Two relays are preferred because of how the bulb works...both high and low beam will be on when you are on high. The relays will allow the current through the switch to be lower so you don't need to clean the switch as often and you wont have any excess wear from higher voltage.
Take a 14-16 ga wire with 20A fuse near battery to the headlight bucket.
Terminal 30 is the relay's power input...this connects to your new wire from the battery positive terminal.
Terminal 87 is your relay's power output...this connects to your headlamp's respective high beam and low beam, using two relays...one of the relays has terminal 87 powering the high beam and one relay has terminal 87 powering the low beam.
Alternative connection for Terminal 30 is from the voltage regulator Positive output... With our bikes the connection is rather short to the battery and it can be easier to tap into the battery directly or at the fuse box... What ever you choose to get the positive from you are going to want to fuse it immediately after picking up that connection to your power source.
Take this time to find and clean every ground connection on the bike and put some dielectric grease over the connection after you have cleaned it to bare metal and removed any trace of oxidation that is seen. Brass brush works well for this, battery disconnected of course. If you have rust, then you may need something a bit more aggressive. A coating of dielectric grease keeps the air out so you block oxidation of that connection.
Terminal 86 is your relay switch input, this is from your original headlamp wiring, if you want to have it return to stock one day (don't know why you would...) then connect a blade quick disconnect into the high or low beam feed wire that goes on the socket on the back of the headlamp. These drive your relay's switch...
The output of the relay's switch is terminal 85 and that returns to ground. Choose to use either the closest ground or use the negative terminal feed from the headlamp...but I really advise against using that wire...you need an equally large return to ground as the gauge wiring used for the feed side of the power (+ terminal).
Connect the new Positive feed line from your voltage source to terminal 30 of both relays with that 20A fuse in the wiring right after you pick up the new positive voltage run. On cars you pull a lot more current and I would run high powered bulbs and use 10 ga wire, really overkill, or 12 ga at the minimum...depending on the wattage I intend to operate at and I would use 30A relays. A good relay is worth the money. Since you don't want a relay failure in a curve or other area at speed on a dark night it is worth spending more than the $2.50 for a cheap relay. I favor the Hella relays, they are reliable and easily found. They have more expensive ceramic base relay designs that are designed for high current systems. We aren't dealing with those currents on our bikes. A 20A relay more than handles our lighting needs.
Having a larger than needed wire doesn't hurt, having too small a wire leads to voltage drops which you are trying to fix/avoid.
Run a 14-16 ga wire from terminal 87 of both relays to the terminals on the socket for the H4... as mentioned, one from 87 to low beam and the other relay output terminal 87 to the high beam.
The ground is important as I mentioned...
You do need to clean your grounds to spotless conditions and tie into the closest ground if you opt for that method of getting the ground. (often under a coil on frame, the coil tower post, or under the seat...)
I always was of the opinion if your + feed needs to be at least 16 ga. then the return to ground needs to be equal to that gauge. Some slight gains may be seen going up to a high quality wire that is high count multi-strand wire one size larger than minimum needed.
For a 550 the charging system is mediocre or low output so, you can't expect to run a high wattage bulb in a H4 lamp setup. Stock lamps were what? 35W. So the 55W is pushing the system a bit already. Going with a LED bulb even running lower wattage benefits from the addition of relays as you minimize the resistance and voltage drop of the stock wiring.
Check for voltage from the battery with your + input of your meter (red probe) (VOLTAGE SETTING, never current) and the other probe (black probe) to your High beam input on the back of the bulb or the Low beam input on the back of the bulb. Write those values down. This is the voltage loss of the wiring on the positive side of the circuit. Go from the - terminal of the bulb with your red probe of your voltmeter and connect the negative probe (black lead) to the battery's negative terminal...
Write down that value. Add the two values your obtained and this is your total voltage loss of the wiring circuit. Check these same readings after you have installed your relays...this will let you see the improvement.
According to Hella's tests a halogen bulb in a automobile for the European point of view is more conservative than what Americans typically use.. They consider full voltage to the lamp to be 13.2V typically with the car running. 13.2v to be "100%" of lamp's output. (14V is the upper limit of voltage and higher voltages leads to dramatically shortened bulb life for a halogen bulb.) The loss curve is the same, though. When operating voltage drops to 95 percent (12.54v), headlamp bulbs produce only 83 percent of their rated light output. When voltage drops to 90 percent (11.88v), bulb output is only 67 percent of what it should be. And when voltage drops to 85 percent (11.22v), bulb output is a paltry 53 percent of normal!
Your voltage should be higher with the relay in the circuit and your voltage loss numbers should be lower.
Enjoy your new light!
