Email and other apps fetch data quite often. But the more they fetch, the more they drain the battery. So try increasing the fetch intervals or fetch data manually. On the iPad, go to Settings > Mail, Contacts, Calendars > Fetch New Data and select "Manually." Alternatively, you can increase fetch intervals to hourly. To turn off auto-sync in Android 4.0, 4.1 or 4.2, tap Settings > Accounts & sync, and touch or slide the switch at the top of the screen to "Off."