File this under "Don't try this at home kids" but it works.
Its a very simple capacitive battery charger from scrap parts but if you have little respect for electricity or batteries its best you go buy a ChinaMart charger ... and more batteries.
I have restored three batteries to service in the last week with this charger, the first was my ATV battery, took about 4 hours. My regular charger would not wake up a dead cell that refused to take a charge (notice the other cells were bubbling but #3 cell refused). 4 hours later the cell woke up after being stung by 150 volts of buzzing 120 cycle power.
Second battery was my trolling motor battery, an overnight charge did the trick. Was 9.7 volts all sufated up, looked real bad. The next morning it showed 14.8 volts and all cells bubbling with a hot charge. After taking it off the charge in the morning that afternoon the voltage settled down to 12.8, a good hot battery.
Third was my lawn tractor battery, took only about 6 hrs.
Be damn careful, use the charger outside of your garage or shop, always hook up the battery leads BEFORE plugging it in, always unplug the charger BEFORE checking battery leads. This thing can KILL you if you are not careful. Stay away from the battery while charging. Do not charge a battery while wired in the vehicle, unless you want 150 vdc smoking your voltage regulator, alternator and electronics ...
My charger is fused to 5 amps with a neon indicator lamp to show when its hot. Its built in an Army ammo can.
I use a 50 mfd ac motor run capacitor for a 2 amp current limiter, just enough juice for a slow overnight desulfating charge. I used a 25 amp/400 volt bridge rectifier. This charger will charge any lead acid battery, no matter what the battery voltage is ... 6, 12, 24 volt ... it doesn't matter. My cost to build it was $12.
The problem with regular voltage limiter chargers is they cannot equalize a battery or desulfate cells. Not enough electrical pressure to bust up the lead sulfate that's insulating the cell plates.
The power output is limited by capacitor size - 25 mfd per amp output. If you want a 1/2 amp charger use a 12 mfd capacitor, if you want a 6 amp charger use (3) 50 mfd capacitors wired in parallel , ect , ect, ect ...