Teardrops and small campers are towed by SUVs and midsize pickups as often as by full-size trucks. On the Ridgeline, Pilot, Outback and Frontier owner boards, the question is the same: will the trailer plug charge my lithium battery? That depends on the vehicle. Some have a fused charge circuit, some have one that only works once a fuse or relay is fitted, and some have none. Even where it works, a lithium battery barely charges from it. The fix is the same in every case: a DC-DC charger with its input current set below the vehicle's fuse.
The charge circuit, and the plug that lacks one
The round 7-way connector carries an auxiliary 12 volt feed, the black wire in the standard color code. That is the circuit meant to top up a trailer battery while you drive.
A 4-pole flat connector carries only four circuits: tail lights (brown), left stop and turn (yellow), right stop and turn (green) and ground (white). It has no 12 volt feed at all. If your vehicle only has a 4-pole flat, nothing reaches the trailer battery.
What each vehicle provides
| Vehicle | What we could verify | Source |
|---|---|---|
| Honda Ridgeline | Fuse 11, "TRL CHARGE": 15 A in the 2017 manual, 20 A in the 2021 and 2024 manuals. The 2024 manual lists it as not on all models. | Honda owner's manuals; etrailer quoting the 2021 manual |
| Honda Pilot | "Trailer Charge" fuse, 20 A, in the 2023 and 2025 manuals, beside a 30 A Trailer Main and a 20 A Trailer Brake fuse. All three are marked not on all models. | Honda owner's manuals |
| Subaru Outback | A 4-pole flat from the factory harness, so no charge circuit. Adding one means a 4-way to 7-way kit and a new feed from the battery. | etrailer, 2018 Outback |
| Toyota 4Runner | Factory 7-way. The 12 volt circuit should be live from the factory. If it is dead, look for a missing fuse or relay under the hood. | etrailer, 2014 4Runner |
| Nissan Frontier | Factory 7-way. A relay in the underhood fuse block and a fuse marked "trailer tow" or "trailer 12V", rated 20 to 40 A, must both be in place. | etrailer, 2016 Frontier |
Ratings change by model year and trim, as the Ridgeline shows. Read the label on your fuse box lid and meter the circuit at the plug, key off and engine running, before you buy anything. An empty fuse position is the most common reason a charge circuit is dead.
Why lithium barely charges from it
A lithium iron phosphate battery rests at around 13.2 to 13.4 volts and wants 14.2 to 14.6 volts to charge fully. A vehicle's alternator holds its own battery at around 14 volts, and many newer vehicles run lower once the starting battery is full.
Then the wire takes its share. As an example, take 25 feet of 12 gauge wire each way, from the fuse box to the trailer battery and back on the ground. That is about 0.08 ohms. At 15 amps it loses about 1.2 volts, so 14 volts at the fuse box becomes about 12.8 volts at the trailer. That is below the lithium battery's own resting voltage. The current falls until the loss is small, and the battery gets a few amps at most.
Two side effects matter more than the slow charge:
- A flat lithium battery pulls hard at first. Lithium has very low internal resistance, so a deeply discharged battery can draw right up to what the wire and fuse allow. On a 15 or 20 amp circuit, that is how fuses blow and plug contacts get hot.
- If the circuit stays live with the key off, the trailer can drain the vehicle's battery overnight. We could not confirm how each of these vehicles switches the circuit, so check it with a meter.
The fix: a DC-DC charger with its input limited
A DC-DC charger near the trailer battery takes whatever the plug delivers, boosts it, and charges the lithium battery with a proper profile. It also isolates the trailer from the vehicle, so nothing flows back.
The Orion XS suits these vehicles because its input current limit is a setting. Victron's manual gives an adjustable range of 1 to 50 amps, set in the VictronConnect app. Set it below the fuse, with margin:
| Fuse on your label | Input limit to set |
|---|---|
| 15 A | 10 A |
| 20 A | 14 to 15 A |
| 30 A | 20 A |
Expect about 12 amps into the battery from a 15 amp input limit. That is our estimate after wire and conversion losses. It takes a 100 amp-hour battery from half to full in roughly four hours of driving.
The Orion-Tr Smart 12/12-30 is rated for 30 amps out, which takes more than 30 amps in. That is too much for a 15 or 20 amp fuse. Save it for a dedicated heavy line.
Use engine detection or the remote input so the charger only runs with the engine running. That protects the vehicle's battery whatever the circuit does with the key off.
Wire it from the charge wire in the trailer's 7-way plug and junction box to the charger's input, through a fuse near the junction. Then run the charger's output to the battery through a fuse sized to the charger.
Outback owners: build a dedicated line instead
An Outback has no charge circuit, so you are running a new wire from the battery anyway. Make it a heavy one. Run a cable from the battery to a 50 amp Anderson-style plug at the hitch, fused at the battery with a MEGA fuse in a holder. Size the fuse to the cable, and size the charger to the alternator rather than to the battery. The same approach works on any of these vehicles if your trailer carries 200 amp-hours or more. Our truck article covers cable sizing for the heavy line.
Which setup for which trailer
- A breakaway battery or a small lead-acid battery: the 7-way as it is.
- A 100 amp-hour lithium on a Ridgeline, Pilot, 4Runner or Frontier with a working circuit: an Orion XS on the 7-way, input limited below the fuse.
- An Outback, or 200 amp-hours or more: a dedicated line, an Anderson plug and an Orion XS.
Whatever you fit, add a SmartShunt to the trailer battery. Lithium voltage does not tell you state of charge. The shunt shows exactly what the vehicle is delivering, in the same app as the Orion.
Related guides
- Charging a lithium trailer from a Ram, Ford or GM truck's 7-pin.
- Charging a lithium camper from an electric truck's 7-pin, for the Lightning, Rivian and Tesla.
- Orion XS versus Orion-Tr Smart.
- 7-pin trailer connector guide.
Send us the vehicle, its model year, the rating on the fuse label and the battery in the trailer, and we will tell you which setup fits. Everything ships from Upland, California, and the counter is open if you would rather collect it.