My wife's car is dead. It's in the garage, a hard push uphill to get it out to jump-start, so I figured I'd build a battery charger. We still have our old DSL modem, and it has a huge 21-volt, 1.1 amp wall-wart, so I hooked that up. 3.1 amps... if I left that on all night, it would probably die... seems to be a 60 Hz xfmr-type, so probably has a thermal fuse. Adding my Makita belt sander in series, tie-wrap on the trigger, gets down to 1.3 amps, good enough in a cool garage. Maybe it will start in the morning.
So I should buy a real battery charger some day soon. Are they still big klunky transformer-rectifier things? I'd think that some nice small switcher could manage a few amps at least.