I think I have a power problem

I am asking the VCC to do quite a lot and based on one experience probably too much. On a MEGA 2560, I have an xBee 900 MHz Pro, a GPS, an SD serial data logger and on I2C I have a CMPS09 tilt compensated compass and a 4x20 LCD. The only thing I have had trouble with is the compass. I felt sure it was just wigging out on me because it failed even when testing with some of the worst power hogs (LCD and GPS) removed. But then I was wiring an ESC in (because I thought the MEGA didn't have enough to do ;D ) not realizing that it provided regulated 5V rather than requiring it. When I connected the battery for the motor to the other side of the ESC, the Arduino powered up and the compass works. Disconnect the ESC, power up on the USB and the compass fails. Power up using a very similar battery to what the motor uses, only through the barrel jack, and the compass fails. Everything else works every time in all power situations.

Should I beef up the on board power? If so, how? An alternative would be to power a bunch of the stuff with an external regulated battery and share grounds. That could also be the answer to my "how?" question - just use a good strong regulated 5v (I assume 5.5 is okay) battery to provide system power.

A related question is whether I should connect the 5v from the ESC or not. I would certainly connect GND.

The regulators on the mega are only 1A regulators. Saying that you will not get 1A from them due to the power dissipation in taking 12V down to 5V. That is a 12-5 = 7V drop you are asking the regulator to take and at 1A that is 7 Watts you are asking it to dissipate. This is way too much for that package. See:- http://www.thebox.myzen.co.uk/Tutorial/Power.html and http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html

You NEED to know what current each module is taking either from data sheets or measurement. The most efficient form of regulator is a switching one. This is an example of the sort of thing I guess you need. http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=152-524&x=0&y=0

This is what happens when you lower the barriers far enough to let unruly software guys be mostly successful wiring things. :D

One of the mechanical hardware guys told me some 3' flaming war stories before he gave some LiPo 7.4v batteries to power my controllers with and he achieved his goal of making me respect them. I wired 1A fuses in line on the + side from the batteries and this unit (the MEGA with all the toys on it) has blown a couple. One time was motor testing and I can blame that one partially on the hardware guy. He wired a beefy servo so that I was powering it, not just controlling it. I overdrove it while we were figuring out values for turning and blew a fuse. But more recently, I blew one for no apparent (until now) reason. I just figured I must have accidentally shorted something and was happy to only be out a fifty cent fuse.

So what about using a regulated battery? I was also thinking about contacting the vendor of the ESC to be used in the "real" unit (some drive train parts are being custom made, so the stuff I am using now isn't final) to get the characteristics of that 5v supplied by the ESC. My fear about it is that I might see a drop when I gun the engines.

"regulated battery" already available: http://www.pololu.com/catalog/product/783

I was actually thinking of a LiPo or other light and rechargeable battery. There are several with regulated power out and in (charge pretty much magically anytime power is coming in another way).

There are several with regulated power out and in

Are there? A battery by itself does not provide a regulated output. You can put a regulator on the output voltage to burn off or switch out the excess but it is external circuitry.

charge pretty much magically anytime power is coming in another way

It might come as a shock but there is no such thing as magic, it's all Physics.

Next you will be telling me that those SmartLipo batteries can't think for themselves!

Just the other day, Santa assured me magic is real; next time I see him I'll ask which list you are on...

;D ;D ;D