I use old 9v smoke detector batteries to charge really dead car batteries (well, not directly). My battery charger has a "feature" where it won't start charging your cars battery until it detects that you have connected the cables correctly. It does this by sensing the polarity of the voltage from your connection. But what if your battery is so dead that there is no voltage to detect? In this case, the charger will refuse to begin the charging process. So I connect my charger and then use some wires to temporarily jumper my old smoke detector battery across my cars battery. The "smart" (???) charger then says, "Hey, I've got voltage, and it's the correct polarity!" and it begins the charging process as I remove the jumpered smoke detector battery fake.
I should just buy a car battery charger that has an override button for this "smart" polarity check, but I'm too cheap. My existing charger is a good one - save for this brain-dead "safety" feature it has. And I always have mostly dead 9v batteries laying around from smoke detector replacements (although my most recent smoke detector purchases use two AA batteries and my 9v detectors are slowly being replaced). Two AA batteries are a heckuva lot cheaper than one 9v battery BTW.
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