A blocking diode installed on the positive wire going from the charger to the battery would totally prevent this from happening. (By this I mean the problem of destroying a battery with a charger attached but not plugged in.)

http://www.the12volt.com/diodes/diodes.asp

I've done some poking around. The trickle charger I have that I might install is a 1A trickle charger. This diode, http://www.radioshack.com/search/index.jsp?kwCatId=&kw=1n4001&origkw=1N4001, is rated at 1A continuous and 30A surge. It can withstand 50V applied to it of the wrong polarity without allowing reverse current. It is 59 cents from Radio Shack. For less than a dollar, plus some soldering I can make sure that my charger will not kill the battery.

-Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.