Reserve capacity gives you some idea of how long the battery will last (in minutes) if the charging system dies. However, there are sometimes other reasons for specifying a high-reserve capacity. For example, the '84 Sunbird Turbo had a high-reserve capacity battery. One of its devices could turn the electric cooling fan on after the engine was shut off. This was to help control the underhood temperatures. Of course, this electric motor was a major draw on the battery at a time when it wasn't being charged.
By the way, 4-cylinders are harder to start cold than 8-cylinders. Why? Ask me when you see me.
Well, by now the author of this article and editor of the magazine are probably sorry that they asked me to write a sidebar, so I'll stop here. - John Sawruk, P.E.