Every electric-truck forum has the same thread. An owner buys a travel trailer with a lithium battery, plugs into the seven-pin, drives four hours, and arrives with the trailer battery no better than when they left. Then the question: what does this connector actually put out, and why is it not charging?
The answer is different for each truck, and in every case the fix is the same device. Here is what the connectors deliver, from the owners who have measured them and the documentation they quote, and how to set up a DC-DC charger so it works within those limits.
What each connector delivers
| Truck | Charge pin | What owners and documentation report |
|---|---|---|
| Ford F-150 Lightning | Pin 4, black auxiliary wire | Fed through a 30 amp fuse. Live only when the truck detects a trailer with a battery, ignition is on, and the truck is not shedding loads. On some trucks it was off from the factory and had to be enabled with Ford's diagnostic software. |
| Rivian R1T and R1S | Pin 4 | Owners cite a 35 amp fuse on the charge circuit; other documentation shows smaller electronic fuses on the non-brake pins, so the real ceiling is uncertain. The line is live only with the vehicle on and drops out a few minutes after parking. The community's own advice is a DC-DC charger no larger than 20 amps. |
| Tesla Model Y, Model X and Cybertruck | Pin 4 | On vehicles built with the lithium low-voltage battery, from 2022 on, the owner's manual lists pin 4 as "not supplied". There is no charge output at all. |
| Chevrolet Silverado EV | Pin 4 | Owners report the same "no power on the charge pin" question; we could not verify the circuit rating. |
Three things follow from that table.
None of these is a charger. A 30 amp fuse on a long run of factory wire is a maintenance feed, designed to keep a lead-acid trailer battery topped up. It was never meant to recharge a depleted bank.
A lithium bank will pull everything the circuit can give. Lithium has very low internal resistance, so a discharged lithium bank connected by a plain wire to a 12 volt source draws whatever the wire and fuse allow, indefinitely. On the Lightning that is a blown 30 amp fuse or a hot connector. On the Rivian it is the same at 35. That is the failure owners are describing when they say "it charged for a while and then stopped".
On a Tesla there is nothing to connect to. The trailer has to charge itself.
The fix: a DC-DC charger with its input current limited
A DC-DC charger between the seven-pin and the trailer battery solves both problems at once. It presents a controlled load to the truck, never more than the current you set, and it delivers a proper lithium charge profile to the bank instead of raw 12 volts through a fuse.
The Orion XS 12/12-50 is the unit for this because its input current limit is a setting, adjusted in the VictronConnect app. You tell it how much the truck can supply and it never asks for more.
| Truck | Input current limit to set | Why |
|---|---|---|
| F-150 Lightning | 20 to 25 amps | Stays well under the 30 amp fuse on a circuit whose wire length you do not control |
| Rivian | 20 amps | Matches the community recommendation and stays under either of the disputed fuse figures |
| Tesla | Not applicable | Pin 4 is dead; see below |
Set the limit below the fuse, not at it. A fuse carries its rated current only in a cool connector on short wire, and the seven-pin is neither.
The charger's engine detection matters here too. The Orion XS decides whether the truck is "running" from the input voltage, with a smart-alternator mode for vehicles whose 12 volt supply sags and rises. On an electric truck the 12 volt system is fed by a DC-DC converter from the main pack, so it behaves like a very steady alternator when the truck is on and drops when it sleeps. The Orion XS backs off and stops rather than dragging the truck's 12 volt battery down when the feed disappears, which is the behaviour you want on a Rivian that cuts the line a few minutes after you park. Our Orion XS versus Orion-Tr Smart guide lists the detection thresholds.
If 30 amps is all you want, the Orion-Tr Smart 12/12-30 non-isolated does the same job with a fixed lower ceiling. The XS is the better buy because the limit is adjustable and its efficiency is higher, so less of the truck's 30 amps is lost as heat.
Wiring it
Run the charge feed from the trailer-side seven-pin junction to the charger's input, then from the charger's output to the battery through a fuse sized to the charger. The trailer's own seven-way plug and junction box is the starting point if the existing one is corroded, which on a trailer bought used it often is.
Two points that decide whether it works:
- Ground. The charge circuit's return is the white wire through the seven-pin. A poor ground at the plug drops the voltage the charger sees, and it may decide the truck is off. Clean the pin and the trailer-side ground stud.
- Wire gauge on the trailer side. Twenty amps over the length of a trailer frame needs heavier wire than the factory used for a maintenance feed. Our cable and fuse sizing guide has the table.
An Anderson-style connector for the charge circuit, separate from the seven-pin, is the better arrangement if you are rewiring anyway. It keeps the high-current feed off the small trailer plug pins, which is what the off-road camper builders do as standard.
The Tesla case: charge the trailer from itself
With no supply on pin 4, the trailer needs its own charge sources. For a small camper that means:
- Solar on the roof through an MPPT controller, which covers a fridge and lights in most weather.
- Shore power at every stop through the trailer's converter, which should be a lithium-capable one; our converter upgrade guide covers that.
- A portable power pack at the campsite if the trailer has neither.
Some owners run a DC-DC charger from the Tesla's 12 volt accessory socket. Those sockets are fused low and switch off with the car, so the useful current is small and the arrangement is fragile. We would not build around it.
How far the truck's 12 volt system reaches
The reason these trucks limit the feed is that everything on the 12 volt side is produced by the truck's own converter from the traction pack, and the manufacturer decides how much of that to share. It is not wired to an alternator that can be upgraded. Respect the limit, set the charger below it, and use the trailer's solar and shore charging for the bulk of the work. The seven-pin becomes a top-up while driving, which is all it was ever going to be.
Know what you are getting
A SmartShunt on the trailer battery shows exactly what the charger is delivering while you drive, and shares it with the Orion XS in the same app. On a first trip it tells you within an hour whether the feed is live, whether the truck has cut it, and whether your current limit is right. Without it, you find out at the campsite.
Send us the truck, the trailer's battery capacity and where the solar lives, and we will confirm the charger and the current setting. Everything ships from Upland, California.