30/40 relay.....amp draw

[bigbadchev84]

Ok this is a little unusual question, but I want to install a anti-theft system on my gauges. I have a full autometer set-up and a convertible. I was thinking about wiring up a 30/40amp relay so that when the gauges are disconected it drops the coil out on the relay and sets off the horn.

So will the coil on the relay be a huge drain on the battery with the truck not being started for weeks at a time?

 

[Fordum]

Any drain is going to be big when you don't drive it for weeks....
And I understand your idea of letting the relay drop out. There are a few better ways, but if you need the current capability of the big relay, and want to keep it simple, then use two relays.
One very small one that stays pulled in all the time, and let it trigger the big one when it drops out and have the alarm come on when the big one turns on.

 

[bigbadchev84]

Is the coil in a smaller relay really gonna be that much less than the 30/40 one? I have about 10 of them I picked up off ebay and use them for everything. Just havent used one for when the truck is turned off. I could wire one through the door/dome light, but that wouldnt help if they hopped on the tire and up over the bedrail.

 

[Fordum]

Is the coil in a smaller relay really gonna be that much less than the 30/40 one? I have about 10 of them I picked up off ebay and use them for everything. Just havent used one for when the truck is turned off. I could wire one through the door/dome light, but that wouldnt help if they hopped on the tire and up over the bedrail.

Well, its all relative. There are some relays out there that will draw over an amp.
And some that draw a whole lot less.
My problem is, I don't know how much yours draws. If you have a good digital voltampmeter, you can put it in series with the coil and see how much it draws.
If there are make and model numbers on your relays, you might be able to look up the specs.

For instance here are a couple of relays.

This one is a common automotive relay.
http://www.alliedelec.com/search/productdetail.aspx?SKU=70175709#tab=overview

If you look up the spec sheet and do the math, you will find it draws about .162 amps, or about 162 milliamps.


This one, is a fairly low current one. There are lower, but this one is not bad.
http://www.alliedelec.com/search/productdetail.aspx?SKU=70176173#tab=overview

Doing the math on it, comes out to about .0166 amps.

BTW, I got lucky on those numbers. I did not set out to find a relay that was 1/10 current of the other.

Battery are rated in amp/hours among other ways. A big deep cycle marine battery might be rated at 100 amp/hours.

The heavy relay will draw 1.62 amp/hours every 10 hours.

100 amp/hours divided by 1.62 is 61.7.
Times the 10 hours it takes to get there, equals 617.2 hours. Divided by 24 equals 25.7 days to totally dead.

The lower draw relay would take 10 times as long.

None of those numbers are all that accurate, or even close for lots of reasons.
But it gives you an idea of the difference in relays.