It depends. Essentials can be wired direct to the battery. But theres no need. The only thing that is a problem may be the radio code. So you connect this to the battery before the gadget if you cant remember it. Or write it in a hidden place.
This is an emergency measure only.
The best safety options are:
a) A maintainance charger connected if you are not using the van. Or a sensible sized solar charger on your dashboard when unused.
b) 12V Anderson option on van dashboard AND chair, and one externally in case you need to power ramp with a totally flat van battery.
Like this:
http://www.wheelchairdriver.com/faster- ... arging.htm