[quote=demon]
You can't check voltage drain like that. The minute you put the leads on the post and cable it completed the circuit through the light bulb. It'll read 12 volts too if you use a meter hooked up that way. You need to check for amps.
I disagree, in a non automotive situation try to get a bulb to light when its connected negative to negative.
In the case above think of the bulb as acting like an amp meter as it will illuminate as a current flows through it. It is connecting to positive though the device that is using the current.
Well as a union electrician I can tell you are mistaken. It is not going negative to negative. The positive cable is still hooked up. All you're doing is taking off the negative cable and hooking it back up with a light bulb in it. The bulb itself is load. Unless you unhook every harness off the battery it will light the bulb.
If you don't believe me go out to the garage an disconnect your negative cable and then get out a volt meter and put 1 end on the battery and the other on the disconnected cable. The meter will read 12 volts.
Only if there is something "ON" somewhere causing current to flow. No current flowing, no light. Measuring amps will accurately tell you how much the draw is pulling. An incandescent test light will also be useful by observing how bright the bulb is.
You have to be careful with newer cars that have auto-diming interior lights, chimes, and anything else that stays on for a period of time after the ignition is shut off. "Keep alive" circuits are very small, will take weeks to run a battery down and won't turn a test light on.
