Thats pretty much it, extract from rave below
You need to be really careful as if you connect the battery and then unintentionally disconnect it for a second the sat nav can die.
I found the article on testing battery drain on cars with ECU's but have never tried it myself.
There is a gadget called a fuse buddy that some have used whereby you replace the fuse with the tool and it tells you if there is a current flowing.
http://randysrepairshop.net/testing-for-battery-drain.html
Battery disconnection
It is imperative that the key is removed from the
ignition before disconnecting the battery. A time of 2
minutes must also elapse before disconnection.
Failure to do so could result in:
l Navigation computer hardware damage
l Incorrect fuel gauge reading
Before disconnecting the battery, disable the alarm
system and switch off all electrical equipment. If the
radio is to be serviced, ensure the security code has
been deactivated.
CAUTION: Never disconnect the battery with the
ignition switched on.
CAUTION: To prevent damage to the navigation
computer hardware, a waiting period of two
minutes must elapse after the ignition is
switched off before the battery leads are
disconnected.
CAUTION: To prevent damage to electrical
components, always disconnect the battery
when working on the vehicle's electrical system.
The ground lead must be disconnected first and
reconnected last.
CAUTION: Always ensure that battery leads are
routed correctly and are not close to any
potential chafing points.