Dug,
Lots of good ideas, but nobody has answered your questions yet. I'll forget alternatives to your plan and just answer your questions for you. See below..
Now that I have LED tail lights and turn signals coming cheap, I think a head light upgrade might be a good idea.
I looked into 24 volts LEDs and I don't want to spend that kind of money.
I'm thinking about going 12 volts. I think the selection and prices are better on the 12 volt side. Any suggestions for bulbs that throw more light yet don't break the bank?
I don't want to hack my wiring so I think I would unplug the drivers side and tape it/zip tie it out of the way. Then unplug the passenger side and use it to control two relays.
Although not a question I do have a comment; This sounds like a good plan to me. Very easy to convert back to stock should you (Or the next owner) ever see a need to do so.
Using two 24 volt relays with the proper military connectors added to mate to my disconnect harness I would run the low beam wire to one relay and then high beam wire to another. When energized they would let the 12 volts on through to a custom wiring harness I'll make just for the head lights.
You can get headilght sockets from most any auto parts store for under $5.00 each.
If I have a 12 volt failure I could simply reinstall the original headlights since I didn't hack anything up.
Would the 25 amp x 2 relay I used to turn my 12 volt system on and off be heavy duty enough?
Those realys are as the spec sheet states, one of the most popular in the automotive industry. Yes, they are plenty for your needs.
What gauge wire will I need to run from the 12 volt battery to the relays and head lights?
That will depend on 3 things. The voltage being used, the amperage drawn by the load (your lights) and the length of the wire. Here is a link to a chart for both 12v and 24v use. It is very easy to use. The left column is the amperage your load will use, the top line is the gauge of wire to use. The center of the chart is the length of the wire you will need. One key thing to remember, if you ever have any question, the larger size the better. Other than costing more (Not really an issue in the smaller sizes) and being harder to work with (Again, not really an issue in the smaller sizes) there really are no problems using a wire that is larger than recommended.
CHART:
Wiring Size Guide
To calculate the amperage your lights will draw you use this simple formula: Watts divided by voltage. I believe most headlights are 55w on high beam. 55 divided by 12 = 4.58 amps. Your running 2 lights so 4.58x2=9.16 Round that UP to 10 amps for the circuit.
On the chart 10 amps up to 11 feet total you use 14 gauge wire. You wanna be extra safe/sure and insure your going to get absolutely the most out of your lights you possibly can? move up to the next larger size (12 gauge) no need to go any larger than that but if you have some 10 gauge in your shed that is not doing anything and no 14, there is no problem using it.
What size in line fuse should I use?
That is determined by the maximum safe capacity of the wire you have used which of course was determined by your math of the load. Your usually safe adding 15% to the load number to determine your fuse size. You don't want the fuse the same size as your calculated load because it will likely not last long and you'll be replacing them all the time. (Think of rope; working load and breaking load, you need your wires to work, but when it breaks you have a fuse). Your safe going SLIGHTLY larger. So, your 10 amp load on 14 gauge wire (or larger) would enjoy being protected by a 11.5 amp fuse. Good luck finding that. Use a 15 amp and your good to go for 100 years or a short/overload of some kind.
Not sure if I'll actually do it, since I'm not positive I want to "count" on the 12 volt system for anything important. I guess if I found a safe place to stash my old lights and was willing to reinstall them to get home if needed that could work.
Thoughts/suggestions?
Suggestions.... Hum...
I do see some advantages of changing to the 12v system. Obviously they do work and work well but even more importantly, you can get them literally anyplace. I have even seen them at the local 7-11. I can not recall seeing a 24v headlamp at the local Auto Boys, Pep Zone or -Mart store.
As for the wiring harness; I have taken several "older" cars/trucks (Civilian 12v) that had functional headlights and made a new harness for the headlights. First how, then why.
How: Just like you I did/do not want to cut up the factory wiring so I used the factory light socket for one of the lights to act as the trigger for my relays, 2 of them, just like you thought about using. I run a 10 gauge wire directly to the "+" battery terminal to a 15 amp fuse within 10" of the battery. (You can get very nice in line covered "Water RESISTANT" fuse holders for about $5 at most auto supply stores). I use the 10 gauge wire to feed both relays and from there both the high and low beams on both sides and I run a ground directly back to the battery, (No fuse). Mount both the relays to the fender/firewall/grill. As you noted, use the factory socket for the trigger for the relays. Be sure to put your wires in split loom for a professional look and to protect the wires from any damage and of course, be careful in your routing to avoid any moving or hot parts.
An added bonus bit of info: They make a socket for the relays your using but the sockets are usually difficult to find or are special order. Some NAPA stores keep them in stock. I HIGHLY recommend using sockets. It makes replacing a bad relay 100% easier, gives a cleaner looking install and it is safer since the socket will insulate the individual relay terminals from each other, the fender and anything that may hit/drop or otherwise contact the terminals. If you have trouble finding the sockets you can use normal (old style 3 blade) headlight sockets! They will fit on 3 of your terminals and you only have 1 remaining wire that will use a normal "Spade lug crimp terminal". Not quite as clean looking as the correct socket but still very usable and since it will only go on 1 way, it will eliminate any confusion as to which wire goes to which terminal if you have to replace the relay later.
WHY: frequently on older vehicles the wiring and connectors are ... old. Often times it is also undersized, especially if you upgrade to new brighter lights. By using the stock wiring to control the relays and going directly to the battery for both power and ground AND using what is actually an over sized wire (the 10 gauge I use) your guaranteeing that your going to get 100% of whatever your headlights have to offer.
After doing this modification I have seen marked improvement in the amount of light provided by the same lights. Change the lights to a modern reflector and lens design and halogen filament and your all but guaranteed to get more light.
One last thing. Those relays are not sealed so if you mount them in an area that is going to get wet, the insides will get wet. Be sure to mount them as close to vertical as you can to allow any water to drain out. Don't forget about steam.. You mount the relay under the hood and drive into water, your going to get some steam under the hood. This can get into the relay. Make sure that once the steam inside the relay cools and turns back into water it can drain/run out.
These relays are inexpensive. Once you start using them you'll find more and more uses for them. Buy an extra and keep it in your box. Sooner or later you will need it.
I hope this helps some. Let us know what you decide to do in the end.
