I am thinking about installing a 500w halogen (HDX brand) on the roll bar in my jeep as the dome light because often I work in the back when it is dark or when I would like more light. The 500w light will be drawing about 40amps from my car's electrical system (battery or alternator).

Currently I have an 800w inverter that I can just plug it into. If I am using the light while my jeep is idle the alternator will be producing more power to power the 500w light, right?

Will the engine RPM automatically rise so the alternator can output more power?

3 Answers 3


I don't know of any vehicle (as equipped from the factory) that increases the idle speed to compensate for a large draw on the battery. If the alternator starts bogging the engine down then the computer should compensate to maintain the set idle speed but not make it increase.

It's easy to find out if the alternator will put out enough current at idle to run the light. With the light turned on and the engine at idle check the voltage at the battery. If the voltage is 12.6 or higher then alternator is putting out enough current to run the electrical loads. If the voltage falls below 12.6 volts the alternator is not putting out enough current to run the electrical loads.

If your alternator doesn't put out enough at idle you have a couple of options install a high idle device to raise the RPM. Install a larger amp alternator, there may be a larger one available from your parts store or you can have your alternator rebuilt to a higher amp version.

Some other options to consider:

Hard wire the inverter and put an outlet that's easy to access from the bed of the truck. Then you could just clamp the light on the roll bar (or wherever) and plug it in whenever you wanted to used it. This would eliminate any issues with vibration causing the light to prematurely fail.

With the price of fuel and the unnecessary wear on the engine you could consider a small generator. Honda makes a small quiet 1000 watt generator that would work great for this application. Some of them have a Halogen flood light attached already.

As already suggested you could use a 12 volt light to get the same effect. LED lights are expensive but they will have a lower amp draw for the same amount of light.

  • Thanks mate. I have a 160amp (2k rpm) alternator, so even if it is not at its full speed it should still deliver quite a lot.
    – Sponge Bob
    Commented Jan 4, 2013 at 20:20

Most modern engines should increase RPM's when the load on the electrical system increases, as the Engine Management system should monitor voltage and identify it being drawn low, so increasing RPM to increase output from the alternator.

Even many older engines would do this.

  • 1
    I don't think you're wrong, but I've driven many reasonably modern cars where the revs would drop noticeably when under load at idle. I think the only way to know with any certainty is to try.
    – Dan
    Commented Jan 3, 2013 at 9:37
  • 1
    They will drop a bit, until the voltage drops below whatever threshold is set. Mine does this too, until the revs get down to about 800, then you can hear the surge as the throttle is opened slightly
    – Rory Alsop
    Commented Jan 3, 2013 at 9:40
  • 1
    Ah, that's the interesting, thanks. The only things I've pushed very hard electrically are old non-FI engines, so it was all moot.
    – Dan
    Commented Jan 3, 2013 at 9:46
  • 2
    I forgot to add, is that not a by product of the engine preventing a stall rather than trying to increase electrical output?
    – Dan
    Commented Jan 3, 2013 at 11:10
  • It's a bit of both really:-)
    – Rory Alsop
    Commented Jan 3, 2013 at 15:45

I have a couple concerns with your plan. These types of lamps (bulbs) are fragile. If it is mounted to your rollbar there is a good chance it will be broken if you do any type of off roading. The most effecient means of powering extra lights is the 12volts of the Jeep. Because of losses in the inverter you will use more power than an equivilent 12v light. This means you will get more light for the same electrical load. I would compare the lumen output of some offroad halogen, highoutput LED and HID lights, then do a cost per lumen comparison. From what I have been able to find it is like comparing apples to oranges, no one uses the same standard when giving specification. Some list only watts (power used), or the color spectrum etc. I would check with some brands in your price range and contact their customer service and ask how many lumens the light puts out. This will at least give something to compare.

  • That is true. But the home depot light that I would like to install cost about $6... for the housing, reflector, and bulb. I did give some thought to the filament possibly braking when I'm 4 wheeling. I only go 4 wheeling for recreation so I can just take the light out when I do that.
    – Sponge Bob
    Commented Jan 3, 2013 at 22:23
  • Filaments are relatively strong when not powered, weaker (due to being incandescently hot) when powered. He probably won't be going off-road with a big light on in the bed of the truck, so I think it is fine. Halogen lamps actually have much thicker filaments than a standard bulb (draw more current and run hotter), so they are very tough. The "halogen" part is pressurized gas that prevents the filament from burning up, which it would in a vacuum because it is thick and draws a lot of current --> gets really hot --> melts or explodes.
    – user15009
    Commented Jun 29, 2016 at 2:14
  • If incandescent lamps are that fragile why are they used everywhere on a car? Headlights, interior, rear, brakelights, indicators, fog lights. All incandescent on most cars. Point is, the accelerations you can achieve through rough driving are in the order of +/-1g and at low frequencies. No problem for the bulbs. Virbrations may be more of a problem, e.g. if you bolted the bulb to the engine for whatever reason.
    – JimmyB
    Commented Nov 9, 2016 at 14:45

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