Once I was in a similar situation.
If you can, connet the item to a 12 v car battery and measure the
current draw.( the battery will have 12 to 13.5 v and provide whatever
current is needed)
Then if it is about 400ma then it would be ok to use the adaptor
because anything drawing about that current will cause the voltage to
drop to about 12-13 volts.( as per the specs of the adaptor).
Ali Tonto