For those wondering here is the difference in battery life between the new and old models...
Model number: HAC-001 (Old Model)
(product serial number starts with “XAW”)
Approx. 2.5 to 6.5 hours
*The battery life will depend on the games you play. For instance, the battery will last approximately 3 hours for The Legend of Zelda: Breath of the Wild.
Model number: HAC-001(-01) - Available starting in mid-August 2019 (New Model)
(product serial number starts with “XKW”)
Approximately 4.5 - 9 hours
*The battery life will depend on the games you play. For instance, the battery will last approximately 5.5 hours for The Legend of Zelda: Breath of the Wild.
Source: https://www.nintendo.com/switch/tech-specs/