Usually any portable device will warn you or the download info will have the warning to plug in the power cord or to at least have a full battery.
The problem is that sometimes the battery is old or not fully charging and you think you are ok, but then you go down while you are installing firmware and you are really "cooked."
I think a GPS once warned me that if the charge failed while making a firmware update that you would have to send it back to the factory and they would charge you at least for shipping, and it would take a week to 2 weeks, so hook up the power cord for safety.
I have taken their advice and can report good results.
