I'm no legal expert so don't take this as a definitive answer, but given they provide the cable for the mount with a standard 12v plug then I can't see how they could enforce such a warranty policy.
Personally I'm not very electrically skilled so I got my Celestron power tank in a package with my grab-and-go scope. It's the 7Ah version and the torch function is useful and it has 2 power sockets for powering the scope and another, for example a dew heater tape. But as the other guys have mentioned, there are cheaper and more beefy solutions depending on how creative you are.
As for lasting the night...it depends on how long your sessions are
my 7Ah power tank has lasted repeated 6 hour sessions of my Nexstar 8SE, the AVX and my bigger scope on the EQ6 (note: not at the same time) without the low power light coming on, so I don't really know how long it lasts.