This problem is not limited to vehicles. I've had to rewire low voltage power systems on ships and at remote locations such as navigation beacons and communication relays also. It's pretty common with all commercially produced low voltage power system wiring. In my experience, most battery power systems and backup power systems are inadequately wired for long term reliability.
Your example of 14.4 volts being dropped to 12.7 volts, is actually fairly typical and is perfectly acceptable to most manufacturers. The problem is, they don't bother to take into account the characteristics of bulb filaments when the charging system is overloaded or shut down. Bulb filaments are rated by wattage because they operate at such a high temperature that they draw reduced current when operating normally. A lower voltage will actually cause the current to increase. In your example, if the engine is idling, providing reduced charging current so that the electrical load exceeds the voltage regulator's output current, the battery terminals will drop to 12.6 volts. That results in the headlight voltage dropping to 10.9 volts, causing the two 55 watt bulbs to try to draw a total of 10.1 amps instead of the expected 8.7 amps. Typically, bulbs designed for 12.6 volts will draw increased current all the way down to 10 volts, before starting to draw less current. That means the headlight ground wiring has to be able to handle at least 11 amps for a pair of 55 watt bulbs. While fire prevention tables call for 346 circular mils per amp (AWG16), for wiring exposed to weather and vibration AWG14 would provide better long term reliability.
Installation of HID adds another design factor. While bulb filaments just draw more current with slight voltage reductions, electronic circuits are much more unstable and can stop working or even be destroyed by voltage drops that bulbs can handle. The addition of an electronic module (ballast) requires that the wiring be upgraded to allow less voltage drop as the bulb curent increases. Most designs consider that a 1 volt total drop in the wiring is acceptable, with half that being dropped in the connectors and the wire size being calculated to drop the other .5 volts. Personally, I have found that increasing the wiring size to allow only .25 volts being dropped in the wire at peak load, gives much better long term results. Assuming that we are still specifying two 55 watt bulbs, with one of them being HID, and half of the wire voltage drop is in the positive wires and the other half, .125 volts, is in the ground wire, the common ground wire would need to have less than .011 ohms of resistance. This specifies a minimum wire size of AWG14 for a ground wire up to 4 feet in length and AWG12 for a ground wire longer than 4 feet but less than 6.5 feet long. Of course, this does not include any calculations for inrush current that the ballast may require, since I don't know what that current may be.
Originally Posted by voxmagna
The most common issue on motorcycles and cars is skimpy wiring which causes voltage drops on the high load circuits like headlamps, horn, rear stop and tail. That's why you get the light dimming and once they wrap the wire in the loom, you have to consider adding a decent 12 gauge wire externally. BMW must use a computer to design their looms for minimum copper content because they have so many connection points, some inside the loom where you can't get to them.
I read a write up once where a guy was complaining about his poor headlights. With the motor running he measured the normal about 14.4 volts across the battery but 12.7 volts across the headlight lamp. With both lamps lit he was losing something like 15 watts just in the wire. Of course his headlamp bulbs lasted forever because a 12% reduction in lamp voltage is a lot less light.
The best option not in the list is to run a separate ground in 12 gauge from the battery negative up to the cockpit, tie the existing brown headlamp ground across to it and use that connection point for the headlamps. If you go HID a relay isn't needed, but it helps with the handlebar switch life if you want to do it. That halves voltage drop on the oem wiring and would be adequate for HID. If you want to go the next step, partiucularly with bigger tungsten headlight bulbs, that involves adding a 'must have' relay, looking at the hot wire route and beefing that up. But HID's is the way to go. Whatever you do, don't start making 'ground' connections to the bikes metalwork. In places up front, the electrical conduction on anything but the clutch housing is unreliable, some metalwork is painted and things can stop working when bike frame parts are removed.
When you buy the aftermarket so called 'brighter' lamps, they just assume there are these kind of wiring losses and de-rate their lamp voltage so they overrun by up to 10% and of course don't last very long. Even less life if you thicken up the wiring.