I have a normal Google Android phone that i noticed runs out of batteries so quickly even if turned off. So I did a quick experiment.
I removed the battery for a few days. Wouldn't you know it, the charge was exactly as I left it, unlike before. Makes you wonder what exactly that phone is doing using power while supposedly turned off.
There are many apps that try to phone home with data even if the phone is turned off You would have to wipe these spying apps to reduce power usage.
I have a Samsung phone I haven't registered nor bought data time for, I bought it for a backup, and keep it powered off. But when I power it up, it keeps complaining that it can't reach its masters. Which include Google and Samsung, and even though it wasn't connected, it keeps telling me that there are new updates to download.
They are active and trying to connect even if powered off.