Alas, a missing battery doesn't cause the sad mac on a Plus. But to answer your question about the battery itself, it's a 4.5V unit. These are now getting hard to find (Radio Shack used to sell them, but I've been told that this is no longer the case). You can fashion an equivalent from the little button cell-based batteries found in cordless telephones. Many of these batteries put out 3.6V, but that seems to be enough for a Plus.