A cheap, simple gel-cell charger

I added an LM317 circuit to the top of a Radio Shack "lump-in-a-line" 12 VDC power supply.  This made a gel-cell battery charger that is thermally limited to about one amp, and automatically switches to trickle-charge when necessary.  NOTE:  You CANNOT use this charger for NiCd or NiMH batteries.  Total cost to build is under $20; if you have a well-stocked junk box, you can do it for zip.  Here is a picture (177K).  See May 1994 in the article index.