18 volts, eh? Well, let's do a little math. Remember, filament bulb life is exponential to the power ~-13 (negative 13) with voltage, and output is exponential to the power ~3.4. So let's take an ordinary HB3 (9005) bulb rated 1700 lumens and 320 hours at 12.8v.
Lifespan:
(18 ÷ 12.8)^-13 = 0.0119
0.0119 × 320 hours = 3.8 hours' lifespan at 18v.
Output:
(18 ÷ 12.8)^3.4 = 3.19
3.19 × 1700 lumens = 5418 lumens.
5418 lumens sounds nice, and maybe through some kind of twisted logic we decide a 3.8-hour bulb lifespan is OK with us, but that's before we remember two other important things:
1) That 3.8-hour lifespan is with the bulb perfectly still on a lab bench. Filament sensitivity to vibration increases with filament temperature; in actual vehicle service you've got a great deal of vibration, so you're almost certain to have adjacent filament coils arc and/or physically short to each other, greatly reducing bulb output (permanently), and you're also going to have a vibration-induced filament failure much sooner than when the filament's at normal operating temperature. The actual in-service lifespan at 18v is going to look more like 38 minutes than 3.8 hours. And if you happen to kick this magical overvolt device on before the filaments are preheated by operation at normal temperature, then filament lifespan will be on the order of 3.8 milliseconds (they'll pop like flashbulbs and you'll have nothing but little balls of melted tungsten where the filament ends used to be.)
2) Headlight halogen bulbs are pressurized to between 6 and 12 atmospheres (about 90 to 200 PSI) cold. The pressure inside the bulb goes much higher than that when the bulb's hot. And the hotter it gets, the higher the pressure goes. Any tiny flaw or thickness variation in the bulb glass, and POW, your bulb can very easily become a grenade even when operated at normal voltage. Hot, sharp glass flies in a spherical field at extremely high speed. If you're lucky and the bulb's inside a headlamp, all that happens is you destroy the headlamp's optics. If you're playing around on the workbench or otherwise operating the bulb outside of an enclosure, you will get hurt or you will get blind or you will get dead. Increase the voltage to ~50% above design voltage and you are practically guaranteed to create a grenade sooner or later.
As for the other suggestion, running lows with highs can be done safely if (and only if) the vehicle uses separate bulbs for low and for high beam. It is several different kinds of unsafe if attempted with two-filament bulbs (like H4/HB2, 9004/HB1, 9007/HB5, H13). For one thing, you're overheating the bulb again, pow, see above. Also you're throwing a 100% overload on the common lead and the wire it's connected to, so you stand an excellent chance of creating significant and costly damage to the vehicle's wiring and switch. Finding the changeover point in your car's high/low beam switch and holding it there is a poor idea. The contacts in those switches are barely adequate to handle the current for one pair of filaments, let alone two.
Better wiring, yes. Relays, yes. Better bulbs, yes. Better headlights if you need them and they can be had for your particular car, yes. Auxiliary lights if you need them and can find a place to put them, yes. But this overvoltage thing is just dumb.
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