitors battery voltage. Once it sees a voltage above its closing threshold, it connects both batteries together.

With a LiFePO4 battery installed, that higher resting voltage can keep the relay closed even after the engine has been shut off.

Instead of disconnecting like it should, the relay can continue tying the batteries together. The result is that the LiFePO4 house battery begins back feeding the lead-acid starting battery until both batteries eventually settle at roughly the same voltage.

That's obviously not what you want.

For that reason, the charging relay should be removed from the charging circuit and replaced with a DC-to-DC charger.

Why use a DC-to-DC charger?

A DC-to-DC charger solves several problems at once.

It:

  • Properly charges the LiFePO4 battery using the correct charging profile.
  • Isolates the two battery banks when charging is not taking place.
  • Prevents the house battery from discharging into the start battery.
  • Allows you to control charge current, helping protect your outboard's alternator.
  • Can be configured to only operate when the engine is running.

For most outboard applications, this is by far the cleanest and most reliable solution.

Why I recommend the Victron Orion XS

I'll admit I'm a little biased since I'm a Victron dealer, but I was recommending Victron products long before I became a reseller.

The Orion XS has become my go-to DC-to-DC charger because it offers:

  • Excellent build quality
  • Fully programmable charging profiles
  • Adjustable current limiting
  • Bluetooth programming and monitoring
  • Engine run detection or ignition-controlled operation
  • A 5-year warranty
  • Outstanding reliability

Simply put, it checks all the boxes.

Installation

The good news is that installing an Orion XS is usually pretty straightforward.

If your boat already has an ACR installed, there's a good chance you can reuse portions of the existing wiring, including the battery cables and fuse protection. Whether that's possible will depend on how the ACR was originally installed and whether the cable size and fuse ratings are appropriate for the new DC-to-DC charger.

Every boat is a little different, but most installations are much less intimidating than they first appear.

Wiring Diagram

Words are great, but sometimes a picture explains everything.

Below is a typical wiring diagram showing how I recommend installing a Victron Orion XS to charge a LiFePO4 house battery from an outboard.

Need Help?

Every boat is a little different. Battery sizes, alternator output, electrical loads, and charging requirements all play a role in selecting the right components.

If you're planning a LiFePO4 conversion and aren't sure what size battery or DC-to-DC charger you need, feel free to reach out. As an ABYC-certified marine electrician and Victron/Epoch dealer, I'm always happy to answer questions and help design a system that's safe, reliable, and built for your boat.  Contact Us – DIRECT CURRENT BOATWORKS

We also offer complete Orion XS installation packages to make your conversion even easier. Our packages include the Victron Orion XS, custom-made marine battery cables, and properly sized fuses, so you don't have to spend time sourcing compatible components or figuring out the correct cable and fuse sizing. Reach out to discuss your boat and electrical system, and we'll help put together a package tailored to your application. Contact Us – DIRECT CURRENT BOATWORKS

 

 

****Disclaimer: The information in this article is intended as a general guide based on common marine electrical practices. Every boat is different, and there is no one-size-fits-all solution. Before modifying your electrical system, verify that the wiring, overcurrent protection, charging equipment, and battery configuration are appropriate for your specific application. If you're unsure, don't hesitate to consult a qualified marine electrician.