I'd think a deep cycle could jump start a car without harming it ..
but the more evolved battery guru's can address that.

here's what I do for a 'simple, manual, nothing complicated or wierd, two
battery system' that has full versatility.

I just put a heavy duty battery switch in either the hot or the ground cable
of each battery.
That way I can manualy choose which battery to start off of.
I can choose exactly when charging # 2 battery starts to happen.
Full versitality, no electronic gismo's.........
and the switches have a large key you can take with you when the batteries
are turned off ....as in anti-theft device.

there is only ONE caveat ....need to be sure never to have both batteries
disconnected when the engine is running.

Granted, you have a few more feet of battery cable running around with this
system ..
but for 'pure robust simplicity and full manual control' .....can't be beat.
I always noticed that about basic small aircraft like older Cessna's say ...
*everything* is full manual control...
fuel mixture, mag control....flaps ..*everything'  is total manual control.
Not 'idiot proof' so much ...but far more versitle and simple/robust.
  Sorry if the aircraft comparison bothers anyone- just illustrating a point
..
one that will contribute toward more reliable vanagons and more fun trips
and fewer hassels with electron management.
scott
www.turbovans.com