Trailer Breakaway Systems: The Battery Nobody Tests

A breakaway system is three parts: a battery on the trailer, a switch, and a thin cable that connects the switch to the tow vehicle. If the trailer separates, the cable pulls a pin out of the switch, the switch closes, and the battery applies the trailer's electric brakes.

And the battery is the part that fails silently. A breakaway battery that has quietly died looks identical to one that works, right up to the moment it is needed and does nothing.

What it is for, precisely

If a braked trailer comes off the hitch, everything that would normally stop it is gone. The brake controller in the vehicle has no connection. The vehicle's brakes are not attached to it. A loose braked trailer would otherwise roll free, and it would be heavy.

The breakaway system exists to bring that trailer to a stop on its own.

It is not the safety chains. The chains are meant to keep the trailer attached to the vehicle if the coupler releases, and to hold the tongue off the road. The breakaway system is what happens if the separation is complete.

And it is not an anti-theft device, although some people treat it as one. It is a brake of last resort.

The three parts, and what goes wrong with each

The battery

A small sealed battery mounted on the trailer, usually in a box near the tongue.

This is the failure point. It is outdoors, it sits through summers and winters, it is discharged and recharged by the vehicle's charge line whenever that line is working, and it is out of sight.

Three ways it fails:

• It ages out. Batteries have a service life whether they are used or not.
• The charge line never charged it. The seven-pin's auxiliary circuit is what keeps it topped up, and if that circuit is not connected on the vehicle, or the fuse has gone, the battery has been running down since it was fitted.
• The terminals corrode. Outdoors, in salt, unprotected.

And none of those announces itself.

The switch

A small plastic body with a pin in it, mounted near the tongue.

What goes wrong: the pin corrodes in place, the switch fills with water, or the body is cracked from having been pulled once and never replaced. A switch that has been pulled and reinserted should be inspected, because they are not all designed to be reused indefinitely.

The cable

The thin cable from the pin to the tow vehicle.

Where it is attached is the part people get wrong, and it matters enormously.

The cable attaches to the tow vehicle, to a solid point on the vehicle or the hitch receiver, and it must not be attached to the safety chains.

Here is why. If the trailer separates and the chains hold, the chains keep the trailer attached and the breakaway does not need to trigger. If the chains also fail, the trailer departs and the cable pulls the pin. A cable clipped to a chain goes with the chain, and the pin never comes out. The failure the system exists for is exactly the failure that renders it inoperative if you clip it to the chain.

It also must not be so short that it pulls the pin on a tight turn, and not so long that it drags on the road. Enough slack to turn fully, and no more.

Testing it, which takes two minutes

Do this before every trip, or at the very least before every long one. Almost nobody does.

The simple check, with the trailer stationary and chocked:

1. Chock the trailer wheels. Every part of this test involves the trailer's brakes and you are about to apply them.

2. Disconnect the trailer from the tow vehicle's electrical connector, so the vehicle is not powering anything.

3. Pull the breakaway pin.

4. Try to move the trailer, or have somebody attempt to turn a wheel by hand where that is possible. The brakes should be applied. On many trailers you will hear the magnets engage.

5. Put the pin back in immediately. Leaving it out drains the battery, which is the exact failure you are testing for.

If the brakes do not apply, the system is not working and the trailer should not be towed until it is fixed. That is the whole point of the test.

Some breakaway units include a test button or a monitor. Use it if you have one, and understand what it is testing: some indicate battery voltage rather than confirming that the brakes actually engage. Voltage present is not the same as brakes applied.

And how long the system has to hold the brakes for is set by the applicable rule and by the equipment maker. We are not printing a duration. If yours holds for a moment and fades, the battery is not doing its job.

Maintenance, which is short

Check the battery is being charged. If your vehicle's seven-pin auxiliary circuit is not live, the battery has been discharging. Check the vehicle's fuse for that circuit.

Replace the battery on a schedule rather than on failure, since failure is invisible. The battery maker publishes a service life.

Keep the box closed and the terminals protected. Dielectric grease on the terminals, per the product's own instructions.

Inspect the cable for fraying and the clip for corrosion, at the same time you check the safety chains.

Replace a switch that has been triggered, or at minimum inspect it and confirm it still holds and releases correctly.

Whether you are required to have one

In most US states a trailer that is required to have brakes is also required to have a breakaway system. The thresholds, the definitions and the specific requirements are state level, they differ, and they are amended.

We are naming no state and quoting no statute. Your state's DMV or department of transportation publishes it, and if you cross state lines, every state you drive through applies its own.

Whether your trailer needs brakes at all is the decision that usually determines this one, and it has the same answer: your state publishes the number.

The other things at the coupler

While you are at the tongue, three items belong to the same pre-trip check.

Safety chains. Crossed under the tongue so they cradle it if the coupler releases, attached to the vehicle at rated points, with enough slack to turn and not enough to drag.

The coupler latch and its pin or lock. A latch that is closed but not pinned can open.

The jack, fully retracted and secured.

Together with the breakaway test, that is a four-item check that takes three minutes and covers the failures that end worst.

Where you stop

• Chock the wheels before any breakaway test. You are applying the trailer's brakes deliberately.
• Do not leave the pin out. It drains the battery.
• Do not tow with a breakaway system you have not confirmed works, and do not tow with a dead battery on the assumption the chains will hold.
• Never clip the breakaway cable to the safety chains.
• Trailer brake work at the axle, magnets, drums, shoes, bearings and wiring, is a trailer shop job. If the test fails and the battery is good, the problem is downstream and it is not a roadside fix.
• Battery handling: sealed batteries still carry current and can be damaged. Follow the maker's instructions, and do not short the terminals.

Before any of this, the calculation that establishes what your combination can carry comes first, because a rig within its numbers is a rig whose brakes were specified for the load.

The short version

A breakaway system is a battery, a switch and a cable, and it applies the trailer's brakes if the trailer comes completely off. The battery fails silently, so test it: chock the wheels, unplug the trailer, pull the pin, confirm the brakes apply, put the pin straight back. Attach the cable to the vehicle or the receiver and never to the safety chains, because a cable clipped to a chain leaves with the chain and never pulls the pin. Whether you are required to have one is your state's rule, and every state you drive through has its own.