0 is a fine number, as long as an unconfigured ADC channel doesn't trigger an error, but a configured channel outputting 0 does trigger the error. We should also be careful about displaying "0" in the GCS when it actually means "unconfigured". 

For that reason, I would prefer a magic number, but I'm not sure what it could be. Nan is a bad choice because it contaminates math further down the line. -1 isn't a better choice because current can flow back into the battery on solar-powered planes. If we can't find a magic number then 0 is okay.