Trailer Breakaway battery charging

[MikeMl]

I was working on a several-year old car-hauler double-axle flat-trailer (replacing one of the electric brake actuators) when I tested the breakaway battery/switch only to discover that the 12V 5Ah SLA battery was stone dead.

In looking at the standard 7-pin trailer connector, I noticed that pin 4 is supposed to have 12V supplied by the towing vehicle. That is normally used to charge the breakaway battery in the trailer, but the black wire in the trailer harness was missing, so the trailer manufacturer didn't provide a means of charging the battery. No current is ever drawn from the battery unless the breakaway switch closes, but how did they expect a lead-acid battery not to self-discharge after a year or two??

I next checked the 7 pin connector on the towing vehicle, only to discover that pin 4 is apparently not hooked up in the vehicle, either. I did notice that, by default, every time the towing vehicle is started, the tail/running lights (brown wire) pin 3 has about 14.2V on it with the engine running. That is consistent with all of the trailer running lights being on when the towing vehicle is connected to the trailer with engine running.

I added three components into the trailer breakaway battery box as shown below. This should float the breakaway battery at ~14.2V - the forward drop of the Schottky diode, or about 13.7V The 2Ω will prevent from blowing the fuse in the towing vehicle (and limit the initial charging current to the SLA) if the battery gets seriously depleted. The capacitor is so the Schottky doesn't get zapped due to transients.

What have I forgotten?

Should this be posted as a "project"?

 

[KeepItSimpleStupid]

A mechanism to disconnect the battery when the trailer is towed. The simplest would be a connector on the trailer if it could be done that way.

e.g. When connected to the car, the brakes activate.
When the cable is disconnected, the brakes activate.
When the cable is plugged into a connector on the trailer, the brakes are disabled.

You would sort of like it to be automatic to be of any use. A sophisticated mechanism might be a wheel speed switch to charge when moving.

If the towing vehicle has a lamp monitor, then arrange the system to "think" a bulb us burned out if the charging switch isn't thrown on the trailer.

I could be missing something too. It could already be incorporated in the system.

 

[MikeMl]

A mechanism to disconnect the battery when the trailer is towed. ...

The industry does it this way: The breakaway switch is mounted on the tongue of the trailer. The switch is normally-open. It is held open by a thing that looks like a plastic clothes-pin. If the clothes-pin is pulled out of the switch body (takes a bit of force), the contacts close, immediately locking the trailer brakes. To rearm the switch, you have to push the clothes-pin back into the body of the switch.

The clothes-pin is attached to a small-diameter (0.1") steel cable. The other end of the steel cable is clipped to the rear-bumper of the towing vehicle, independant of the normal trailer coupling, the safety chains, or the electrical connection between the trailer and the towing vehicle.

If the trailer were to become uncoupled from the towing vehicle at speed, and the safety chains are not able to keep the trailer hooked to the towing vehicle, then the electrical connection would be severed (leaving nothing to power the electric brakes if there is no battery in the trailer). As the clothes-pin is pulled out of the breakaway switch body by the cable presumably still attached to the towing vehicle, that is supposed to lock the brakes on the trailer to stop it.

At that moment, the source of the 12V required to apply the brakes (about 2A per wheel) must come from the trailer-mounted 12V 5Ah SLA battery. Note that supplying these type of electric wheel brakes with a steady 12V will lock up the wheel so that they skid. During normal towing, the brake controller in the towing vehicle modulates the braking by varying the Pulse Width of the braking current. The braking controller is actually quite sopisticated, and bases the PWM to the trailer brakes on the rate of de-acceleration of the towing vehicle (using a MEMS accelerometer).

 

[shokjok]

Whether or not the chains can keep the trailer from disconnection is irrelevant. The breakaway switch activates the electric brakes, though I question the effectiveness of a 0.1" steel cable if the connection is insecure. Traffic laws are strict on trailers and towed vehicles, roadside tests can be conducted anytime by law enforcement ( in my area it's called STEP - Selective Traffic Enforcement Program). Most captives are big rigs or RV-towed trailers or vehicles. No minor detail is too passive for safety's sake on our roads.
My licensed bike trailer has only one chain connected to the bike seatpost below the crossbar by a U-clamp. There is no breakaway switch, it drags on the rear tire if the towblock breaks. Missing from this trailer is a highway triangle.