Thanks, everyone. I think you're all in agreement, and I'm also comfortable with something I found on the internet. It says that car voltage changes as the engine races and idles, and that 13.8 is about normal. It also said that modern equipment is marked with a tolerance figure in addition to the nominal voltage and this is usually documented as "13.8 +/- 15%." (Unfortunately, my CB radios and car stereos are all ancient things, mostly bought second hand, and I don't have the documentation.)
nacl is right - batteries are called "12 volt batteries" but it's really more like 13.8.
And, micro: ribit is right that I'm talking about car electronics that I want to run indoors. But he's wrong about grits. If maggots are so smart, they wouldn't be getting eaten, right?
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