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DaveE, No problem! The circuitry is designed for 12V batteries and can handle up to 15V or so to accommodate for a fully charged lead-acid batter. We just didn’t want anyone to daisy-chain a pile of other batter types much beyond 12V, which is why we listed 12V.
We hope to be providing full documentation on the new v.5 Mayfly boards soon!
That’s a good question, thanks for asking: The new v0.5 board will handle an absolute maximum input voltage of 16v, so you should be okay. The 12v limit is the maximum “recommended” voltage. A charger for a 12v battery will output 13 or 14v, but it’s still safe to use that with the Mayfly (version 0.5 only). The older v0.3 and v0.4 boards get way too hot if you put more than 6v on the external input because the onboard solar charging regulator has a max input of 7v. The switch on the new v0.5 disconnects the charging regulator as well as protects the USB port (and your computer) from any accidental overvoltage coming from the external source.