Any portable GPS with a lead to an external power source may be powered from any external power source (typically within the 10v to 28v DC range) as well as the aircraft power. Eg power guerilla /monkey, sealed lead acid battery, L/ion battery etc . I personally use a 12v 7ah SLA which is completely portable, lasts many hours longer than other options, is reasonably compact (if weighty) and is as cheap as chips to buy and without the (alleged) fire risk associated with L/ion types