Electrical troubleshooting.....

[daveSanborn]

I hate it, but I'm getting better at it.

My sons '68 FB slowly drains the battery.  10 days'll go by and the Optima Red Top won't have enough juice to crank the motor.  It's done this since the "professional" audio guys installed his sound system/alarm/motion sensors/radar detector/and "another" radar device.  IMO, their work was second rate.  One of the "black boxes" was just stuffed up under the dash and became entangled in the WS wiper linkage.  I had to re-do some of their work and wasn't too thrilled.  With him living overseas and not driving the car, this hasn't really been an issue, but I'd like to correct this before he returns.  I've been leaving the trunk mounted battery disconnected from the cars positive lead and keeping the battery on a trickle charger to keep it fresh.  I've noticed when I connect the positive cable to the battery that there is a rather large "spark".  This tells me that something is placing an immediate drain on the battery while everything, but (supposedly) the stereo presets/memory should be "off".

What's the best way forward here?

Start pulling individual fuses for all of the different electrical components and see which disconnected circuit stops the positive cable from sparking when it's connected to the battery?

[Sluggo]

That is the best way to at least ID what circuit you are dealing with. Then unplug individual components.
Many aftermarket devices have keep alive memory circuits, digital clocks, radios, amps, etc. any device that could need power to retain memory,  ie. radio station presets, clock memory, etc.

The arc on connect could be meaningless. What you need to do is check the draw on the battery by doing the following.
While batt is disconnected, hook test light to disconnected cable, hook up batt, then touch test light to batt post and watch for lighting of light.
If the light comes on for a few seconds and goes out it you likely have some component that has a capacitor or back up power source.

If it stays light beyond a few seconds after connect you likely have a draw. Replace the test light with a DVOM set to 1mv scale and measure the draw. anything above .5 millivolt is unacceptable.

I personally think .5mv is a little too much, but I did not come up with the number.

Start with those two simple tests.

[Midlife]

Sluggo: how can you measure draw by putting the DVOM on voltage settings?  Surely you meant DC current?  That's what I do, and my criteria is anything above 100 milliamps (clocks and radio setting devices can add up!).

Dave: when you connect the battery, are you sure all of your doors, lights, and stereos are off?  Any one of those will draw sufficient current to cause a spark when re-attaching a battery cable.  Otherwise, your first instinct to pull fuses one by one to find the offending circuit is a good one.  Try that first (it's cheap and easy), and report back what you found.  I can guide you from there.

[Starfury]

Yeah, if you put it in series like that, you need to set it to measure milliamps, not volts.  Otherwise you'll simply get battery voltage.

[Fast68back]

Dave: when you connect the battery, are you sure all of your doors, lights, and stereos are off?  

My car will throw a spark with the door open and connecting the battery

[Sluggo]

Sluggo: how can you measure draw by putting the DVOM on voltage settings?  Surely you meant DC current?  That's what I do, and my criteria is anything above 100 milliamps (clocks and radio setting devices can add up!).

Dave: when you connect the battery, are you sure all of your doors, lights, and stereos are off?  Any one of those will draw sufficient current to cause a spark when re-attaching a battery cable.  Otherwise, your first instinct to pull fuses one by one to find the offending circuit is a good one.  Try that first (it's cheap and easy), and report back what you found.  I can guide you from there.

Damn...you caught me. I meant 50 milliamps.
100 milliamp draw will make quick work of a battery's soc.