On every truck board, the most-asked towing question is the same one: does the 7-pin charge my trailer battery? The short answer is "a little, and less than you think", and with a lithium battery in the trailer it is "barely". Here is why, what your truck actually puts on that circuit, and the two ways to charge a lithium trailer properly while you drive.
What the 7-pin charge circuit is
One of the seven circuits in the round trailer connector is the auxiliary 12 volt feed, the black wire in the standard color code. It carries battery voltage from the truck through a fuse, on a wire that is usually 10 or 12 gauge, over twenty feet or more to the bumper and then several more feet to the trailer battery. It was designed to keep a breakaway battery or a lead-acid house battery topped up and to run a trailer fridge on 12 volts while driving.
Two things decide what it can do: the fuse that protects it, and the voltage it loses on the way.
What Ram, Ford and GM put on that circuit
| Truck | Charge-circuit fuse | How it switches | Notes |
|---|---|---|---|
| Ram 2500 and 3500 | 30 A; on 2015 and 2016 trucks, F30 in the underhood power distribution center | Owners report it stays live with the engine off, fed from the battery | A relay on an ignition feed makes it switched |
| Ford Super Duty, without factory brake controller | 30 A "Trailer Tow Battery Charge" maxi fuse, plus a relay | Through the relay | Fuse and relay positions vary by year |
| Ford Super Duty, with factory brake controller (2017 on) | Handled by the trailer module | Charging is enabled only after the module detects a trailer and a brake-pedal input | The separate relay and fuse are not needed |
| Chevrolet Silverado and GMC Sierra HD | 40 A trailer battery fuse | Varies by year | On some 2020 to 2022 trucks the 40 A maxi fuse has to be installed before the circuit is live |
Model years differ, and a dealer or a previous owner may have changed things. Read the label in your fuse block and put a meter on the circuit, engine off and engine running, before you buy anything. A dead circuit is often an empty fuse position, not a fault.
Why lithium barely charges from it
A lithium iron phosphate battery sits at around 13.2 to 13.4 volts for most of its range and wants 14.2 to 14.6 volts to charge fully. The truck's alternator holds its own battery at around 14 volts, and newer smart alternators drop lower once the truck battery is full.
Now add the wire. Around 17 meters of 10 gauge, out on the charge wire and back on the ground, has a resistance of roughly 0.06 ohms. At 20 amps that loses about 1.1 volts, so 14 volts at the truck becomes about 12.9 volts at the trailer, which is below the lithium battery's own voltage. The current falls until the loss is small, and the result is a few amps at most. The circuit is not broken; it is limited by physics.
Two side effects matter more than the slow charge:
- On a truck where the circuit stays live with the engine off, a flat lithium battery can pull the truck's battery down overnight through that fuse.
- Owners on GM boards report trailer brake module faults on 2023 to 2025 trucks when the trailer carries solar or lithium that feeds voltage back up the 7-pin. Whatever the exact cause on a given truck, a charger that isolates the two systems removes it.
Fix one: a DC-DC charger in the trailer, fed from the 7-pin
A DC-DC charger mounted near the trailer battery takes whatever the 7-pin delivers, boosts it, and charges the lithium battery at its proper voltage with a proper profile. It also isolates the trailer from the truck, so nothing flows back and nothing drains the truck with the engine off.
- Set the input current limit below the truck's fuse, with margin: about 20 amps on a 30 amp circuit, 25 to 30 amps on a 40 amp one. The Orion XS lets you set that limit in the app; the 30 amp Orion-Tr Smart non-isolated suits a smaller battery.
- Use engine detection or the remote input so the charger only runs when the truck is running. On a Ram whose circuit stays live, that is the setting that protects the truck's battery.
- Expect roughly 15 to 17 amps into the battery on a 20 amp input limit. That takes a 100 amp hour battery from half to full in about three hours of driving.
Fix two: a dedicated heavy line and an Anderson plug
For a 200 to 400 amp-hour bank, or long drives between camps, the 7-pin is simply too small. Run a heavy cable from the truck battery to an Anderson connector at the bumper, fused at the truck battery, and plug the trailer's DC-DC charger into it.
- Fuse it at both ends: a MEGA fuse in a holder at the truck battery, and a fuse at the trailer end. At a lithium bank, the main battery fuse needs an interrupt rating to match, which is Class T territory; our van fuse sizing guide covers the rule and the Lynx Class-T Power In that mounts it.
- Size the cable for the run. A full-size truck's bumper line is often 10 to 12 meters out and back. At 50 amps over 12 meters, 4 gauge loses about half a volt, which a DC-DC charger tolerates easily; 6 gauge is fine at 30 amps. Our cable and fuse sizing chart has the full table.
- Pick a connector rated above the charger's current. Our 50 amp Anderson-style plug suits a 30 amp charger with margin.
- Size the charger to the alternator, not just the battery. A rule of thumb on the boards is no more than about a third of the alternator's rating; read yours off the label.
Which setup for which trailer
- Breakaway battery or a small lead-acid battery: the 7-pin as it is.
- A 100 amp-hour lithium, weekend trips: a DC-DC charger on the 7-pin, input limited to 20 amps.
- 200 amp-hours or more, long drives: the heavy line, an Anderson plug and a 50 amp Orion XS.
- A GM truck with brake-module faults when towing a lithium or solar trailer: a DC-DC charger regardless of battery size, for the isolation.
Whatever you fit, put a SmartShunt on the trailer battery. Lithium voltage does not tell you state of charge, and the shunt shows exactly what the truck is delivering, in the same app as the Orion.
Related guides
- Charging a lithium camper from an electric truck's 7-pin, the same question for the F-150 Lightning, Rivian and Tesla.
- Orion XS versus Orion-Tr Smart.
- Victron Orion versus REDARC BCDC from the tow-vehicle side.
- Brake controller and 7-pin troubleshooting.
Send us the truck, its model year, the fuse rating on the label and the battery in the trailer, and we will tell you which of the two setups fits. Everything ships from Upland, California, and the counter is open if you would rather collect it.