DC power for D7000

> Model: A7000|Nighthawk AC1900 WiFi USB Adapter - USB 3.0

   "A" != "D".  D7000[v1] or D7000v2, or what?  Look for "Model" on the
product label.

> When I use a 12 volt car battery [...]

   With or without the engine running?  Cable length?

> [...] Continuously cycles. [...]

   That's a little vague.  The LEDs flop around as in a normal power-on,
but don't get stable, or what?

   I've never tried it, so I know nothing, but I'd expect a bare battery
to work well.  A running engine (generator) might add some potentially
troublesome (electrical) noise, but I wouldn't expect it to matter.  A
sufficiently long cable might make a local filter/bypass capacitor
desirable close to the load.

   I happened to have handy an adapter cable stub which was deformable
into compatibility, so I've now tried it with a D7000[v1].  Worked as
expected (although without Internet access).  The LEDs look dim
outdoors, but that's true with the AC adapter, too.  My cheap-Chinese
meter said 12.3xV ("x" varied), for what that's worth, with the engine
off.

   It did take about a minute and a half to initialize (either way).