i used to use an 18 volt power supply for a PC, to charge my starter box batteries, 10xD cells, 12volt, and it had no protection or any mods, i just plugged it in and it pumped a continuous 2.3 amps into the battery,
the battery was acting as its own voltage regulator, it souns dodgy but if you had seen my starter box you probably wouldnt question it.
it worked fine until one day i plugged it in backwards and blew the supply