Capacitive Battery Charger
From Mikey Sklar:
I do have a favorite capactive charger design that I use. It takes 120VAC in and breaks it down into a nice pulse charger for any DC voltage up to 120. It only requires two components. A capacitor and bridge rectifier. I use these to charge all of my diesel vehicle starter batteries and my electric vehicles battery banks. We also use these cheap chargers on all our battery based power tools. I will release a open source kit for this charger in the next few months that can not only do any voltage, but can also offer multiple current settings down to 200mA safe enough to recharge a single AA Ni-MH cell or up to 2A for larger lead acid cells.