Those car-power-adapters would be nice. However, those you can get are probably rated for 500mA, and might not even be able to provide that (at a stable 5V).
That said, they are built for 12V input. They must tolerate 10-14V, but below or above that, nu guarantees.
The ebay DCDC converters are much better. They can easily provide 1A, and will do 2A, or 3A provided it doesn't have to be very long at a time.
For input they require anything more than 0.5V above the output voltage (5.5V in this case) and anything up to 30V. So you can build an 8 cell AA battery, providing about 12V but varying between 13V when completely charged (depending on the battery) and 8V when depleted.
Either cut a microUSB cable and solder the wires to the output of the DCDC converter, or hook it up to the GPIO connector, but be very careful as a mistake will blow up the 'pi in no time.
(If you hook up a component with reversed power connections, many will act as a diode. Possibly two diodes in series. If the current that flows in that case is powerful enough to damage the device, it will blow up. If the current is limited enough then the device might survive. I hooked up one of those DCDC converters the wrong way around, and it survived. The current was limited to 1A and as that's a normal current for those devices, it handles 1A the "wrong way" just fine. However for a 'pi there are lots of "delicate" components that might just blow at a low current, and it is very likely that your powersupply isn't currentlimited as my professional lab powersupply is....)
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