That said, a plane will keep flying fine with no battery and a failed alternator, since the spark plugs are driven by two redundant magnetos; in fact, there are still planes flying that don’t use batteries and have to be started by hand propping. Granted, the loss of the electrical system would cause awkwardness in controlled airspace (I’d have to talk to ATC on my handheld radio or cell phone), and would be a serious risk in IMC (since I’d have no backup to the vacuum pump).

On a nice VFR day, I would not hesitate to fly a plane with a weak battery back home for repair (assuming I could get a boost to start the plane). I wouldn’t knowingly fly outbound in that condition, or fly at night or in IMC or marginal VMC.