I have been using cheap chargers I find at thrift stores for years. Usually find them for $5-$15. They are used for wheel chairs, forklifts, etc. They are made to charge 24v batteries and I can bring a dead pair of 12v car/light truck batteries up to the point where they'll start the deuce in an hour or two, usually. Some are automatic chargers and some you need to remove after a few hours. I grafted a single pin male plug onto the wires usually, or sometimes just use a pair of alligator clips. I'm an electronics guy and understand ohms, volts, and all that stuff. Works fine, and for good reason, too.
Additional info: The reason they are so cheap is that most folks have no use for a 24v charger, so they kinda dump them. The wheel chair chargers take longer to charge, of course. I got a nice big forklift charger, with a built in timer and ammeter for $10 a while back. Works really well.