Restricted items - American Airlines

What the official page says

Portable chargers (power banks): Carry-on - Yes, Checked bags - No. Limit 2 portable chargers per passenger while onboard. Each charger must not exceed 100 watt hours (Wh). Keep portable chargers within reach and visible while in use. Store them in an easily accessible place, such as: a personal item under the seat, the seatback pocket. Do not: use in seat power outlets to recharge portable chargers while onboard; store portable chargers in overhead bins or other enclosed spaces. Lithium ion battery limits (by watt hours, Wh): Less than 100 Wh: Up to 4 spare batteries in carry-on. 100-160 Wh: Up to 2 spare batteries in carry on. 160-300 Wh: Contact Special Assistance before traveling. Large portable power banks or lithium ion generators are not allowed.