For the latest iPhones (every little thing after the iPhone 7) you can purchase a battery with no less than a three,000-mAh capacity, which must Supply you with not less than one total charge for a device. With the exception on the Atom XS, most of the products highlighted earlier https://modernsmarthealth.com/product/computers/power-bank/battery-capacity/15001-20000mah/pisen-fast-charge-power-bank-20000-mah-2-usb-portable-charger-powerbank-18650-external-battery-for-xiaomi-mi-a1-phone-poverbank/