Your lucky it's under warranty, that should be covered. Before you arrange a warranty replacement, try these steps:
1) Next time the icon shows that it's not charging, remove & re-install the battery while laptop is running and plugged in. Make sure it's locked into place.
2) Open Device Manager > Expand Batteries > Uninstall Microsoft ACPI-Compliant Control Method Battery > Reboot
Once rebooted, the computer will automatically re-install the missing driver.
3) Check the ASUS website for a BIOS update for your model (usually found with drivers & downloads). If available, download and install according to their instructions.
4) Contact ASUS - warranty issue!