An often-asked question about HID lighting kits installed in cars is “will it drain the battery?” The answer is yes, and no; it depends on what phase of operation the lights are in.
To understand HID voltage draw, it helps to understand how they turn on and operate. Instead of passing current over a filament causing it to glow, as in traditional headlights, HID uses an arc of energy between two electrodes to generate the glow. To better visualize this, think of an arc welder; high current between the target metal and the welding electrode cause enough heat to fuse the metal while generating a very bright light. The first problem is that you need to get electrical energy to “jump” the gap, and the only way to do that is with force. The force required to cause electrons to move across a conductor is called electromotive force, (emf) and is commonly measured in volts.
For a given conductive material, the larger it’s cross section, the more electrical energy that can pass with low resistance. In the case of a filament bulb, current flows easily in the wires to the bulb, but meets resistance when it attempts to flow across the thin filament. The resistance causes the filament to heat up and glow. The 12 volts of a car’s battery is quite able to supply that energy.
Since an HID lamp has no filament, you need to have the electrical energy bridge the two electrodes to cause ignition. 12 volts is not enough, you need to be in the kilovolt range (kv 1kv = 1000v) for that to occur. The only way to get that kind of voltage is to step-up the battery voltage. For this reason, it is not recommended to ignite HID lights without the vehicle running. Repeatedly igniting the lights with the vehicle off can rapidly drain the battery.
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Now for the no part; once the HID lights are ignited, it takes a lower, stable current to keep the arc alive. The ballast in the HID circuit takes care of providing that stable current to the lamp. Typically that current is lower than the current required to keep a filament glowing, so in this phase of operation, current draw from the battery should be at the very least no more than a traditional headlight.
So yes, HID lamps can drain the battery if they are continually ignited with the engine off. However, their current draw is lower once they are operating, and this should not drain the battery any more than a standard headlight would. Of course if the kit is not installed properly, a path for current could be formed and that would drain the battery.