Outer Limits 4x4 Board

I know not another bloody spotlight question.
I put 4 Spotlight on my roll bar of my ute, now when going down the road @ around 80kms they are great, but when I slow down all the lights drop. I have 2 on the front bar but they still stay bright at idle and at speed. Is my alternator not big enough?
I am running a std alternator on a 2L 2.4 Diesel.

They 'drop' as in physically point downwards or they dip in brightness?

The 2L alternator probably isn't enough to power those lights (more info on lights please)

Froon wrote:They 'drop' as in physically point downwards or they dip in brightness?

The 2L alternator probably isn't enough to power those lights (more info on lights please)

They dip in brightness they are 4 X 100watt Ziels

Leighroy1984 wrote:

Froon wrote:They 'drop' as in physically point downwards or they dip in brightness?

The 2L alternator probably isn't enough to power those lights (more info on lights please)

They dip in brightness they are 4 X 100watt Ziels

Depending on the manufacturer, 100W@ 12V may be more at 14V. Estimate 10Amps each. You need a 40Amp alternator just to run the lights on top. Also Halogens are very sensitive to voltage drop, so they are probably getting 12V when everything else is getting 14, and when you slow, your heavily loaded system drops to 12 and they drop to 10.5 or 11 - yellow.

Quickest solution is to swap to 55W + 50's and you'll get similar brightness without the current. Or go HID.

Paul

I'd say Paul's pretty much nailed it. A combination of voltage drop at the alternator at idle, and voltage drop caused by higher current (400W worth) on the longer wiring up to the roof lights.

How are the roof lights wired? How many supply wires, what size, return via wire or body/chassis? For spot lights, don't select wire based on current capacity - choose it based on voltage drop.

Hi Leighroy, I’m guessing here but I’d say you had both a small current alternator and an old one at that.

The most obvious is that the alternator is not really big enough for what you are trying to do and older alternators will not produce large current out puts till they are running at high revs and the reason for your drop in light power as your speed reduces.

Newer alternators tend to produce far more current at idle than older ones did so you can get much power even at low speeds.

Paul and Scott have pretty well covered the short term fix but a new higher capacity alternator might be something to consider, particularly if you are planning to do some night time off roading where speeds and engine revs are much lower than typical highway us.

Cheers

"...I have 2 on the front bar but they still stay bright at idle and at speed....."

Start by upping the guage of the wire going to the roof.

Leighroy stated that the spotties on the bulbar are OK at idle, so the Alternator is putting out enough voltage at idle to produce white light from the globes. What is happening is the resistive losses in the roof power cable is allowing the voltage at the roof spotties to drop off the voltage required for "white" light from the globes with the difference from high speed to idle (normal anyway for most alternators).

To work out the current needed

Current = Wattage / Voltage

so

100/12 = 8.3 amps per light but as someone stated may be more at 14.4V

6 lights = atleast 49 amps to run all you spotties

on the 2L the standard alt should put out 40 amps.

If the wiring is smaller than 4mm that will not be helping either.

Also to throw a spanner in with the kettle of fish, one manufacturers 100w bulb is different to another, ie one so called "100w" halogen bulb running at 12.5v supply can draw 6.5 amps, whilst the next "100w" bulb that you grab and measure can chew 9+ amps.

So the so-called marketed "100w" bulbs can vary in actual wattage greatly.