Basically, in the Smart Action you are going to set the Trigger to be display off and the Action for disable cellular data. I disable wifi as well. This way no matter what the situation, the battery isn't draining.
Now, if you're worried about not being able to receive emails or notifications, you can set up time frames within the Trigger setting so that periodically the data or wifi will turn back on even when the screen is off. This way you can still sync. Go into triggers for time frames and start creating new blocks of time, name them whatever, but make sure there's a check mark next to each one and that they're each set to run everyday. You can make the time data turns back on whatever you'd like. I set mine to turn the data back on for 15 minutes every 4 hours. So my first time frame is called A and it's set for 8:15 am to 12:00 pm. Second one is called B and it's set for 12:15 pm to 4:00 pm. And so on and so forth. Just create a full 24 hours of time frames and you'll be good to go. Your Triggers should read: Display off, Time Frame A or B or C or D or E or F. Your Actions should read: cellular data disable, wifi off. And last but not least to create all of this you have to make the rule from scratch. It's one of the options when you tap the green plus button within Smart Actions. Really long winded explanation and set up, but it's well worth it.
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