a slow trickle charge is good, as long as...
you dont over charge it then it will wipe the battery clean.
I use a cool one called Battery Tender which don't overcharge and you can leave it on all winter to keep it conditioned and crap. It dont do a huge amp quick charge but I have went from dead to start in about 40 minutes or so. I'm sure there are different kinds of these now but when I was shopping this was pretty much the only choice for an advanced charger.
95 E34 530I V2.37
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Those who make peaceful revolution impossible will make violent revolution inevitable.
John F. Kennedy