relay's

[89gqpatrol4x4]

This may be a simple/stupid question but when do you use relays? From what i know you need one when the current draw required is more than a simple toggle switch can handle. i.e. fridge. Is this correct? obviosly you can get diiferent sized relays? and how do you know when to use one and what size to get? I am going to get a 750mm led light bar for the bullbar, do I need a relay for this? I was going to take power from the battery to a switch on the dash and back to the lights, is this right?

[Yendor]

There are many different reason to use a relay, such as, reduce voltage drop, enable smaller size wiring to be run to switches, able to use lower rating switches, for different switching type circuits....etc.

I would use a relay for all motor type devices such as thermo fans, air compressor (if running via a toggle switch) and extra light above 55 watts.

Normally you don't need to use a relay for a fridge.

You will need a relay with your light bar. You will then run the wiring from the battery (via) a fuse to the relay and then from the relay to the light bar.

You can then run lighter gauge wiring from the relay into your toggle switch.

[megatexture]

You can do that providing the switch you use will cope with the max amp draw of the light bar but if you were to link it to your headlights you will need to use a relay.

[the evil twin]

snip
I am going to get a 750mm led light bar for the bullbar, do I need a relay for this?

Yes, unless you want to spend significant bucks on heavy cable up to the dash and back.
and...
Yes if you want Hi-beam control as well. If it is only for Off-Road then no BUT you are back to having to use heavier gauge cable again.

Most 30 inch Bars are at least 90 watts single row LED or 180 watts double row. Some are significantly more if they are 5 or 10 watt LED chips.
It is also pretty rare for the manuf not to include a prewired loom these days (with a relay in it already)

[threedogs]

X 3 and put your fuse {Correct amperage} as close to pos+ on Aux battery as possible.
try to run accessories except winch off AUX battery and leave your start battery for just that starting

[Mtiehatten]

Using a relay in this situation might also help in detecting problems when they occur. I'd always use one.