[AC Question]

using a lot of data on the way to work when its charging in the car

[hallux]

Based on Nexus menus on Nougat - if you go into settings, then data usage then tap on cellular data usage it will show you what apps or services are using data and how much they're using. From there, you can determine what the next steps are. If you have a different device (you didn't say what device you have) your menu structure will be different.

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[Rukbat]

Charging is a hardware function, backing up is a software function. They're not connected.

[B. Diddy]

Welcome to Android Central! You can go to Settings>Data Usage to see what has been using your data. These days, phones are often refreshing data on a frequent basis, even when idle--it could be 3rd party apps, preinstalled apps, or the system. If it's too much for you, you can go to Settings>Data Usage and Restrict Background Data--but keep in mind that this can make you miss email notifications (and other notifications) when your phone is asleep.

[hallux]

Charging is a hardware function, backing up is a software function. They're not connected.

Actually, the Photos app CAN detect charging/not charging and decide to back up based on this setting.