Originally Posted by Mike Romain
I am like jeephammer and would use a volt meter to check the draw. My meter is only rated for ten amps on the ammeter and I blew enough fuses to now use volts to isolate the draw.
I prefer to use the ammeter circuit on my multimeter. Here is a tip to avoid burning out the 10 amp measuring circuit in your meter. I suppose I could have mentioned it earlier, but it slipped my mind. Got a lot going on over here.
Get a generic tail light bulb and socket for $5 or whatever small amount they cost. You can easily slip that in series with your ammeter. The resistance the light provides will keep the current below burning out your meter. Gives a nice visual cue of current flow and you'll never cook another meter again. If the light will not illuminate, or is very dim, it means there is not much current flow taking place. You are safe to use the meter directly.
In this case the OP is just trying to find out what is draining his battery overnight, so we are not interested in measuring the actual current flow. It could be calculated if you know the construction of every portion of the circuit you are measuring, but since it's seeing the entire vehicle we don't care. Just interested to see if it is flowing enough to drain the battery overnight and finding out what is causing it.
Of course a person could just hook up the light to see if it would illuminate. Don't need a meter at all. If the light shines when you hook it up as previously described, just start disconnecting things till it doesn't shine anymore.