I'll just say this: don't do it. There's a really good reason why batteries need to be monitored on planes. They can explode in the cargo hold, thanks to the air pressure. This happened on a Delta Airlines flight: a battery melted along with a t-shirt next to it. Luckily staff caught the smoke and everyone was safe, but it's not much of a stretch to imagine what would happen if a battery burst into flames mid-flight.