Will my trailer's LiFePO4 battery push current back into my Ridgeline through the 7-pin?

The 7-pin's black auxiliary wire is just a wire. Current flows from whichever end has the higher voltage. A charged LiFePO4 battery rests around 13.2 to 13.4 volts, higher than a parked truck's lead-acid battery. So if the Ridgeline's charge circuit is live while the truck is off, the trailer can flow back toward the truck, and you are relying on the truck's fuse and wiring to sort it out.

Check your truck first

The Ridgeline's trailer charge feed is fuse 11, labelled TRL CHARGE: 15 amps in the 2017 manual and 20 amps in the 2021 and 2024 manuals, and the 2024 manual lists it as not on all models. An empty fuse position is the most common reason the circuit is dead.

With a meter at the trailer plug, check the charge pin three ways: engine running, key on, and key off for a few minutes. If it is live key-off, a plain wire lets the two batteries talk both ways. We could not confirm how each model year switches the circuit, so measure rather than assume.

Why the plain wire is a poor setup anyway

Even with the engine running, a lithium battery barely charges through a long 7-pin run. As an example, 25 feet of 12 gauge each way is about 0.08 ohms; at 15 amps that loses about 1.2 volts, so roughly 14 volts at the fuse box becomes about 12.8 at the trailer, below the lithium's own resting voltage. And a deeply discharged lithium battery can pull right up to what the fuse allows, which is how fuses blow and plug contacts get hot.

The fix: a DC-DC charger at the trailer

A DC-DC charger near the trailer battery takes what the plug delivers, boosts it to a proper lithium profile, and isolates the trailer from the truck, so nothing flows back. The Orion XS suits the Ridgeline because its input current limit is a setting: 10 amps on a 15 amp fuse, 14 to 15 on a 20 amp fuse. Expect about 12 amps into the battery from a 15 amp limit, our estimate after losses.

Use its engine detection or remote input so it only runs with the engine on. Wire it from the charge wire at the trailer's 7-way plug and junction box, fused near the junction. A SmartShunt on the trailer battery then shows exactly what the truck is delivering, in the same app.