Here's a quick question for the techs here... I have a couple trailers that have their own 12 battery on board, to operate onboard hydraulics. Those batteries are wired in a way so that the trailer plug supplies +12v power to keep them charged. It simply connects the 12V aux power pin on the truck directly to the battery positive terminal. The wire is 12 or 10 guage. The problem is when the battery is a little discharged, plugging it into the truck you get enough inrush to melt wire insulation, etc etc Most vehicles have a 30 amp fuses on that circuit. Sometimes you blow that fuse, sometimes you just melt wire. I could put a fuse inline on the trailer. I would prefer there was some sort of current limiting regulator device I could put on that circuit to protect the wire. Is there such a thing? My 12V knowledge is weak. Ken
The easy way would be a 20 amp self resetting circuit breaker instead of the fuse.
The old way would be a wire wound resistor in series with the power lead,ohms law will enable you to calculate the value and wattage required to limit the current.That is not very efficient by todays standards as the resistor dissapates energy in the form of heat,they do it these days with solid state junctions.Log on to NTE Electronics,they supply the aftermarket with just about anything that you would need to repair consumer electronics and have a very thorough catalog online.You can even download their cross reference to install on your computer if you like.Ron G
Resistor in series will cause difficulty charging under normal conditions. I agree with the self resetting CB although 30 amp is a better value than 20. 12 gauge is good for 30 amps in car wiring. The breaker will allow some charging to take place before it trips, then once it cools off it will cut back in and allow some more charging, etc. without letting anything get too hot until inrush current drops to such a level that the breaker can stay in indefinitely. This has been my go-to solution for many years.
For one thing, you might want to install an isolator if you don't have one in the truck. If your Aux lead is coming off the truck batteries you have almost unlimited amperage there, the isolator will only feed alternator output through and the regulator in the alternator should help limit current since the truck batteries will usually be near full charge all the time. Of course I may be all wet too, lots of variables.
sounds like something else going on to me.. bad battery.. hydraulics not shut down.. bad ground... something... there are many thousands of RV's running around with 12 and 10 gauge charge wiring (and smaller) with out burning it up.. .. what portion of wire is melting down.. truck charge wire... or tow cable wire.. or both.. does it have a dedicated ground wire of same or larger size as charge wire.. or is it trying to use the hitch to ground? (never a good idea) I also would recommend 30 amp auto reset circuit breaker rather than a fuse.. Also I would check (and clean) all ground wires where ever they go. If it is trailer frame grounded that might be your fix right there... also check to see if the tow rig plug is frame grounded usually just at the tow plug.. don't look at it.. clean it up.. lots of these type problems are ground related. Good luck..
All good advise.My next suggestion was to be the battery isolater to charge one battery at a time.Bad grounds can cause all sorts of problems that do not respond to reason when troubleshooting,Ron G
Lots of good ideas here. I don't know if the truck wiring is having any issue. All I know is on a standard 7 blade trailer plug in the bumper of most any truck there is a +12v pin that is always on with the ignition. On two trailers that pin is wired straight to the positive terminal of a 12v car battery on board the trailer. There is a hydraulic power pack wired with heavy cable the car battery as well. Often the 12 gauge wire is found with melted insulation. I can't say if the trailer is backfeeding during truck engine start, or if there is a surge during power pack operation, or if it is simply inrush during battery charging. The 30 breaker sounds like a low cost idea. Ken
Lantraxo's thoughts about the battery isolator are spot on to do with current from the alternator and will prevent discharging the truck battery in the event of a power/fault/draw from the trailer.. however the alternator may still supply enough amps to melt some wire. As for the self resetting breaker... it will work to protect the wire... however if running the trailer battery down is a common occurrence in your operation you may find the self resetting breaker to wear out soon as they are not designed for the continuous resetting that will occur when the battery is drained enough I use these DC to DC modules for lots of other projects ---> http://www.ebay.com/itm/361265316970 not sure how they would work in this application but I may try one in a dodge p/u we use that has it's 12v trailer circuit burned out at the moment this particular module is only good for ~8 amps but I know there are bigger ones out there.
That looks just about perfect ^^^
I agree that a self resetting breaker might give out eventually. They are cheap and easy to replace. I do not like semiconductor (diode) battery isolators that require you to tap into the alternator wiring. They have a forward voltage drop of 0.7 volts or more. This can be an issue depending on circumstances. Additionally many alternators these days are 200 amps plus, and that is a lot to put through a diode. I like the continuous duty relays that look like a starter solenoid with a shiny case. The thing on ebay I cannot comment on but I would not sell one to a customer until I had proven it very well first.
That thing on Ebay has a published forward voltage drop of 1.5 volts. Me personally I would prefer the diode isolator, I weld every day with machines that pump 300 amps or more at 28 volts through diodes and they rarely fail....... I do agree though, I don't like butchering harnesses to install an isolator, but then I've always run Dodges and it always seemed straightforward on the trucks I've owned.
Some of the modules I have used are about 0.5 volt drop in to out even though they are rated at 1.5 volt drop... it will vary a bit on what kind of current you are drawing, like I said I haven't tried it in this application but probably will soon another thing you could do if it did have too much drop to get enough charge voltage up is wire in another buck converter in front of it and step it up to 16+ volts... kind of a hack but it will work.