Installing HIDs today…
A word about Power
The XenonDepot kit included a very nicely made wire harness that provides power directly from the 12v battery main and uses the existing H1 bulb socket to switch the relay. This allows the kit to draw power from the battery mains instead of the existing headlight power circuit.Unfortunately, in the Elise, the battery is far in the back of the car, while the headlights are (quite sensibly) in the front. I could find no easily-accessible source of high-current power in the front crash structure. Arno on EliseTalk explained that there is a 50A power lead available somewhere under the dash, but running wires to it would be quite a bit of work, requiring removal of the dash top and likely drilling.
For this reason, I wanted to find out if a separate power source (with relay) is not actually necessary, meaning I could power the HID ballasts directly from the headlight circuits. This is a controversial approach, and I encourage you to do your own research before doing the same.
Power Demands
The HID ballasts are rated “20A Max” each according to the silk-screen labeling. However, HID lighting in general takes less power than normal Halogen (incandescent) lighting. Hard numbers are hard to come by, so measurements were in order.
First, the stock headlight bulbs draw 4.5A each. There is a large inrush current as the filament first heats up, but it’s very fast – too brief to measure. With the engine off, the stock wiring provides 12.0v to the H1 connector under load – excellent voltage, so I’m not worried about sagging below the 9v minimum required by the ballast.The HID Ballasts draw a large amount of current when first powered on as they ignite the arc and heat the salts in the capsule. It is this startup current that could blow fuses or cause damage to underrated wiring. I measured the highest startup current when the bulbs were cold (green line on chart), and it quickly drops off as they warm up. The highest peak reading was 17A!
Restarting the HID arc while the bulb is hot takes considerably higher voltage (up to 23 kV) to ignite, but actually less current. You can see the red line in the graph showing a lower current peak after the bulb has been resting one minute off and then restarted.
This is clearly beyond the capacity of the 10A fuse… or is it? The automotive fuses are “slow-blow”, so they can take a bit more than the rated current for a brief amount of time. According to Bussmann’s spec for the ATC fuse, we should be able to withstand around 17A for a full second without melting. The current spike is actually shorter than that, and the full time spent above 10A is about two or three seconds. In my sample set of two fuses, they have not blown at startup. Your mileage may vary, especially if you see subzero temperatures or have a different ballast core. I will be carrying a few spare fuses in the car, but I’m comfortable with that for now. I will say that I have heard three first-hand reports of HID installations on the existing power wiring, and none have reported power or fuse problems.
