Driving lights and power while driving.

[Damian m]

G'day guys,

I was hoping that someone would be able to help me with a silly question.

While driving does the battery or the alternator provide the power to run my driving lights? And does that also mean all of my accessories are also being powered the same way.

And with the car turned off is it the same or opposite?

Thanks

 

[Kranzy]

My understanding, and I'm happy to be corrected is that the battery powers everything and the alternator charges the battery while driving.

So when the engine is off, the battery will lose charge as there is nothing charging it.

 

[bods]

^^sort of. Generally, from the factory they put an alternator in that has enough output to run all the stock accessories while the car is running and have a little bit left over to charge the battery. the problem is when you start adding things like lights, uhf, subs, amps, etc and use more power.

if you add up how much power the accessories all use flat out, including running the heater/aircon, rear window demister, wipers on full, headlights, highbeams, etc plus how much power any accessories you have added use, including the size of any auxiliary batteries as these need power to charge if running a fridge, compressor, etc, then find out how many amps your current alternator puts out. then subtract the power usage calculated from above from your alternator output...

chances are all these will use a lot more than what it does put out, so then you need to think if there's ever a chance you might need to use all of that at once (which is probably not very often) and decide if you should spend the money to put a bigger alternator on...

as above, when the car is off everything is powered from the battery, obviously the more you discharge the battery when the car is off, the more you need to charge it back up, which adds to all of the above when allowing for power usage...

probably a very long winded answer, but hopefully it makes sense......

 

[Old.Tony]

What they said. The alternator CAN power the car, but not very well at idle especially with large demands. Lots of driving lights can be troublesome. Let's say two on the bullbar, plus 2 fog lights, plus 4 on the roof = 6x100W + 2x55W = 58A and the D22 alternator is going to struggle to put that power out even at midrange (don't forget, the ECU, instruments and ordinary lighting needs to be powered too). At idle, you're drawing most of your power from the battery.

 

[Damian m]

So I'll give you the scenario, I want to add 4 driving lights and a light bar.
I do a lot of long drives, mostly at night.

I will be running,
UHF
Radio
Heater/aircon
Phone charger
iPad charger
Plus occasionally the windscreen wipers.

If I add the lights am I better off also adding a better aux battery, or getting the alternator rewound?

I travel 800 Ks to get to work of which 5 hours is definitely after the sun goes down.

I've been thinking about getting a third battery set up to run the fridge and chargers off.

How do you work out how many amps you consume? And also how to relate that to any battery capacity.

Thanks

 

[bods]

to work out amps from lights, divide the wattage of the lights by 12v, so a 100w driving light would be 100/12=8.333a. another way is to get a multimeter and put it on the amperage setting and run it as part of the circuit to work out the particular amp draw. keep in mind these are usually limited to 10a, but you can get clamp on amp meters, or even a needle gauge type that you could fit in the circuit of what you want to test. if you look at the battery it should have a sticker stating cca and rc (cold cranking amps and reserve capacity) the rc is how long in minutes the battery will last with a constant 25a draw on it.

i have a hid setup on my motorbike and wanted to fit heated grips. i thought of tapping into the headlight circuit so they would turn off when the ignition was turned off, so they couldn't flatten the battery. so i did a cold start test for the hid using a multimeter on amps and to my surprise it drew less than 1a, even on a cold start and that is a 35w hid setup.... so i guess they do use a lot less than halogens, considering the headlight circuit has a 10a fuse in it...

if you want to reduce power usage on the lights, get hid's for the spotties, they use a lot less power than halogens of the same wattage, as well as putting out a lot more light. light bars (depending on the size) don't use a lot, to give you an idea of mine, when i put the high beams on (22" light bar, 2x hid spotties and 2x halogen spotties) the volt meter i have in the cab only drops .2 - .3 of a volt, that's with the engine at idle. then once the hid's are warmed up it goes back up to where it was before turning them on, even though they are still running.

the more batteries you add, the longer it will take to charge them all. what model have you got? you could price up getting yours reconditioned or see if they have one on the shelf they could mod and do an exchange on yours. to reduce more power usage, you can change globes to led also, but if you do indicators you will need an electronic flasher unit to stop them blinking fast as if a globe is blown...

as for working out how many amps you consume, above is the worst case scenario. realistically you won't have the heater going flat out all the time, the rear demister going all the time, phone chargers don't use a lot of power, maybe a couple of amps...

in all honesty you will be fine. if you don't have a big stereo cranked up with subs, etc going the alternator should keep up with no problems. highway driving, the rpm you are doing is close to the peak output for the alternator anyway, so you will be getting plenty of power out of it...

 

[Old.Tony]

You could try an alternator off a Patrol, there's one (I think from the GU30) that delivers 120A that bolts straight on. That's more charge up front.

It does depend on your choice. Heater might draw 3-4A for the fan, phone and iPad would be lucky to take a little over 1A together (2.1A + 0.5A @ 5V = 2.6*5 = 13W / 12V = 1.1A). So that is only 5A. Add an amp for the UHF radio, 3-4A for the dash radio, about 5A for the wipers, you're looking at 15A. Normal headlights say 60W = 5A each so the total is 25A. Tail lights and front parkers say 12W each = 4A together, 29A total. The ECU could draw no more than 10A (check the fuse, it probably draws 5A but we'll call it 10A for safety) so there's 39A to control the car. Add more power to open injectors and more to run the instrument panel - say another 10A - that's 49A to keep your car going at night.

Your alternator will put that much out while cruising, so that's not a problem. However, you want to add more lights ...

4 spotties at 100W each (about 8.3A) will need 48A on their own! That actually exceeds the output of your alternator (90A @ near 5000rpm) so you'd have to look elsewhere.

The best alternative for driving lights would have to be HIDs, particularly if the reflector is designed for HID. If it's designed for halogen globes, the HID globe will cause a fair amount of light scatter - you could get better distance light by getting something designed for HID.

You could consider LED lights, but they're still lacking distance (unless you grab those ARB Intensity lights which are stupefyingly bright). HIDs would be my choice, and only two of them - no need to light the roof up.

If you DO want to have a heap of lights up on the roof, put up 4 pairs of 20W dual CREE LED lights. They'll be bright, but you probably won't get the distance vision you're looking for.

 

[Damian m]

Thanks for the replys Bods and old.tony.

Now I know how to work out how much extra amps it will all draw.

I am looking at light force xtg hid's and the 170 strikers.

In all honest I know I'm up for about 2k for the lights I want, so what's an extra couple of hundred to get the alternator sorted out as well. That way I know I won't have any issues.