Electronic – Replace Lithium Battery with Permanent Power Source

In place of the LM7805, using a Low Drop Out (LDO) regulator such as MIC29300-5.0WT would allow operation at as little as 5.37 volts, providing 3 amperes at that voltage, enough for your Beagleboards but not the motors.

The more expensive MIC29500-5.0WT at $5.46 apiece on Digikey would provide the 5 amps you might want instead.

• Tapping between batteries is best avoided with unbalanced loads.
• And yes, the batteries loaded higher would deplete earlier, with unpredictable outcomes.
• In stepping down 6 to 7.5 volts to 5 volts at 5 amps using an LDO, up to 12.5 watts of heat would necessarily be dissipated at the regulator: Power dissipated as heat = voltage difference * average current = 2.5 * 5

However, realistically the batteries won't be at 7.5 volts under load, and not for long anyway. Also, the continuous current drawn would not be 5 amperes through the regulator. Your design would ideally provide the motors with unregulated power, so your "step down about 4-5 amps" deserves a revisit. Besides, voltage is stepped down, not current, at least in this context.

Switching (buck) regulators would waste less heat, roughly around 15% that of the LDO, but they usually need more voltage headroom to operate. YMMV, searching DigiKey might yield what you need.

Why is your hub needing to be powered from the same power as your car motors? Your hub should be at your base station, the car would just carry a WiFi dongle of some sort, ideally?

Suggestions:

1. Edit: [15-Oct-2012] Provide regulated power only to the logic parts, drive the motors directly off the battery, and add a hefty decoupling capacitor before the regulator to work around motor inductive spikes. If your batteries can handle 4 motors of 2 amp each, they can handle the initial surge the capacitor will need.
2. Provide a diagram of some sort, of what is being attempted, and some clarity on how the powered hubs got aboard the moving car. This will enable more helpful answers.

The power connections above certainly seem to match the USB pinouts which is the most important thing to get right for a start. Maybe once constructed you could test that the charger is working with a cheaper device with a micro USB port to make sure you've got the pin ordering right.

I haven't verified the other information you've researched on Google Glass in particular but even if the device is in USB data mode I think it's relatively unlikely that placing headphones across the D+ / D- lines to ground would cause any damage. If you wanted to be cautious you could place a 1K resistor between the headphone ground and USB ground on the first test to limit the maximum possible current down to the sort of range used during normal USB communication. I'd expect that should be barely audible just for an initial test.

Finally I'm not sure how criticial it is for that device but to ensure that it is in the right mode it probably is worth shelling out for something closer than a 470k resistor. Most hobby type electronic shops won't sell them but 499k is the closest E192 value. Here's one at Farnell / Element 14 in the UK. I noticed the delivery charge is quite high but they probably have some other stuff you need or you may be able to pick them up (not sure how that works in the UK).