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"?
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"?
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