If you have a car cigarette lighter equipped and a small 8A charger it's
OK to connect the charger to that cigarette lighter socket. This socket
is connected to permanent +12V. A charger of 8A does not exceed maximum
current of that socket and it's fuse*.* This way you may use that socket
in reverse direction.*
*

Helmut, '80 Vanagaudi

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My Vanagon got T-boned on passenger door, hard ($15K damage); step-in
area crumpled, seat pedestal compressed such that Passenger seat is
immovable, so no access to battery.  It has taken months to arrange
frame straightening & body work, and still might take a while longer
until work begins.  Meantime, Vanagon has been sitting in my garage; I
drive it about every 2 weeks (windshield is buckled and smashed about
1/3....so am always on lookout for cops).  If I have more bad luck and
battery discharges to the point of no-start, what would I do?  I have a
small trickle charger available.  Could I just hook the positive onto
the junction of big red cable at starter (I presume big red cable goes
directly to starter battery) and charge battery that way?