Driving lights off aux battery?

longjake82
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Driving lights off aux battery?

Unread post by longjake82 » June 2nd, 2019, 2:06 pm

G'day guys, I'm planning on swapping out my old 100w halogen driving lights for a pair of Domin8r 7" LED spotlights, or similar, on my FZJ80R Landcruiser.
The Halogens are currently running off a 40a relay which takes its power from the factory power outlet box on the RHS of the engine bay. This outlet has only a 20a fuse, which brings me to a few questions. Is the amperage rating for driving lights, like the 10.5a for the Domin8r's for each light or for the pair? Assuming it's per light, my plan is to take the power to the relay straight off the auxiliary battery instead since it's literally only inches away (with a 30a fuse?) and run earth from the lights back to the negative terminal of this same battery. I'll probably upgrade the power wire to the relay and lights to heavier cable, but since I'm not confident with the dark art of negative switching on Toyotas and how the rest of the relay is actually wired, I'd really like to leave the rest of it alone if possible.
Am I missing anything, or does anyone have a better idea they'd like to share? Cheers.

Boots33
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Re: Driving lights off aux battery?

Unread post by Boots33 » June 2nd, 2019, 7:50 pm

The 10.5A quoted for the Domin8r 7" LED spotlights would be for the pair.

longjake82
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Re: Driving lights off aux battery?

Unread post by longjake82 » June 3rd, 2019, 4:37 pm

Thanks Boots33: so 2x 100w halogens should be drawing 13.8a for the pair @ 14.4v right? So I'd actually be using less power switching to the Domin8rs not more? That makes things easier, cheers!

Boots33
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Re: Driving lights off aux battery?

Unread post by Boots33 » June 3rd, 2019, 5:59 pm

Without getting too deeply into the subject ............

When working out current draw in automotive situations you should use the nominal voltage that the item is labelled with. For most automotive items that will be 12v.
So if your halogen light is rated at 100w at 12 volts then it will draw around 8.3A (16.6A for the pair). You cannot use the same formula when you are running the light on 14v because the bulb will actually be dissipating more power. You can see that in a practical sense in that you will notice the light will be brighter at 14v than it will be at 12v.

longjake82
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Re: Driving lights off aux battery?

Unread post by longjake82 » June 3rd, 2019, 8:53 pm

Aah of course, I was fixated on amps=volts÷watts but when you put it like that it's obvious; the halogens won't be putting out 100w the whole time if the battery slowly goes flat either will they... just glad I won't have to change too much wiring to swap them out. Just out of curiosity, if I did swap to a set of lights that drew 20a or more, would my original plan work, or should they really be powered off the main battery? Thanks!

Boots33
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Re: Driving lights off aux battery?

Unread post by Boots33 » June 4th, 2019, 8:59 am

Technically (in a broad sense as I do not know exactly how your aux battery is setup) there is no reason you couldn't run your lights off of your auxiliary battery, they would be no different to running any other type of device.

Probably the main consideration would be in your systems ability to supply the required power to both maintain the aux battery and the extra load of the lights at the same time.
For example many vehicles use some form of dc-dc charger to service their aux batteries, If the dc-dc charger was only rated at 25A it would not leave much available to recharge the aux battery and power other devices likes fridges that may also be connected.

longjake82
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Re: Driving lights off aux battery?

Unread post by longjake82 » June 4th, 2019, 2:12 pm

The aux battery is hooked up to an older looking isolator solenoid; not sure of it's rating but it's wired up with 4 gauge cable at least so I've got high hopes... but if I end up getting lights that draw less than 20A it's all moot anyway as I might as well leave the relay exactly how it is.
Thanks again for your help!

hoyks
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Re: Driving lights off aux battery?

Unread post by hoyks » June 4th, 2019, 7:48 pm

Fuse selection should really be based on the wiring, rather than the lights. An over sized fuse and the lights won't care, they will just use the power they use, and an under sized fuse will be at risk of blowing all the time.

But a 30A fuse on wire that can only handle 20A, if the insulation fails and the wire shorts out, then the wire will get hot, insulation will burn and the fuse may not pop as intended.

longjake82
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Re: Driving lights off aux battery?

Unread post by longjake82 » June 5th, 2019, 5:12 pm

hoyks wrote:
June 4th, 2019, 7:48 pm
Fuse selection should really be based on the wiring, rather than the lights. An over sized fuse and the lights won't care, they will just use the power they use, and an under sized fuse will be at risk of blowing all the time.

But a 30A fuse on wire that can only handle 20A, if the insulation fails and the wire shorts out, then the wire will get hot, insulation will burn and the fuse may not pop as intended.
Cheers, yeah that's why I was going to run power to the relay straight from the battery if my new lights draw close to, or more than 20A total. Because the factory power outlet that's currently powering the relay has only a 20A fuse: presumably the factory wiring to it is rated for not much more than 20A as well. Can the amp rating of cable match the fuse, or should the cable rating always be greater?

hoyks
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Re: Driving lights off aux battery?

Unread post by hoyks » June 6th, 2019, 5:34 pm

should be greater

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