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Step 10Follow up
If you are comfortable with this type of set up, but would like to use a Firewire cable instead, use an adapter that has an output voltage of 12v to 30v. I have several adapters that output 14v constant, these are ideal to splice with a Firewire female wire. Coincidently, this cable (http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=100&products_id=1701) would also allow you to mount the connector in the car (in dash, flush mount, console, or otherwise) and you can also hardwire if you wish.
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USB chargers should output between 4.75 and 5.25 volts, and for full charging efficiency 1900 mA (Any set of batteries will provide more than that. Over-amperage won't damage your devices, as they will just pull what they need). Low power USB devices should be able to work with as little as 4.4 V, and all devices should be able to signal, if not work.
Now, a standard disposable battery is rated as outputting 1.5 V, and will at most of it's useful life, but when they are brand new they use to output as much as 1.6 and 1.7 volts, and they go down as they discharge. Most devices are aware of this and will work at 1.2 volts and up. Rechargeable batteries output at less voltage (Fully charged at 1.4 or less and for most of its use before recharging at 1.3 - 1.2), but have a less step voltage diminution graph.
Down to the math:
Brand new alkaline outputting 1.7:
3x1.7=5.1 (OK)
Brand new alkaline outputting 1.6:
3x1.6=4.8 (OK)
Less batteries you are too low, more you are dangerously high.
Used alkaline (1.5):
3x1.5=4.5 Low, but usable (-0.25v)
4x1.5=6, way too high (0.75v)
fully charged rechargeable (1.4):
3x1.4=4.2, too low (-0.55)
4x1.4=5.6, too high (0.35v)
Somewhat used rechargeable:
3x1.3=3.9 too low (-0.85v)
4x1.3=5.2 (OK)
Used rechargeable (1.2):
3x1.2=3.6 way too low (-1.15v)
4x1,2=4.8 (OK)
Most devices will have higher tolerances than the USB standard requires, but the question is ¿Do you feel like risking? In any case, under-voltage is much less likely to do damage than over-voltage.
Check this out:
http://www.ehow.com/how_4900110_reduce-voltage-resistors.html
for details on how to calculate the amount of resistance needed.