Google has officially updated the Maps for Android with a Wi-Fi only option and the update adds the ability for users to download offline areas on SD card. it allows users to save the downloaded offline areas on the internal storage of phone.
You will see a now see a new “WiFi Only” option in the Settings Menu which replaces the “Offline Areas”. When you enable the WiFi Only option, the app will stop using the cellular data and load majority of the data through WiFi.