Roadmaster Towed-Car Battery Charge Lines: 156-25 vs 156-75

Some cars use electrical power while being flat towed, even with the ignition in the specified towing position. A Roadmaster charge line provides a connection from the motorhome’s charging system to the towed vehicle’s battery so its charge can be maintained during a trip. Roadmaster sells two separate kits: 156-75 for the motorhome and 156-25 for the towed car.

Which side needs which kit?

156-75: Adds the charge circuit to the motorhome’s towing connector when the RV does not already have one. Many newer motorhomes have a factory charge line. If yours does, confirm it works rather than automatically buying another motorhome kit.

156-25: Connects the towed vehicle’s front towing socket to its battery through the supplied thermal breaker. It needs an operating charge circuit on the motorhome side; installing 156-25 alone cannot supply power if the motorhome socket has no charging feed.

Follow the charging connection

See the two charge-line kits

156-75 — Motorhome charge-line kit

For the motorhome / towing vehicle; includes 12 AWG wiring and a thermal circuit breaker.

View Roadmaster 156-75

156-25 — Towed-vehicle charge-line kit

For the car being flat towed; includes 14 AWG wiring and a thermal circuit breaker.

View Roadmaster 156-25

Manufacturer wiring instructions and connection drawings

Roadmaster’s 156-75 motorhome installation drawing (PDF) identifies the motorhome socket’s charge-line connection and breaker location. The separate 156-25 towed-car wiring diagram (PDF) shows the front socket, 14-gauge lead and connection to the towed vehicle battery. Both use an automatically resetting thermal circuit breaker and specify mounting it close to the applicable battery, within approximately 8 inches.

The installation requires matching front and rear sockets and a six-conductor or compatible six-to-seven-conductor towing cord. A four-wire lighting-only cord does not provide the extra charge-line connection specified by these kits. Roadmaster’s diagrams identify the correct pins for each socket; confirm your actual connector wiring rather than relying on wire colour alone.

Before ordering or installing

1. Consult the towed car manufacturer’s flat-towing procedure. Some vehicles call for battery disconnects or other specific electrical steps, which must still be followed.
2. Check whether the motorhome already provides a working, protected charging feed at its towing socket. Buy 156-75 only if that side needs the circuit.
3. Check the towed car’s front socket and battery connection. 156-25 supplies that vehicle-side wiring.
4. Confirm both connector ends and the intervehicle power cord support a separate charge circuit, then follow the respective Roadmaster installation sheets.
5. After installation, use a voltmeter to verify charging voltage with the motorhome operating. Secure the harness away from fuel lines, hot parts, sharp edges and moving equipment.

A charge line is intended to maintain a battery during towing, not replace a defective battery or instantly recharge a deeply discharged one. The 15-amp capacity is the maximum Roadmaster specifies for these kits; actual current depends on the vehicle and electrical system.

For the remaining flat-tow electrical system, see our Roadmaster taillight wiring guide and supplemental braking guide.