My '95 Ford Ranger (4L V6) has done this for awhile but it's getting more frequent. When it's sitting at idle, I hear a small click and the engine speed bumps up just a bit. Then it clicks off and the rpms go back down. Looking under the hood it appears to be the alternator switching on for a few seconds then back off. Last week the battery got real low, it's a good battery (I think, couple years old) but it was very cold and I had moved the truck a couple times without driving it anywhere and I thought maybe it just got worn down. Maybe it's more than that. Anyway I charged it up overnight and it ran fine Saturday and Sunday. The red battery indicator came on a couple times in the first half hour while driving on the highway but that stopped. Drove it an hour or two with several stops Saturday and everything seemed fine, then again Sunday (no trouble lights at all). But sitting still, that alternator was clicking on a lot more often and for shorter times, like 5 seconds each. Seems like the system thinks it needs to charge the battery but when it comes on, it changes its mind and shuts it back off. Seems like I ought to look into it. Is it the alternator, the battery, or the charging control system?